Like clockwork - nest attendance patterns and foraging ... fileLike clockwork - nest attendance patterns and foraging behaviour of Snares penguins Eudyptes robustus as a function of

Like clockwork - nest attendance patterns and foraging

behaviour of Snares penguins Eudyptes robustus as a function

of daylength and oceanography

Thomas Mattern Corresp., 1, 2 , Ursula Ellenberg 2, 3 , David M Houston 4 , Lloyd S Davis 1

1 Department of Zoology, University of Otago, Dunedin, Otago, New Zealand

2 Global Penguin Society, Puerto Madryn, Chubut, Argentina

3 Department of Ecology, Environment and Evolution, La Trobe University, Melbourne, Victoria, Australia

4 Science and Policy Group, New Zealand Department of Conservation, Auckland, New Zealand

Corresponding Author: Thomas Mattern

Email address: [email protected]

The breeding routines and foraging behaviour of many pelagic seabird species is

influenced by environmental factors. Seasonality greatly affects the temporal prey

availability for many marine species while the spatial distribution of prey often correlates

to oceanographic features. We examined the influence of such environmental factors on

the nesting routines and the foraging behaviour of Snares penguins Eudyptes robustus

that is endemic to the Snares island group south of New Zealand. Nest attendance and

foraging patterns were studied during the incubation stage of three consecutive breeding

seasons (2002-2004). Nesting patterns observed in one of the biggest colonies (ca 1200

nest) were highly synchronised, with male penguins leaving the colony to forage within a

five-day period around 13 October each year. The males stayed at sea for a mean 11 days

before most males returned within a 7-day period around 24-26 October which also

marked the main departure period of the females. The females’ foraging trips were

considerably shorter and ranged from 5-8 days. The females’ return occurred around the

same dates in 2002 and 2003 (late October) but was markedly later in 2004 (early

November). Nevertheless, the female’s return was always in sync with egg hatching.

Foraging ranges and dive behaviour of six male and two female penguins was examined

with GPS dive loggers and time depth recorders. Four of the six males foraged mainly in

the cooler waters south of the subtropical front (STF), some 200 km east of the Snares.

Dive behaviour of all males indicates primarily travelling behaviour during the first two

days at sea. Two males remained in warmer Central Tasman Water (CTW). Movements of

three birds determined from GPS suggest that the penguins targeted sea areas with

elevated chlorophyll a concentration. Dive behaviour was also related to water mass with

dive depths being on average deeper in the cooler waters of the STF. Both females

remained only within warmer CTW; temperature data suggests that both birds foraged

PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.26653v1 | CC BY 4.0 Open Access | rec: 10 Mar 2018, publ: 10 Mar 2018

north to north-east of the island. Dive data indicates that females travelled continuously

throughout their trips. The Snares penguin’s foraging behaviour is dictated by oceanic

productivity which in turn depends on environmental factors such as day length. Thus,

foraging and, consequently, nesting patterns of incubating Snares penguins are also to a

great extent a product these factors.


1 Like clockwork - nest attendance patterns and foraging

2 behaviour of Snares penguins Eudyptes robustus as a function of

3 daylength and oceanography

4 Thomas Mattern1,2, Ursula Ellenberg2,3, David M. Houston4, Lloyd S. Davis1

5 1 Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand

6 2 Global Penguin Society, Marcos Zar 2716, Puerto Madryn (9120), Chubut, Argentina.

7 3 Department of Ecology, Environment and Evolution, La Trobe University, Melbourne, Australia

8 4 Science and Policy Group, Department of Conservation, Auckland, New Zealand

9 Corresponding author: Thomas Mattern

10 Email address: [email protected]

11


mailto:[email protected]

12 Abstract

13 The breeding routines and foraging behaviour of many pelagic seabird species is influenced by

14 environmental factors. Seasonality greatly affects the temporal prey availability for many marine

15 species while the spatial distribution of prey often correlates to oceanographic features. We

16 examined the influence of such environmental factors on the nesting routines and the foraging

17 behaviour of Snares penguins Eudyptes robustus that is endemic to the Snares island group south of

18 New Zealand. Nest attendance and foraging patterns were studied during the incubation stage of

19 three consecutive breeding seasons (2002-2004). Nesting patterns observed in one of the biggest

20 colonies (ca 1200 nest) were highly synchronised, with male penguins leaving the colony to forage

21 within a five-day period around 13 October each year. The males stayed at sea for a mean 11 days

22 before most males returned within a 7-day period around 24-26 October which also marked the

23 main departure period of the females. The females’ foraging trips were considerably shorter and

24 ranged from 5-8 days. The females’ return occurred around the same dates in 2002 and 2003 (late

25 October) but was markedly later in 2004 (early November). Nevertheless, the female’s return was

26 always in sync with egg hatching. Foraging ranges and dive behaviour of six male and two female

27 penguins was examined with GPS dive loggers and time depth recorders. Four of the six males

28 foraged mainly in the cooler waters south of the subtropical front (STF), some 200 km east of the

29 Snares. Dive behaviour of all males indicates primarily travelling behaviour during the first two

30 days at sea. Two males remained in warmer Central Tasman Water (CTW). Movements of three

31 birds determined from GPS suggest that the penguins targeted sea areas with elevated chlorophyll

32 a concentration. Dive behaviour was also related to water mass with dive depths being on average

33 deeper in the cooler waters of the STF. Both females remained only within warmer CTW;

34 temperature data suggests that both birds foraged north to north-east of the island. Dive data

35 indicates that females travelled continuously throughout their trips. The Snares penguin’s foraging

36 behaviour is dictated by oceanic productivity which in turn depends on environmental factors such


37 as day length. Thus, foraging and, consequently, nesting patterns of incubating Snares penguins are

38 also to a great extent a product these factors.

39 Introduction

40 The distribution of seabirds in the world’s oceans is to a product of dynamic physical processes.

41 Currents and oceanographic features such as fronts play the most important part in the distribution

42 of nutrients and, linked to that, primary production (Chang & Gall, 1998). Phytoplankton is an

43 essential determinant for the occurrence of zooplankton and subsequently higher trophic levels of

44 the food web (Bradford-Grieve et al., 2003). The rate of primary production depends on solar

45 energy, so that at high attitudes the concentration of phytoplankton fluctuates strongly with season

46 (Murphy et al., 2001). Consequently, any biological processes at higher trophic levels follow the

47 dictate of seasonality.

48 Penguins are important top-level consumers in the marine environment (Croxall & Lishman, 1987).

49 The biology of most penguin species reflects the environmental conditions that determine the

50 abundance and distribution of their food. This is particularly apparent in primarily planktivorous

51 species like most crested penguins (Eudyptes sp.). The annual breeding cycles of most crested

52 penguins is highly synchronised and coincides with the seasonal increase in food availability and

53 day length (Williams, 1995). At the same time, the distribution of penguin prey is often linked to the

54 presence of oceanic fronts. Here concurrent water masses accumulate nutrients that fuel primary

55 production which makes such areas an attractive, spatially predictable source of food for penguins

56 (e.g. Hull, Hindell & Michael, 1997; Tremblay & Cherel, 2003).

57 The Snares penguin (Eudyptes robustus) is endemic to the small Snares island group some 200 km

58 south of the New Zealand mainland. Just as in other crested penguins, the timing of breeding as well

59 as the general nesting patterns are similar between years (Mattern, 2013). This is particularly

60 apparent during the incubation stage, where nesting patterns of Snares penguins are well


61 structured. Egg-laying in late September is followed by a two-week period during which both

62 partners stay at the nest. In mid-October, some 25.000 breeding males leave the island within a 48

63 to 72-hour timeframe (the ‘exodus’) which leaves the females in charge of the incubation. The males

64 return after a two-week period at sea and take over incubation duties from their mates who leave

65 the colonies for one-week foraging trips (Warham, 1974). Chicks also hatch fairly synchronous

66 within a 3 to 5-day period at the end of October and beginning November. The females usually

67 return within 24 hours of the hatching of their chicks (Warham, 1974). While the synchrony of

68 these events might be a result of social interactions (e.g. Fishman & Stone, 2006) environmental

69 factors are likely to play an equally if not more important role (Davis & Renner, 2003).

70 The incubation stage (September-November) coincides with the onset of the spring bloom of

71 phytoplankton in New Zealand waters (Murphy et al., 2001). One of the main components of the

72 Snares penguins’ diet is the euphausiid Nyctiphanes australis (Mattern et al., 2009), a species of krill

73 that feeds predominantly on phytoplankton (Ritz, Hosie & Kirkwood, 1990) and, thus, provides a

74 link between Snares penguin foraging and marine primary production. However, particularly

75 during the early incubation stage (September & October) the oceanic productivity around the

76 Snares still tends to be low (Murphy et al., 2001). Under these circumstances, it seems likely that

77 the penguins utilise an important oceanographic feature in relative proximity of the Snares, the

78 Subtropical Front (STF). At the front, warm, saline subtropical Central Tasman Waters (CTW) and

79 cool, less saline subantarctic waters (SAW) converge (Heath 1983, see Fig. 1). Convergence zones

80 create mixing processes that have been found to accumulate planktonic prey of seabirds

81 (Schneider, 1990).

82 The aim of this study was to examine whether the breeding synchrony in Snares penguins can be

83 explained with the seasonal developments in their marine environment. We predicted that (a) the

84 timing of the male exodus is connected to the onset of the phytoplanktonic spring bloom in the

85 Snares region; (b) the male penguins travelled to the cooler waters at Subtropical Front to


86 compensate for still low productivity closer to the islands; (c) the female penguins benefit from

87 advanced productivity closer to the island during their incubation foraging trip; and (d) that chick

88 hatching coincides with the spring bloom setting in directly around islands.

89 To test these predictions, we studied the foraging ranges and dive behaviour of breeding Snares

90 penguins using newly developed GPS dive loggers and standard time-depth recorders. At the same

91 time nest attendance patterns were recorded to determine the level of synchrony of keystone

92 events during breeding and their relationship to seasonal changes of environmental conditions (as

93 determined from satellite-based measurements of chlorophyll a concentrations and see surface

94 temperatures).

95 Material and methods

96 Timing of field work and study site

97 Nest attendance patterns and foraging behaviour of incubating Snares penguin were studied during

98 three consecutive breeding seasons from 2002 to 2004 on the Snares’ main island, North-East

99 Island. All observations and logger deployments occurred in the second largest Snares penguin

100 colony (A3) that comprises ca. 1200 breeding pairs. This colony was chosen not only for ease of

101 accessibility but also because some basic information on nest attendance patterns for the same

102 colony is available in Warham (1974).

103 In the first two years, the research team arrived on the Snares on 06.10.2002 and 09.10.2003,

104 respectively, and allowed timing of onset and end of the exodus of breeding males in colony A3 in

105 both years. In 2004, field work commenced on 17 October after males had already left the island.

106 Duration of expeditions ranged from five weeks in 2002 to six weeks in 2003 and 2004.

107 Nest attendance patterns


108 To determine nest attendance patterns, observation plots in colony A3 were established. In 2002,

109 only data for one plot comprising 42 nests is available, while in the following two years three

110 observation plots were established in different areas of the colony encompassing 40 to 60 nests

111 each. In total, 154 nests were monitored in 2003, and 166 nests in 2004. Each observation plot was

112 assigned to a single observer who conducted two to six-hour long observations every day

113 (generally between 1100 and 1700 hours). During observations nest status and sex of attending

114 adult were recorded. Additionally, change-over times of breeding adults as well as other

115 behavioural activities in the plot were noted. Marking penguins was not permitted so that

116 identification of adults had to be achieved by passive means only. Male Snares penguins have a

117 markedly heavier bill than females so that the sex of each mate in a pair could be determined

118 visually when both partners were together at the nest. Individual features such as melanisms on a

119 penguin’s breast, overall body composure or scars were additionally used to identify birds. The

120 return of a male to its nest was easy to notice as males were considerably fatter than the incubating

121 females and quickly produced numerous scat marks radiating from the nest bowl. Females

122 returning from their long incubation trips generally spend several hours on their nests – even if

123 eggs had not yet hatched – before leaving for another shorter foraging trip (Warham 1974, own

124 observations) so that the likelihood of females returning and departing before the arrival of the

125 observer was minimal.

126 GPS loggers and Time Depth Recorders (TDR)

127 Foraging and diving behaviour was studied using two types of data loggers. GPS data loggers (GPS-

128 TDlog, earth&OCEAN Technologies, Kiel, Germany; dimensions: L100xW48xH24mm, mass: ~70g)

129 recorded information of the penguins’ at-sea movements and dive behaviour. The device contains a

130 GPS receiving unit that determines accurate geographical position (position error <10m) from

131 signals from orbiting satellites of the Global Positioning System. The device furthermore contains

132 high precision environmental sensors that record dive depth (resolution: ~0.1m) and ambient


133 water temperature (resolution: ~0.005°K). However, during all deployments the temperature

134 sensors failed and recorded no data. GPS and sensor data are stored with precise date and

135 timestamp in the device’s internal non-volatile flash memory and must be downloaded to a

136 computer after device recovery. The loggers’ sampling regime is freely programmable and was

137 setup to record a GPS position after each dive, and to store sensor readings at 5 s intervals. Since

138 acquisition of a GPS fix takes between 25 and 30 seconds (Mattern et al., 2005) no position could be

139 recorded when a penguin stayed at the surface for shorter intervals between dives.

140 Time Depth Recorders or TDRs (MK9, Wildlife Computers, Redmond, WA, USA; dimensions:

141 L67xW17xH17mm, mass: ~30g) were additionally used to study dive behaviour only. The TDRs

142 contain a pressure transducer to determine dive depth (resolution: 0.5 m) and a temperature

143 sensor (resolution: 0.05°C). The TDRs are also freely programmable and were setup to record dive

144 depth and temperature at 5 s intervals similar to the GPS logger sampling regime. Additionally, the

145 device features a wet/dry sensor that triggered the TDR to sample only when wet (i.e. during dives)

146 and stopped sampling while still keeping track of the time when dry (i.e. at the surface). This

147 greatly reduced battery consumption and allowed data to be recorded for complete long-term

148 foraging trips.

149 The devices were attached with adhesive tape (Tesa-tape method, Wilson et al., 1997) to the

150 penguins’ lower backs to reduce drag (Bannasch, Wilson & Culik, 1994). All loggers were deployed

151 at colony A3. The penguins were captured at the nest while their mates were incubating; the birds

152 were then carefully transferred out of the nesting area. Before logger deployment, the penguins

153 were weighed to the nearest 50 g in a cloth bag using spring balances. Deployment involved two

154 persons, one to hold the bird and one to attach the device. During the attachment procedure the

155 penguins head was covered with a cloth hood to reduce stress. After successful attachment the

156 penguin was carried back into the colony and released 5 m from its nest site. The entire handling

157 time (i.e. capture, measurements, deployment and release) ranged between 12 to 18 minutes. All


158 loggers were recovered after the penguins’ long-term foraging trips. Birds were recaptured either

159 at the penguins’ main landing site which is located next to the research huts in Station Cove, or

160 when the birds returned to their nest in the colony.

161 In 2003, four males were equipped with GPS loggers and three males with TDRs before they left on

162 long-term trips. The number of deployments was limited by the number of devices available for the

163 study. This meant that the devices had to be recovered and data downloaded before they could be

164 re-deployed on females. Three of the males returned too late for re-deployment, so that only four

165 females could be fitted with two GPS and two TDRs, respectively, before they left on long trips. Both

166 GPS logger deployments on females failed – one bird returned with a water-logged device while the

167 other female did not return to the colony. In 2004, an attempt was made to collect additional GPS

168 logger data on two females during incubation, but both deployments again failed due to

169 malfunction of one device and water logging of the other.

170 Oceanographic data

171 At-sea distribution of penguins was analysed with regard to sea surface temperatures (SST, °C) as

172 identifier of water masses and chlorophyll a concentration (ChlA, mg/m³) as a measure of primary

173 production. Both parameters were assessed from satellite ocean colour data recorded by NASA’s

174 Moderate-resolution Imaging Spectroradiometer (MODIS/Aqua) programme

175 (http://oceancolor.gscf.nasa.gov). Depending on the satellite’s data coverage which is affected by

176 cloud cover, either weekly (8-day) or monthly data sets were used. The data is available as Level-3

177 Standard Mapped Image (SMI) that give average ChlA concentration and SST in global, equal-area

178 cells with spatial resolutions of 4x4 km (Fantoni et al., 2010).

179 Data analysis

180 All analysis of GPS, dive and MODIS data was done with custom written software (Mattern, 2006).

181 GPS data was used to linearly extrapolate the penguins’ foraging tracks from all recorded fixes, to


182 calculate travel distance as the sum of linear distances between consecutive GPS fixes, and to

183 determine the furthest linear distance between a position fix and the island. The tracks were then

184 analysed with regard to oceanographic parameters that occurred along a penguin’s extrapolated

185 travel route by determining the relative time a penguin spent within a cell of the MODIS SMI. That

186 way daily averages of ChlA concentration and, in the case of birds equipped with GPS loggers, SST of

187 the sea areas visited were determined for each bird. The TDRs temperature readings at the surface

188 fluctuated depending on the weather situation (e.g. sunshine resulted in higher temperature

189 readings) so that sea surface temperature was estimated from temperature data recorded during

190 each dive at depths between 5 and 10m. The TDR temperature readings served as indicator for

191 water mass.

192 From dive data, basic parameters were determined such as dive time, dive depth and duration of

193 post-dive interval the penguins spent at the surface. Three different dive phases were

194 distinguishable, the descent, the bottom phase and the ascent (Wilson, 1995). The start of the

195 bottom phase was defined as the time when the penguins’ descent rate (i.e. vertical velocity)

196 became less than 0.2 m s-1 after a continuous descent. Accordingly, the end of the bottom phase

197 was marked by an increase in vertical velocity >0.2m s-1 followed by a continuous ascent to the

198 surface. The time between start and end of the bottom phase was defined the bottom time. Descent

199 and ascent rates were calculated from the depth change and transit time during the ascent or

200 descent phase.

201 Each dive was analysed with regard to the depth reached during the preceding dive. Consecutive

202 dives reaching similar depths were defined as repeated maximum depth (RMD) dives. Similarity of

203 depths was accepted when a dive reached the previous dive’s maximum depth ±10% (following

204 Tremblay & Cherel, 2000). Under the assumption that after a penguin that has located a prey patch

205 at a specific depth it returns to a similar depth during consecutive dives (i.e. dive bout, cf. Wilson,


206 1995), RMD dives are more likely to represent feeding rather than searching behaviour. To filter

207 out travelling behaviour only dives deeper than 15m were included in the RMD analysis.

208 Additionally, diving efficiency (bottom time/[dive time + post-dive interval]; Ydenberg & Clark,

209 1989) and foraging effort (dive time/[dive time + post-dive interval]) were calculated for each dive.

210 Dive events could only be accepted when the penguins dived deeper than 3m. This was due to the

211 fact that the TDRs pressure transducer showed considerable fluctuations close to the surface. For

212 comparative reasons, GPS logger data was treated similarly although surface fluctuations were no

213 problem. For general comparison amongst birds, all accepted dive events were used, except when

214 the emphasis of the comparison lay on foraging dives. During the analysis of dive data it was found

215 that Snares penguins often performed travelling dives (i.e. dives where the birds covered large

216 distances in short time) at depths between 10 and 20 m. Therefore, if required travelling dives were

217 filtered out by analysing dives >20 m only.

218 All statistical analysis was carried out in Minitab 14 (Minitab Inc, State College, PA, USA). Data were

219 tested for normality using the Kolmogorov-Smirnov test. Details of statistical test employed to

220 compare data are given in the text or table and figure captions. Averages are given as

221 mean±standard deviation unless indicated otherwise. Comparison between group of birds (e.g.

222 males vs. females) were made using individual means for each bird to avoid pseudo replication.

223 Statistical significance was accepted at the <0.05 level.

224 Results

225 Nest attendance patterns and foraging trip durations

226 The departure of breeding males for their long post-laying foraging trip was a highly synchronous

227 event. The exodus was heralded in colony A3 by the departure of the first individuals on 11 October

228 2002 and 10 October 2003, respectively (Fig. 1). In both years, the main portion of breeding males

229 left the colony within a four-day time window (11-15 October) that was identical in both years.


230 Quite possibly, the exodus in 2004 occurred in a similar time frame as – with one exception – no

231 breeding males were present in the colony by the time nest monitoring started on 17 October while

232 return dates that year were similar to 2002 and 2003. The foraging trip lengths determined from

233 nest attendance patterns in 2002 (mean trip length: 11±2 d, range: 8-17 d; n=29) and 2003 (11±2 d,

234 range: 6-21 d, n=126) were identical (t-test: t152=0.819; p=0.414). The return of the males was

235 somewhat less synchronized than departure. But still, 75% of the males arrived back at their nests

236 within four days between 22 and 26 October in all three seasons (Fig. 1).

237 The departure of the females was closely related to the return of their mates (Fig. 1). Usually a

238 female would leave the nest shortly after her mate’s return unless the male arrived back in the

239 colony in the late afternoon. In this case, females generally departed early the next morning. Median

240 departure dates of females ranged between 24 and 25 October in all three years. Foraging trip

241 durations of the females varied between years. Mean trip durations were comparable in 2002

242 (5.5±2.1 d, range: 1-9 d) and 2003 (6.1±2.4 d, range: 1-11 d) but were significantly longer in 2004

243 (8.2±2.7 d, range: 1-16 d; ANOVA with Tukey’s post-hoc: F1,2=23.98, p<0.001). Despite these

244 differences, the return of the females coincided with egg hatching in all years (Fig. 1). Median return

245 and hatching date in 2002 and 2003 was 29 October. In 2004, the median date of return was 31

246 October, the median hatch date was a day later (01 November).

247 At-sea movements

248 The deployments of four GPS loggers on male Snares penguins on long-term trips during incubation

249 2003 resulted in GPS/dive data sets for three of them; the fourth male did not leave its nest for five

250 days and the device was recovered without any at-sea data. The three other birds left the island on

251 the 15 and 16 October, respectively (Table 1). The data recorded before the loggers’ batteries were

252 exhausted encompassed ca. three days (mean operation time: 2.2±0.2 days). This relates to one-

253 third to one-fifth of the complete trip durations (mean trip duration: 12.0±4.4d). During the loggers’


254 operation time, the penguins foraged an average 158.2± 59.7 km away from the Snares and covered

255 distances of up to 226.3 km (Table 1). All three penguins travelled due east from the island (Fig 2

256 map). Two of the birds crossed into the deeper waters beyond the shelf edge of the Snares Rise in

257 the evening of their second day at sea. The third male changed its easterly course while still in the

258 shallower waters (<200 m). The horizontal speeds of all three birds decreased while dive depth

259 increased with trip duration and were lowest during the last day of logger operation (Fig 2a&c).

260 During their third day at sea, two birds foraged in waters of the subtropical front (STF, sea surface

261 water temperature < 10°C, Fig 2b, compare with Fig 3 map) that featured relatively high

262 productivity (ChlA concentration ~0.3 mg/m³, Fig 3d). The third penguin’s course change also

263 coincided with a patch of high primary production similar to the conditions at the front but was still

264 some 80 km short of the cooler waters of the STF (Fig 3 map, b&d).

265 The deployments of three-time depth recorders (TDR) on male Snares penguins resulted sensor

266 data for complete foraging trips. The three birds all left on the same day (14.10.2003) and stayed at

267 sea for 8.8 to 13.4 days (mean trip duration: 11.3±2.5d). While the TDRs did not record any spatial

268 information, the temperature data give some indications about the birds’ general movements at sea.

269 All three penguins foraged in waters > 10°C during their first two to three days at sea (Fig 3, top

270 graph). The temperature profiles of two birds then dropped markedly to surface temperatures of <

271 10°C indicating that the birds entered the cooler waters of the STF to the east of the Snares (Fig 3

272 map). Both penguins stayed at the front for most of their time at sea (6.1 of 11.1 days and 4.3 of 8.8

273 days, respectively) and returned to the island within three days. The third male equipped with TDR

274 foraged for 13.4 days and stayed in waters >10°C all the time. Nevertheless, during its first few days

275 at sea the bird foraged in cool waters (10-11°C) which shows that it nevertheless got close to the

276 front (compare Fig 3, top graph, light grey line with Fig 3 map).

277 After the return of the three males, two of the TDRs were re-deployed on incubating females leaving

278 on long foraging trips. Both females left their nests on the 26 and 27 October, respectively. They


279 both foraged for 4.1 days and, thus, considerably shorter than the males. During their entire time at

280 sea both females stayed in waters >11°C (Fig 3, middle graph). Considering the distribution of the

281 isotherms compiled from satellite sea surface temperature data, the females therefore must have

282 foraged north to north-east of the Snares (Fig 3 map).

283 Diving behaviour

284 Although dive behaviour varied amongst individuals, these variations were not related to device

285 type. When tested with regard to device type basic dive parameters did not differ significantly (t-

286 test of means during first three days at sea; max depth: GPS – 45.0±11.0 m, n=3, TDR – 51.7±4.7 m,

287 n=3, t5=-0.39, p=0.725; dive time: 108.1±18.6 s vs. 125.3±9.7 s, t5=-1.42, p=0.251; diving efficiency:

288 0.27±0.04 vs. 0.26±0.01, t5=0.80, p=0.508; foraging effort: 0.77±0.01 vs. 0.755±0.01, t5=1.91,

289 p=0.151). Therefore, dive data of all six males were deemed comparable regardless of device type.

290 The dive behaviour of males reflected the gradual change from travelling to foraging behaviour

291 during the first three days at sea, regardless of the birds’ destinations (Fig. 3a&b). Most dive

292 parameters differed significantly between the first and third day at sea (Table 2). Higher transit

293 rates to greater depths combined with a significantly longer bottom time furthermore resulted in a

294 higher diving efficiency during the third day at sea which underlines the shift from travelling to

295 prey searching/feeding behaviour.

296 The daily means of dive parameters determined for three males with TDRs over the entire duration

297 of their foraging trips showed strong correlations with the sea surface temperature (Fig 4). The

298 duration of a dive cycle (i.e. dive time, bottom phase and post-dive interval) was shorter in warmer

299 Central Tasman Water (CTW). The birds dived longer and deeper at the front (sst <10°C) and

300 showed a higher frequency of RMD dives (Fig 4). The frequencies of dive depths show a marked

301 bimodality for the two penguins foraging at the STF (Fig. 5 left graph, grey bars). 23% of all dives

302 were less than 20m deep which suggests that one fourth of the penguins’ dive activity might be


303 related to travelling. 50% the dives at the STF were deeper than 85m. Although the birds showed

304 similar bimodal depth frequencies, 30% of the dives were shallower than 20m and only 26% were

305 deeper than 85m (Fig 5 left graph, black bars). The dive depths were much more evenly distributed

306 along the entire depth spectrum in the male that remained in CTW during its entire foraging trip

307 (Fig. 5 centre graph).

308 Contrastingly, the two females showed a strong preference for dives in the upper 20m (54% of all

309 dives) suggesting a strong emphasis on travelling throughout the entire foraging trip (Fig 5, right

310 graph). Consequently, dive behaviour of the females differed from males (Table 3). Dive times of

311 females were significantly shorter than in males and same was true for bottom times. With the

312 exception of descent and ascend rates that were similar, most other dive parameters also differed

313 considerably between sexes. Although statistical significance could not be detected for dive depth

314 and diving efficiency presumably as a result of small sample sizes the obvious differences between

315 the sexes nevertheless represent a strong trend.

316 Discussion

317 In all three years, Snares penguins exhibited nest attendance patterns that were highly

318 synchronized. The synchrony of some key events of breeding – primarily the timing of male exodus

319 and, to a lesser extent, the return of the males and departure of the females – was not limited to the

320 respective breading seasons but was also remarkably similar between the years. This interannual

321 synchrony suggests that breeding patterns are strongly influenced by day length which in turn also

322 determines the onset of the phytoplankton spring bloom and, thus, the availability of penguin prey.

323 Synchronous departure of the males

324 During the first two years, the male exodus ranged around the same median date (13 October). The

325 departure of the males was highly synchronous and the colony was practically devoid of breeding

326 males within 5 days. The timing of the exodus also seems to be consistent with historic records.


327 Warham (1974) reported that in colony A3 the departure of the males “was almost completed by

328 15 October 1972”. Nest attendance and return of males in 2004 suggest that the exodus most likely

329 occurred around the same time of year (Fig 2). Furthermore, the synchrony of the male exodus does

330 not seem to be restricted to birds from the same colony. During the time of the exodus of males

331 from colony A3, the traffic of male penguins leaving the island at Station Cove (i.e. next to the

332 research huts) had increased markedly. From sporadic counts of penguins at this stage between

333 800-1000 birds per day were estimated to leave the island at Station Cove. This volume of penguins

334 greatly exceeds the number of breeding pairs in the study colony (ca. 1200 pairs) and suggests

335 similar departure times and, thus, nesting patterns in other colonies.

336 The highly synchronous departure of the males has also been reported in Erect-crested penguins

337 (Eudyptes sclateri). Davis & Renner (2003) found that the male Erect-crested penguins left the

338 colony within a three-day period independently from egg-laying dates, and suggest that the

339 synchrony of the males’ exodus might minimize the probability of aggressive assaults on lone

340 females that could result in nest failure. While social interactions indeed are believed to facilitate

341 breeding synchrony in seabirds (e.g. Fetterolf & Dunham, 1985; Waas, 1988; Waas et al., 2000),

342 social stimuli might only explain intra-colonial synchrony but offers no explanation for the inter-

343 annual synchrony observed during this study.

344 Instead, the inter-annual – and perhaps inter-colonial – synchrony of the departure of incubating

345 male Snares penguins correlates strongly with the date which suggests photoperiod as primary

346 trigger for the male exodus. Birds are photosensitive and daylength is known to induce hormonal

347 and subsequently behavioural responses (Dawson, 2008). For penguin species living in temperate

348 and polar regions, photosensitivity facilitates the synchronisation of reproduction with seasonal

349 changes in the environment (Cockrem, 1995), most importantly food availability within range of

350 the breeding location (Croxall & Davis, 1999).


351 Foraging of male penguins and influence of Hydrography

352 A likely explanation for the synchronisation of male Snares penguins’ departure with photoperiod

353 is the onset of the phytoplankton spring bloom in the waters around the Snares in October each

354 year (Murphy et al., 2001). Increased phytoplankton biomass often correlates with high

355 zooplankton abundance (Krell et al., 2005) which subsequently makes phytoplankton-rich areas at

356 sea particularly interesting for planktivorous top level predators like whales and seabirds

357 (Bradford-Grieve et al., 2003). Considering that planktonic crustaceans are a dominant prey species

358 in crested penguins (Garcia Borboroglu & Boersma, 2013, but see Mattern et al., 2009) a

359 relationship between chlorophyll concentration (as a measure of phytoplankton abundance) and

360 foraging behaviour seems evident (e.g. Tremblay & Cherel, 2003).

361 Due to subtle interactions of factors limiting primary production in New Zealand’s subantarctic

362 phytoplankton biomass is not evenly distributed in the waters around the Snares (Murphy et al.,

363 2001). Apart from localised occurrences of high chlorophyll concentrations (Banse & English,

364 1997), the subtropical front (STF, Figs 2&3) represents a sea region with probably the most

365 predictable primary production within range of the Snares. The STF is known to feature elevated

366 chlorophyll a concentrations throughout the year (Murphy et al., 2001) which was also the case

367 during the time of this study (Fig. 2). Such properties render the STF a reliable food source for

368 seabirds (compare the “Snares Islands hotspot” in Waugh et al., 2002) and make the front a

369 tempting destination for male Snares penguins too.

370 Indeed, the front was the destination of four of the six males equipped with data loggers. The GPS

371 data recorded on one male that did not reach the front, shows that the bird nevertheless travelled

372 towards the front until it reached a patch of high primary production. Similarly, the temperature

373 readings of one TDR bird that did not enter waters <10°C suggest that it must have foraged close to

374 the front before orienting back into warmer CTW (Fig. 3). Supporting this, all birds’ dive behaviour


375 during the first days at sea reflects primarily travelling behaviour. Dive times and depths increased

376 considerably in all birds between the first and the third day at sea (Table 2) and indicate a gradual

377 shift from shallower travelling dives to deeper dives which indicate prey searching behaviour.

378 Considering this, the benefit travelling to the STF or towards it must have been significant. This

379 implies that the likelihood to encounter productive areas within CTW – and, thus, closer to the

380 Snares – was probably considerably lower than at the STF. In this light it is interesting, that two of

381 the males with GPS loggers apparently “ignored” the area of high productivity that obviously was of

382 interest for the third male (Fig 2 map). A possible explanation for this might be that the first two

383 penguins passed through this area almost 48 hours earlier than the third male, and it is conceivable

384 that a more favourable prey situation developed in the time between the transit of the first two

385 penguins and the arrival of the third bird.

386 The diving behaviour of the male penguins equipped with TDRs was a function of the sea surface

387 temperature (Fig. 4). The penguins dived deeper and longer at the STF than when they were in

388 CTW. More than 50% of all dives recorded at the front were deeper than 85m, suggesting a primary

389 exploitation of prey patches at greater depths. Another, less pronounced peak is apparent in the

390 upper 20m (~30% of all dives, Fig 5) which can be attributed to travelling behaviour. The

391 bimodality is most likely a result of the downwelling mechanisms that are in play at the STF which

392 transport nutrients and plankton to greater depths (Nodder & Gall, 1998). This downward

393 transport at the front is restricted by a vertical temperature gradient which is strongest at depths

394 between 100 and 200m (Morris, Stanton & Neil, 2001). That way, the temperature gradients

395 produce a horizontal as well a vertical barrier at the front which, therefore, acts as catchment for

396 the penguins’ planktonic prey.

397 The bimodality in depth frequencies was also apparent although less pronounced when both

398 penguins foraged in warmer waters (Fig.5). It is possible that the frequency of dives to depths

399 >65m stem from diving activity close to and, therefore, still influenced by the front. In contrast to


400 this, the male that foraged in CTW only, showed a much greater diversity of dive depths (Fig. 5,

401 centre graph). Unlike the STF birds, the penguin obviously did not concentrate its foraging efforts to

402 certain depth classes but rather utilised the entire water column to forage. In CTW, the occurrence

403 of productive patches is less defined than at the front and depends largely on interactions of factors

404 such as local nutrient availability and wind influence (Murphy et al., 2001).

405 Although there are apparent differences in dive behaviour of males visiting the STF and males that

406 remain in CTW, the data does not allow conclusions with regard to the success of either foraging

407 strategies. The TDR bird foraging in CTW stayed considerably longer at sea than the birds that

408 foraged at the STF (13.4 vs. 8.8 and 11.1 days). Conversely, the penguin with GPS logger that

409 apparently did not reach the STF returned earlier than the other two GPS birds (7.9 vs. 10.2 and

410 15.3 days, Table 1).

411 The timing of the males’ return was still remarkably similar between all three years and appears to

412 be independent from hatching – and, therefore, laying dates (Warham, 1975) – which hints at a

413 prevailing influence of daylength on the trip length (Fig. 1). Nevertheless, when compared to the

414 synchrony of the exodus, the males timing of return varied considerably, quite possibly as a result

415 of varying foraging success of the individuals (Davis & Renner, 2003).

416 Foraging of female penguins

417 In comparison to the males, the females face a different situation when they leave for their long-

418 term foraging trips. Probably most importantly, the females’ time at sea is limited by the need to

419 return to the nest around the time of hatching to feed their chicks (Davis & Renner, 2003).

420 Considering that the females’ average foraging trip durations ranged between 5-6 days in the first

421 two seasons, travelling to the STF to forage is an unlikely strategy. Accordingly, the temperature

422 profiles of two females fitted with TDRs in 2003 show that the penguins solely foraged in warmer

423 CTW to the north or north-east of the Snares (Fig 3). Temporally detailed chlorophyll a data is not


424 available for the second half of October 2003 because of increased cloud cover during the satellite

425 passes. So, it is not possible to directly relate the estimated female foraging area to oceanic

426 productivity. However, considering that the chlorophyll a concentration around the Snares tends to

427 increase rapidly from October on each year (Murphy et al., 2001), it is possible that the females

428 benefited from accelerated productivity closer to the islands.

429 Overall, the two females fitted with TDRs showed higher diving activity and performed

430 considerably shallower dives than males (Table 3, Figure 5). A penguin’s chance to encounter prey

431 patches increases with distance travelled (Wilson and Wilson 1990). Given the timeframe to forage,

432 a combination of travelling and prey searching behaviour appears to be a viable strategy for the

433 females. This is furthermore supported by the fact that the females exhibited a much lower

434 proportion of RMD dives indicating that dive bouts targeting prey patches at certain depths must

435 have been brief and interspersed by shallower travelling episodes (Table 3).

436 In 2004, the foraging situation for the females was different. While the males’ return dates were

437 similar to the previous years, the chicks hatched about two days later (Fig. 1) and as a consequence

438 the females could stay at sea for longer. The fact, the foraging trips were indeed significantly longer

439 indicating that the females nest attendance patterns and, thus, time available for foraging is linked

440 to egg status. This, in turn, makes it likely that hormonal mechanisms (e.g. Davis and Renner 2003,

441 Massaro 2004) have a stronger influence on the females’ foraging behaviour than day length and,

442 consequently, oceanography.

443 Conclusions

444 Nest attendance patterns during the incubation period of Snares penguins are a direct function of

445 daylength and the onset of the algal spring bloom along the subtropical front some 200 km east of

446 the Snares Islands. Especially in the male penguins that leave first on a long foraging trip after a

447 shared egg incubation show, a strong influence of the daylength is apparent. Indeed, it appears as if


448 there has been little variation in the mean departure date of male penguins in the past 40 years.

449 GPS dive data shows that the male penguins seek out sea areas of increased productivity to forage.

450 For females leaving on their incubation shift foraging trip, the return date of their mate plays the

451 main role in determining their departure. At the same time, it appears as if by that time the oceanic

452 productivity around the Snares has reached a level that allows the females to find adequate food

453 resources closer to the island, presumably to the north. This, in turn, facilitates the females’

454 remarkable ability to time their return precisely with the hatching of the eggs. Probably the main

455 reason for the remarkably consistent nest attendance patterns in Snares penguins is the species’

456 breeding limitation to the small Snares archipelago, so that all penguins are exposed to the exact

457 same set of environmental variables that influence their nest attendance patterns and foraging

458 strategies during the egg incubation.

459 Acknowledgments

460 We thank Alvin Setiawan and Katrin Ludynia for invaluable help with the observational study in

461 often unfavourable conditions on the Snares. Further thanks have to be expressed to Gerrit Peters,

462 earth&Ocean Technologies, for his unlimited support during one of the first field application of the

463 newly developed GPS loggers. Lars-Gunnar Ellenberg contributed to the development of the

464 analysis software. This study was approved by the Department of Conservation (RES-23444) and

465 the University of Otago Animal Ethics Committee (AEC #02-12) and complies with the current laws

466 of New Zealand. The study was supported by a University of Otago Postgraduate Scholarship to

467 Thomas Mattern and an Otago Research Grant issued to Lloyd Spencer Davis.

468

469

470


471

472


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Figure 1

Interannual synchrony in departure and arrival dates of adult Snares penguins during

incubation, and hatching dates in three consecutive breeding seasons, 2002-2004.

Boxplot derives from nest monitoring data collected each year between 10 October-15

November in one colony (A3) on the Snares Islands. Only data from nests that successfully

hatched at least one egg were included. Medians are given as vertical lines, boxes enclose

first and third quartile of sample, whiskers encompass 95% of sample, dots indicate outliers.

Sample sizes in all years are given in top right corner of figure.



Figure 2

Partial foraging tracks of three male Snares penguins on long-term foraging trips during

incubation in 2003in relation to oceanic productivity.

Map shows the penguins’ at-sea movements in relation to weekly average Chlorophyll a

concentration at the time (16-23 October 2003, from MODIS/Aqua data). Isolines/shading

represent Chlorophyll A concentration (mg/m³) at 0.05 mg/m³ intervals. No ChlA data were

available for white areas (cloud coverage). Dashed line indicates 200m depth contour.

Perpendicular lines intersecting foraging tracks and adjacent numbers give position at

midnight and according date change. Line plots (A-D) summarise mean travelling speed and

mean maximum dive depth (±standard error, left column), and mean sea surface

temperature and mean ChlA concentration (determined from satellite data; ±standard

deviation, right column) along the penguins’ tracks.



Figure 3

Ambient water temperatures (between 5-10m depth) recorded by Time-Depth

Recorders (TDR) on three male and two female Snares penguins undergoing long-term

trips during incubation; and mean sea surface temperatures for October 2003 as

determined from MODIS/

Temperature was sampled at 5s (males) and 2s (females) intervals, hourly means were used

to compile graphs. Grey bars in top graphs and grey area in temperature map highlight sea

surface temperatures between 9-10°C and represent the subtropical front (STF). Dashed line

indicates the 200m depth contour. Note: monthly means used to compile map were skewed

towards cooler temperatures measured in the first half of October; satellite data for the

‘warmer’ second half of the month (i.e. deployment period) was patchy.



Figure 4

Correlations of dive parameters determined for male Snares penguins (n=3) with TDRs

on long foraging trips during the incubation stage in 2003.

Graphs were compiled using daily means of dive parameters and sea surface temperature of

the respective foraging trips (durations: 10, 12 and 14days). Significance was tested using

Pearson’s correlatio.



Figure 5

Frequencies of dive depths in Snares penguins (3 males, 2 females) equipped with TDRs

on long foraging trips during incubation 2003.

Depth frequencies were determined with regard of the watermass the birds foraged in: two

males foraged at the Subtropical Front (STF, grey bars) and in Central Tasman Waters (CTW,

black bars); the third male and the two females remained in CTW for their entire duration of

their foraging trips. Water masses were distinguished via temperature readings (STF <10°;

CTW >10°C) recorded by the dive loggers between 5 and 10m depth (see Methods for

details). Dotted lines indicate the 50% mark of the cumulative depth frequencie.


Table 1(on next page)

Basic foraging parameters of three male Snares penguins on long-term foraging trips

during incubation in 2003.

Due to limited battery life of GPS devices, maximum distance from island and distance

travelled relate to the time of logger operation rather than entire foraging trips. Considering

the much longer duration of the trips, it is likely that at least distance travelled represents a

gross underestimation of the true distance covered by the penguins on their trips.


1

2

Table 1. Basic foraging parameters of three male Snares penguins on long-term

foraging trips during incubation in 2003. Due to limited battery life of GPS devices,

maximum distance from island and distance travelled relate to the time of logger

operation rather than entire foraging trips. Considering the much longer duration of the

trips, it is likely that at least distance travelled represents a gross underestimation of

the true distance covered by the penguins on their trips.

Bird ID T13 T14 T32

Date of departure 15.10.2003 16.10.2003 15.10.2003

Logger operation time

(days)2.9 2.7 3.1

Total trip duration (days) 15.3 7.9 10.2

Maximum distance from

island (km)215.2 96.1 163.3

Distance travelled (km) 226.3 132.5 187.9



Dive parameters of male Snares penguins (n=6) equipped with GPS dive loggers and

TDRs performing long-term foraging trips during incubation in 2003.

Comparison of data was only possible for the first three days at sea because of the GPS dive

loggers’ limited battery time. All values are given as daily means that derived from individual

daily mean values for each bird. Differences were tested using one-way ANOVA followed by

Tukey’s post-hoc comparison. Asterisks highlight significant differences between days;

superscript letters indicate relationship of differences, i.e. values without common letter

differ significantly.


1

2

Table 2. Dive parameters of male Snares penguins (n=6) equipped with GPS dive loggers and TDRs

performing long-term foraging trips during incubation in 2003. Comparison of data was only possible

for the first three days at sea because of the GPS dive loggers’ limited battery time. All values are

given as daily means that derived from individual daily mean values for each bird. Differences were

tested using one-way ANOVA followed by Tukey’s post-hoc comparison. Asterisks highlight

significant differences between days; superscript letters indicate relationship of differences, i.e. values

without common letter differ significantly.

ANOVA

Day 1 Day 2 Day 3 F2,6 p

Number of dives 270±130 a 261±26 a 339±132 a 0.96 0.406

Descend rate (m/s) 0.7±0.1 a 0.8±0.2 a,b 1.1±0.1 b 11.82 0.001*

Max depth (m) 34.4±7.0 a 48.4±15.0 a,b 63.8±9.7 b 10.59 0.001*

Bottom time (s) 27. 9±2.6 a 34.3±5.6 a 48.3±7.6 b 20.55 <0.001*

Ascend rate (m/s) 0.7±0.1 a 0.8±0.1 a,b 0.9±0.1b 4.33 0.033*

Dive time (s) 95.6±10.5 a 115.4±24.3 a,b 140.8±14.3 b 10.20 0.002*

Post-dive interval (s) 26.4±5.0 a 34.4±10.6 a,b 40.5±5.7 b 5.29 0.018*

Diving efficiency° 0.25±0.02 a 0.27±0.04 a 0.27±0.02 a 0.81 0.463

Foraging effort° 0.77±0.03 a 0.75±0.03 a 0.77±0.03 a 0.50 0.615

° Diving efficiency= bottom time/(dive time+post-dive interval)

°° Foraging effort=dive time/(dive time+post-dive interval)



Comparison of dive behaviour of male Snares penguins (n=3) and female Snares

penguins (n=2) equipped with TDRs on long-term foraging trips during incubation 2003.

All values are given as mean±sd that derived from individual means of each bird. Asterisk

marks significant difference.


1

2

Table 3. Comparison of dive behaviour of male Snares penguins (n=3) and female Snares penguins

(n=2) equipped with TDRs on long-term foraging trips during incubation 2003. All values are given as

mean±sd that derived from individual means of each bird. Asterisk marks significant difference.

t-test

males females t 3 p

Trip length (days) 11.3±2.5 4.1±0 6.82 0.021*

Daily dive activity ( dives * day-

1)259±75.5 499±4.24 -7.55 0.001*

Hourly dive activity (dives*h-1) 17.2±4.6 34.3±3.0 -5.99 0.027*

Descent rate (m/s) 0.8±0.2 0.9±0.2 -0.48 0.714

Max depth (m) 54.7±9.8 26.4±10.1 -3.11 0.090

Bottom time (s) 41.0±4.5 18.6±4.7 -5.31 0.034*

Ascent rate (m/s) 0.9±0.1 0.8±0.1 -0.65 0.582

Dive time (s) 125.4±20.43 65.9±15.7 -8.68 <0.001*

Post-dive interval (s) 31.6±5.4 20.1±3.8 -2.81 0.107

RMD dives° (% of all dives) 75.5±6.6 43.4±5.4 6.90 0.020*

Diving effort°° 0.74±0.02 0.78±0.01 -2.32 0.259

Diving efficiency°°° 0.23±0.01 0.28±0.01 -10.02 0.063

° Repeated Maximum Depth dive = dives that return to the maximum depth±10% of the preceding dive

°° Diving effort = divetime/(divetime+post-dive interval)

°°° Diving efficiency = bottom time/(divetime+post-dive interval)


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Like clockwork - nest attendance patterns and foraging ... fileLike clockwork - nest attendance patterns and foraging behaviour of Snares penguins Eudyptes robustus as a function of