GPS tracking of the foraging movements of Manx Shearwaters ...sjrob/Pubs/ShearwaterIbis2008.pdf · GPS tracking of the foraging movements of Manx Shearwaters Pufﬁnus pufﬁnus breeding

Ibis

(2008), doi: 10.1111/j.1474-919x.2008.00805.x

© 2008 The Authors Journal compilation © 2008 British Ornithologists’ Union

Blackwell Publishing Ltd

GPS tracking of the foraging movements of Manx Shearwaters

Puffinus puffinus

breeding on Skomer Island, Wales

T. C. GUILFORD,

1,

* J. MEADE,

1

R. FREEMAN,

1,2

D. BIRO,

1

T. EVANS,

1

F. BONADONNA,

3

D. BOYLE,

4

S. ROBERTS

5

& C. M. PERRINS

4

1

Animal Behaviour Research Group, Department of Zoology, South Parks Road, Oxford, OX1 3PS, UK

2

Microsoft Research Ltd, Roger Needham Building, J J Thomson Avenue, Cambridge, CB3 0FB, UK

3

CNRS – CEFE, Behavioural Ecology Group, 1919, route de Mende, F-34293 Montpellier, Cedex 5, France

4

Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford, OX1 3PS, UK

5

Department of Engineering Science, Parks Road, Oxford, OX1 3PJ, UK

We report the first successful use of miniature Global Positioning System loggers to trackthe ocean-going behaviour of a

c

. 400 g seabird, the Manx Shearwater

Puffinus puffinus

.Breeding birds were tracked over three field seasons during the incubation and chick-rearingperiods on their foraging excursions from the large colony on Skomer Island, Pembrokeshire,UK. Foraging effort was concentrated in the Irish Sea. Likely foraging areas were identifiedto the north, and more diffusely to the west of the colony. No foraging excursions wererecorded significantly to the south of the colony, conflicting with the conclusions of earlierstudies based on ringing recoveries and observations. We discuss several explanations includingthe hypothesis that foraging may have shifted substantially northwards in recent decades.We found no obvious relationship between birds’ positions and water depth, although therewas a suggestion that observations at night were in shallower water than those during theday. We also found that, despite the fact that Shearwaters can be observed rafting off-shorefrom their colonies in the hours prior to making landfall at night, breeding birds are usuallylocated much further from the colony in the last 8 h before arrival, a finding that hassignificance for the likely effectiveness of marine protection areas if they are only local tothe colony. Short sequences of precise second-by-second fixes showed that movementspeeds were bimodal, corresponding to sitting on the water (most common at night andaround midday) and flying (most common in the morning and evening), with flight behaviourseparable into erratic (indicative of searching for food) and directional (indicative oftravelling). We also provide a first direct measurement of mean flight speed during directionalflight (

c

. 40 km/h), slower than a Shearwater’s predicted maximum range velocity, suggestingthat birds are exploiting wave or dynamic soaring during long-distance travel.

Keywords:

climate change, navigation, procellariiform, satellite telemetry, seabird.

Many procellariiforms have an inherently precariouslife-history, with breeding concentrated at a relativelysmall number of islands or other isolated locations,coupled with central-place foraging strategies involvinglong-distance movements over the open ocean.Understanding their foraging movements is crucial

to assessing their conservation status, and indirectlythe health of the marine resources on which theydepend, but their ocean-wandering lifestyle makesthese hard to study with any precision. Satellitetracking has brought huge advances in our under-standing of the precise foraging movements of largerspecies such as albatrosses, bringing into focus theirvulnerability to modern fishing practices, but the sizeof loggers has precluded this for smaller species

*Corresponding author.Email: [email protected]

2

T. C. Guilford

et al.


(reviewed in BirdLife International 2004). In thispaper we report the first use of precision GPSlogging technology to study the individual foragingmovements during breeding of a 400 g procellarii-form, the Manx Shearwater

Puffinus puffinus

.Britain and Ireland hold the majority of the world

population of Manx Shearwaters (Lloyd

et al

. 1991,Hamer 2003), with just two small Welsh islands,Skokholm and Skomer, holding in excess of 150 000breeding pairs, a substantial proportion of the globalbreeding population (Smith

et al

. 2001). ManxShearwaters, which shallow dive from the surface,predominantly for small clupeids and some squid(Thompson 1987, Brooke 1990), are known to flyconsiderable distances during the breeding season;for example, incubating birds of both sexes are awayfrom the colony for an average of about a week at atime (Brooke 1990). Sea surveys have revealed thatthere are concentrations of birds at various places inthe Irish Sea and its vicinity (Begg & Reid 1997).Distribution maps produced from such surveys areinvaluable in pin-pointing the areas where the mainfeeding concentrations of seabirds occur. However,where there is more than one feeding concentrationand more than a single breeding colony, they cannotshed light on the feeding ranges of the birds from thedifferent colonies, nor on the foraging behaviour ofindividuals. Furthermore, in a species as long-lived asthe Manx Shearwater, in any year possibly as muchas half of the population are not breeding; these aremainly immature birds, not yet of breeding age (upto 7 years; Brooke 1990), but also include divorcedor widowed birds searching for a new mate. Whileeach of these birds (without breeding duties) will beassociated with a particular colony, they may notreturn there with the regularity of breeders. Hencetheir foraging patterns at sea may well be differentfrom those of breeders.

Early attempts to determine the foraging move-ments of breeding Manx Shearwaters involvedinterpretation of ringing recoveries, together with off-shore and coastal sightings. These led to the hypothesisthat many birds utilize the sardine fisheries originatingin the Bay of Biscay, which move northwards up thebay during the northern summer (Lockley 1953).Lockley’s hypothesis implies a substantial long-distance southwards component to the Shearwater’sforaging movements. Later analyses, however (Perrins& Brooke 1976), found little support for thishypothesis. In this paper, we use miniature GPSloggers to track the movements of a number of breed-ing birds while they are away from Skomer Island,

during both the incubation and the chick-rearingperiods. This has allowed us to make a first direct testof the current validity of Lockley’s hypothesis, and toinvestigate in some spatial detail how breeding birdsutilize marine resources accessible from their colony.

METHODS

Subjects

Between June and August, in the three seasons2004–6, breeding birds were located in their burrowsusing call playback at two densely occupied sites onSkomer Island, Pembrokeshire, Wales (54

°

44

′

N5

°

17

′

W): one close to the Warden’s House at NorthHaven (2004, 2005), the other close to the OldFarm (2006). Burrows were chosen for the accessi-bility of the nest chamber via the burrow entrance oran inspection hatch. In 2004, a small wire gate, witha one-way trip, was constructed in the mouth of eachburrow to allow capture of returning birds, or, duringincubation, retention of departing birds immediatelyafter changeover. Frequent inspections (every 20–30 min) ensured that we were able to capture focalstudy birds by hand promptly. In 2005 and 2006, asimpler system of small marker pegs placed verticallyin the entrance and frequent inspection (every 15–20 min) was used to identify the comings and goingsof focal birds. To determine whether adults had fedat sea, we weighed them with a spring balance beforeand after each deployment where it was expedientand would cause minimal extra disturbance. Onlybirds weighing 400 g or more were fitted with trackers.

GPS tracking

The global positioning devices (GPDs, adapted fromoriginal designs by von Hünerbein

et al

. 2000,Steiner

et al

. 2000) consisted of an integrated GPSreceiver, flash memory and ground plane (µ-blox Co.,Thalwil, Switzerland), with 4 V, 150 mAh, lithiumpolymer rechargeable battery (Ultralife Batteries Inc.,Newark, USA), and 12.5

×

12.5

×

3 mm ceramicinsulated antennae (Compotron Ltd, Swindon, UK).Total active operation time was about 55 min. TheGPDs were configured to attempt to obtain posi-tional fixes for up to 200 s at 2-h intervals. Thismeant that, when successful, GPDs recorded a briefburst of positional fixes (at 1-s intervals) every 2 h,allowing the type and direction of movement to bedetermined, whilst maximizing the proportion ofthe foraging journey recorded. GPDs were then

© 2008 The AuthorsJournal compilation © 2008 British Ornithologists’ Union

GPS tracking of breeding Manx Shearwaters

3

waterproofed using lightweight heat-sealed plasticsleeves immediately prior to deployment. TheGPDs (which had a flat, streamlined profile of8

×

30

×

60 mm and weighed

c

. 17 g includingattachment material) were attached dorsally betweenthe shoulders using three to five thin strips of blackTESA marine cloth tape, each anchored beneath asmall bunch of back feathers and closed over the topof the device (tape attachments have less impactthan harnesses; Phillips

et al

. 2003). On return, thedevice was removed by peeling the tape away fromthe feathers and the bird was returned to its burrow.Data were downloaded onto a field laptop using pro-prietary µ-

BLOX

software and visualized and analysedusing F

UGAWI

, M

AT

L

AB

, and A

RC

GIS 9 software. ABritish Geological Survey DigBath250 data licencewas purchased for conducting bathymetry analysis.

RESULTS

Effectiveness of the tracking technique

In total, we obtained 50 individual datasets ofvarying degrees of completeness, constituting asuccess rate of about 50% per deployment. Trackswere recorded from 34 different individuals. Eightbirds were tracked twice, one bird was tracked threetimes and two birds were tracked on four occasionsbut across 2 years. No bird was tracked more thanthree times in a season. We obtained six tracks in2004, five from chick-rearing birds; in 2005 weobtained 18 tracks, 16 from chick-rearing birds; and,in 2006 we obtained 26 tracks, 16 from chick-rearingbirds. No tracked bird failed to return to its burrow.However, in many cases trackers failed to acquirepositions, ran out of battery whilst the bird remainedunderground, returned waterlogged, or were lost atsea because the tape attachments eventually fail inseawater (an important welfare failsafe in case wedid not manage to recapture the bird). The meanduration of the foraging trips studied was 71.8 h,but the distribution was heavily skewed, with 68%lasting just 1 or 2 days and the longest trip lasting12 days. Mean recording duration of the devices was31.2 h, so that most trips (69%) of 2 days or lesswere fully recorded, but only the early part of longertrips was recorded before the battery was exhausted.In all, 46% of all trips were recorded completely oralmost so (up to 4 h before recovery), and 89% of allfix bursts contained fixes generated by four or moresatellites, providing highly accurate position (±

c

. 4 m),speed and direction data.

Evidence for foraging

The duration of foraging excursions varies greatly inunmanipulated birds, especially with breeding stageand, to a lesser extent, sex. Nevertheless, trip durationscan give an indication of whether our manipulatedbirds were indulging in normal foraging. Whilst asystematic comparison between manipulated andunmanipulated birds was not possible because wedid not measure controls at the same time, the durationof tracked excursions was broadly consistent withprevious observations. In our study, trips duringincubation lasted a mean of 5.46 days (± 0.92 se,

n

= 13), and during chick feeding a mean of 1.65 days(± 0.15,

n

= 37). Brooke (1990) measured mean tripdurations of 6.88 ± 0.29 for males and 5.42 ± 0.23for females during later incubation (Skokholm 1975and 1976 breeding seasons), and reported means of1.63 days for males and 2.0 days for females duringchick feeding (data from Rum, Thompson 1987).Gray and Hamer (2001, data from Skomer 1999)measured trips during chick feeding at 1.5 ± 0.2 sddays for males and 1.8 ± 0.2 sd days for females. Inaddition, in 33 cases (66%) there was clear evidencefrom an adult’s weight measured before and afterdeployment that it had fed during the tracked excursion.In 13 cases (26%), one or other adult measurementwas missed and we were unable to judge. In four(8%) cases, a tracked adult was sufficiently lighterwhen weighed the second time that it is possibleit had not fed (although in all four cases it is possiblethat it had fed its chick before we weighed it, andthis accounted for the weight reduction). Hence,most or all trips tracked represent successful foragingexcursions, and this also suggests that birds were ableto cope with the drag or the increased wing-loadingresulting from carrying the devices.

Distribution and direction of trips

All tracks recorded were plotted in Figure 1 to showthe overall distribution of foraging movementsduring incubation and chick-rearing. Plotting thedata as tracks connecting each approximately 2-hposition estimate also allowed some visualizationof any pseudoreplication effects (inherent intracking data of this sort) in the following analyses.Nevertheless, it was clear that birds could travel longdistances between each 2-h fix, and hence suchfixes provided a powerful description of the overalldistribution of the birds’ activity when away fromthe colony.

4

T. C. Guilford

et al.


Foraging movements were concentrated northwardsand westwards into the Irish Sea, and not south-wards. This was true both during incubation (red),when birds were often away a week or longer, andduring the shorter trips of chick-rearing birds (blue).Furthermore, birds did not frequent all areas withinreach given the actual trip distances measured.Several areas of activity could be identified, withparticularly dense activity around Skomer, in Car-digan Bay, and at locations in the Irish Sea furthernorth (off Dundalk and the Mull of Galloway) andwest of the colony. Though sample sizes betweenyears were too small to determine definitively whetheror not there were inter-annual effects, informalinspection suggests there was substantial overlapbetween locations visited in different years.

As the logging devices were programmed to obtaina short series or cluster of consecutive fixes at 1-sintervals during each 2-h duty cycle, we obtained notjust a position every 2 h, but a snapshot of the bird’smovement behaviour (principally surface speed anddirection of movement) at the same time. First, weassigned each cluster of fixes a single positionrepresented by the median latitude and medianlongitude of the series, hereafter called simply a ‘location’,and plotted these for all excursions in relation to for-aging trip length. Figure 2a shows that despite ourbeing unable to track birds for the duration of longertrips, there was a clear tendency for longer trips to startin a more northwards direction. Removing all pseudo-replication effects by representing each trip with justa single location taken from the middle of the first dayof the trip shows essentially the same result (Fig. 2b).

We then assigned an estimate of quality to each fixbased on the number of satellites contributing to thefix. Our devices recorded fixes whenever three ormore satellites were successfully accessed, but withonly three satellites, a position fix can be several tensof metres off true position. This has no significancefor the general location estimates presented here,but can greatly affect speed estimates derived frommovement between fixes within a cluster. We thereforefiltered the clusters by quality and calculated meanmovement speeds (‘speed’) whenever there weresufficient fixes based on at least four satellites. Next,we used the data within each cluster to calculateboth the direction of movement from one second tothe next (‘direction’), and the variability in thatdirection across the cluster (‘directionality’ using thecircular statistic

r

, measured from 0 to 1, where 1equates to unidirectional and 0 equates to randomlyoriented; Batchelet 1981). Directionality was thenplotted in Figure 3 against speed, for every location.

Surface speeds were bimodal. We therefore fitteda mixture model of two Gaussian curves to thedistributions shown, estimating the mean andvariance of the two movement modes, which clearlycorrespond to locations where the bird was eithersitting (0.85 ± 0.24 m/s) or flying (11.13 ± 9.55 m/s).The higher variance characteristic of the fast move-ment mode indicated that birds sometimes fly withthe wind and sometimes against it, whereas sittingbirds may drift with the current and probably makeno attempt to counteract their drift. Nevertheless, thecentral tendency of the fast distribution provided adirect measure of flight speed, if we assume thatinto-wind and down-wind effects are roughly balancedacross the dataset.

Figure 1. Plots of all fixes with each trip’s fixes connected insequence by lines to show approximate course of foragingexcursions. Data are classified according to whether the birdwas, at the start of the trip, incubating an egg (in red), or rearinga chick (in blue). The position of the colony at Skomer Island isimmediately off the extreme western end of the headland markedSkomer.



5

Figure 2. Locations classified by foraging trip length: (a) all locations plotted (white = 1 day, light grey = 2 days, dark grey = 2–4 days,black ≥ 4 days): (b) each excursion represented by a single location, that closest to 1300 h, and not outside 1100–1500 h, on the firstday, to remove effects of pseudoreplication.

Figure 3. Gaussian mixture model of speeds during all fix clusters based on four or more satellites, overlaid on a scatter plot of speedagainst directionality for each location, with a box showing values taken to indicate ‘directional flight’ (r > 0.85). The optimal separationboundary between the two distributions is shown as a dashed line at 2.5 m/s.

6

T. C. Guilford

et al.


Using the optimal separation boundary of the twofitted distributions (2.5 m/s) we classified eachlocation as either sitting or flying with maximumprobability. There was also considerable variation indirectionality. For slow moving (sitting) birds, thisvariation was almost certainly attributable to increasingdominance of GPS position error as actual speedapproaches zero, so it was ignored. For fast moving(flying) birds, variation in directionality allowed usto distinguish birds in strongly oriented flight, andtherefore probably travelling, from those in erraticflight that were more likely to be involved in localizedbehaviour such as active searching for food. Informalinspection of the tracks suggested that below athreshold of about 0.85 (normalized directionality),birds were often involved in making major turns,whereas above this threshold flight paths showedundulations, perhaps indicative of wave- or shear-soaring, but no major alterations in overall orientation.In an attempt to distinguish approximately localizedactivity (foraging, resting) from travelling behaviour,Figure 4 plots the locations assigned by whetherbirds are sitting or flying erratically (Fig. 4a), or

flying directionally (Fig. 4b). Fix clusters that did notcontain accurate position data were excluded. Figure 4also plots the British Geological Survey DigBath250bathymetric database for the Irish Sea to provide adetailed visualization of the birds’ activity in relationto underlying water depth.

The patchy distribution across the region clearlyshows several important areas where activity isconcentrated. Comparing the distributions suggeststhat activity on the water and erratic flight may havebeen more locally concentrated than travelling behaviour,although there was no clear separation in most areas.Around the colony itself, the concentration of activitycould have been partially explained by birds waitingto come onshore (which they only do after dark) atthe end of their fishing trips, or preparing themselvesfor a foraging trip immediately after leaving thecolony. To remove activity specifically associated withleaving or returning to the colony, we plotted in Figure 5the distribution of locations after removal of allpoints up to 8 h after deployment or 6 h before return.

Although several relatively shallow areas (e.g.Bristol Channel east of Lundy, and the area north and

Figure 4. Locations, filtered for fix quality, classified as: (a) sitting (red points) or erratic flying (red arrows); or, (b) travelling (greenarrows). Locations are overlain on bathymetry.



7

east of Anglesey) contained no activity, there was noclear overall relationship with water depth. Figure 6(top two panels) compares the distribution of locationswith, as a null hypothesis, a random distribution ofpoints (not on land) generated for the area withinthe minimum convex polygon surrounding theseobservations. A Mann-Whitney

U

test showed nosignificant difference between the depths of actuallocations and randomly created locations (

Z

= –1.09,

n

random

= 600,

n

bird

= 549,

P

= 0.28). We then usedtime of Civil Twilight (U.S. Naval Observatory,Astronomical Applications Department website –http://aa.usno.navy.mil/faq/, accessed February 2008)for the actual times and dates of each observation todetermine whether it was night or day, and segmentedthe data again by sitting versus flying. A Kruskal–Wallis test showed that the depth distributions dif-fered significantly (

χ

2

= 32.84, d.f. = 3,

n

day

= 458,

n

night

= 91,

P

< 0.001), with night-time locations morelikely to be shallow. Further post-hoc comparisonsshowed that there was a significant depth difference

between night-time and day-time locations (

Z

= 5.54,

n

night-time

= 91,

n

day-time

= 458,

P

< 0.001), shown inthe bottom two panels of Figure 6, but not betweenflying and sitting (

Z

= –0.94,

n

flying

= 169,

n

sitting

= 380,

P

= 0.35). It is important to remember that sinceeach bird and each track contributed a series oflocations within the actual data (though not thesimulated random data) there was some inherentpseudoreplication in the tests. However, since loca-tions within a bird’s track were always at least 2 hapart there was in general ample time for birds tohave moved across the entire depth range betweenlocations. Nevertheless, we followed the one signifi-cant result here with a more conservative Wilcoxonmatched pairs test using a single day and a singlenight location (those closest to midday or midnight)for each track. Although the median depth differencewas negative, and hence night locations were shallowerthan day, this was not significant (

U

= 91.0,

n

= 23pairs,

P

= 0.157; median depth difference = –14.74 m).We plotted flying versus sitting activity with time

of day in Figure 7. There was a clear diurnal cycle.Sitting activity was approximately bimodal, withpeaks during the night and in the middle of the day.Flying activity also appeared to be bimodal, with twopeaks dovetailing quite well with the pattern ofsitting activity. Finally, in an attempt to determinewhich locations were most used by birds in the hoursbefore returning to the nest at the end of the foragingtrip, we plotted activity distributions during the final8 h before recovery (Fig. 8).

DISCUSSION

Foraging movements during the breeding season

Early attempts to determine the foraging movementsof birds breeding in the Pembrokeshire coloniesinvolved interpretation of ringing recoveries, togetherwith off-shore and coastal sightings. Ringing recov-eries led to the hypothesis that many birds utilize thesardine fisheries originating in the Bay of Biscaywhich move northwards up the bay during the northernsummer (Lockley 1953). Later, Perrins and Brooke(1976) recognized that Lockley had underestimatedthe age at first breeding, and applied more stringentcriteria for whether birds recovered in Biscay werelikely to be breeders. They found no evidence forforaging excursions this far south after laying, andsuggested that from the breeding population onlyfemales absent from the colony for the 2 weeks or so

Figure 5. Locations at least 8 h after deployment and more than6 h before recovery.

http://aa.usno.navy.mil/faq/

8

T. C. Guilford

et al.


of egg formation were likely to make the journey toBiscay. Because departure from the burrow of such‘honey-mooning’ females is unpredictable, we werenot able to track such birds using GPS and so thishypothesis remains untested. Nevertheless, Lockley(1953) also reported observations of mass birdmovement off-shore and particularly past the head-lands of Cornwall and Finistere (which he termedthe ‘Manx Seaway’). Although the Manx Seawaywas reportedly less intense in June and July than inApril and May, it still appears to provide evidence fora substantial long-distance southwards componentto the Pembrokeshire birds’ foraging movementseven long after laying. Our findings are strikingly atodds with this. We found no evidence for southerly

foraging excursions other than very locally, evenwhen birds were absent from their burrows for manydays. As we were unable to track birds for more thanthe first day or two of each excursion, we cannotknow exactly what happens beyond this time onlonger journeys. Nevertheless, Figure 2 shows thatthere was a strong tendency for birds to start headingin a northerly, and to a lesser extent westerly, directionwhen they were embarking on what turn out to belonger journeys. It is possible that southerly move-ments might have been observed had we tracked birdsearlier during incubation (our tracks of incubatingbirds are from late June and July), but Lockley stillreported strong southwards mass movements alongthe Manx Seaway during July, and this does not

Figure 6. Histogram of locations with depth. Random and actual locations are compared (top two histograms), and actual locations,classified as light or dark using nautical night-time, are compared in the bottom two histograms. Note that there are fewer night-timeobservations because the night is very short at these latitudes during summer.



9

match our findings. Even our earliest birds did not flysouthwards. There would appear to be three feasibleexplanations.

First, southwards moving birds may be non-breeders.Up to half of the birds ‘belonging’ to the colonieswere immature, partnerless, or failed breeders.Presumably, such directional specialization wouldrequire either that breeders and non-breeders seekdifferent resources, or that breeders lack timebetween nest-tending duties to fly sufficiently farsouth to exploit the sardine fisheries. In this lattercase, we expect that non-breeders’ excursions couldbe longer than those of breeders, but this has neverbeen measured.

Secondly, if there is strong local partitioning in thePembrokeshire colonies, and sub-colonies utilizetheir own resource areas, then it might be possiblethat our samples of breeders happen to be takenfrom two areas that only forage to the west andnorth. A division between the neighbouring islandsof Skomer and Skokholm is a related possibility, butthe exchange of ringed birds between the two(Brooke 1990) suggests that mixing across thecolonies might make the formation of stable anddifferent traditional foraging routes unlikely. Atpresent, data are too sparse to test this interestingpossibility.

Thirdly, the species’ foraging ecology may havechanged in the half century since Lockley’s observations,and the birds are no longer utilizing foraging groundsto the south. Contemporary headland sightingscould potentially address this hypothesis. Certainly,Brooke’s (1990) analysis of ringing recoveries since

Figure 7. A histogram showing the frequency of locations containing flying and sitting activity over the 24-h period.

Figure 8. A plot of locations before birds arrive onshore at theend of their foraging trip (white 0–2 h before arrival, light grey 2–4 h before arrival, dark grey 4–6 h before arrival, black 6–8 hbefore arrival).

10

T. C. Guilford

et al.


the 1940s suggested that birds were even thenpenetrating less far south than in Lockley’s time, andit is possible that this trend has continued. Nevertheless,if a northwards shift has occurred, perhaps becauseof warming seas, then it is surprising that we foundno evidence of excursions further north than theMull of Galloway, which is about the limit of north-wards movement proposed by Lockley on the basisof his original data. As the Pembrokeshire populationis probably increasing, we should expect to see adramatic increase in the density of birds in the IrishSea if they are no longer travelling south, and stilltravelling no further north.

Activity within the Irish Sea

Within the Irish Sea itself, observations at sea (e.g.Pollock

et al

. 1997) have shown that Manx Shearwatersare not particularly abundant in March and April,become more common during May and June, andpeak during July and August. Although hard togeneralize, the peak numbers tend to occur in thesouth Irish Sea (including Cardigan Bay and extendingas far west as the south-eastern tip of Ireland), in theNorth Channel (Mull of Kintyre to Mull of Galloway)and in the Irish Sea close to the Irish coast fromabout Dublin north to Dundalk (

c

. 53

°

15

′

N to54

°

0

′

N). The first two of these are close to breedingcolonies (Pembrokeshire islands and Copeland,respectively) and may, at least in part, be associatedwith local movements of birds from these populations.The third is in some ways of more interest becausethere are no significant colonies nearby: Bardseywith about 7000 pairs is the nearest; the colony onthe Calf of Man is insignificant (Newton

et al

. 2004).Yet throughout the summer, large numbers of birdsare found in this area and high numbers are main-tained into September, even after the numbers at thecolonies (and the two other sites of concentrationmentioned above) have started to diminish. Thisarea lies to the north and west of the Irish Sea front(Pollock

et al

. 1997) where high seabird density hasbeen observed (Begg & Reid 1997), presumably inresponse to high marine productivity associated withthe sea front and the stratified waters west of it.

Our tracking data matched these distributionswell, demonstrating that birds breeding at thePembrokeshire colonies are likely to be a major con-tributor to these observed concentrations. The dis-tributions of birds on the sea or in non-directionalflight, activities most likely to be associated with feed-ing, were clearly concentrated in these three areas

(see Fig. 4), whilst birds in travelling flight showed aless concentrated distribution. The concentration inCardigan Bay was sufficiently close to Skomer to bein range for birds waiting to come ashore at night.Nevertheless, considerable activity here remainedeven if we excluded activity in the 8 h after deploymentand 6 h before return (Fig. 5). It is most likely thereforethat Cardigan Bay is also an important foragingdestination. In fact, Figure 5 also shows that manybirds not about to make landfall, and thereforepresumed to be engaged in foraging activity, couldalso be found relatively close to the colony, and tothe west in particular, suggesting that the coloniesare themselves located close to another importantforaging area, possibly associated with the Celtic SeaFront (Pollock

et al

. 1997). In contrast, in the final8 h prior to returning from a foraging trip, birds wereto be found not only concentrated close to the island,but also dispersed much further afield, until veryclose to the time of making landfall (Fig. 8).

Diurnal activity patterns

Birds were most likely to be recorded in flight in themorning from around dawn until around 10:00 h, andthen again, less obviously, during the afternoon andearly evening. Perhaps the most obvious explanationfor this is that they were commuting between fishingand rafting areas in the morning and evening, andthen sitting at night and fishing in the middle of theday. Inspection of the spatial distribution of recordedpositions during actual subjective daylight and darknesssuggests that night-time activity is more clusteredthan day-time activity. Furthermore, whilst therewas no obvious overall relationship with waterdepth, other than an apparent avoidance of shallowareas in the Bristol Channel and north of Anglesey,there was a suggestion of a shift from deeper toshallower water at night.

Flight parameters

The mean surface speed for our birds showing direc-tional travel was about 11 m/s (40 km/h), which isslightly slower than, but still in reasonable agreementwith, earlier estimates of flight speed derived fromobservation (approximately 40–55 km/h, Lockley1953). Calculation suggests that Vmp (minimumpower velocity) should be about 7.5 m/s (27 km/h)and Vmr (maximum range velocity) about 14 m/s(50 km/h) for the Manx Shearwater (Pennycuick1969), so we suggest that birds are flying in the


GPS tracking of breeding Manx Shearwaters 11

middle of their theoretical speed range for purelypowered flight. As long-distance movement mightbe expected to favour flying close to Vmr, it seemslikely here that birds may be flying closer thanexpected to Vmp as they exploit non-powered flightadvantages in the form of wave or dynamic soaring.Birds flying north to the Mull of Galloway to feedtravelled a straight-line distance from Skomer ofmore than 330 km each way, involving a total predictedminimum direct commuting flight time of around16.5 h. This foraging area is much closer to otherbreeding colonies, most notably Copeland situatedabout 30 km away. The foraging destinations ofCopeland birds are currently unknown, but it seemsthat they would stand to gain a 15-h (600 km)travelling advantage over Pembrokeshire birds byutilizing the Mull of Galloway area. The secondmajor northerly foraging area is also approximately75 km closer to Copeland than Skomer, again givinga potential 3.75 h travelling advantage to Copelandbirds. Such an advantage might be expected to resultin increased provisioning and higher breeding success.

Significance for conservation at sea

The Birds Directive (79/409/EEC) requires eachmember state of the EU to set up marine specialprotection areas to protect birds, such as the ManxShearwater, that are considered rare or vulnerablewithin the European Community. Such seabirdshave two key requirements: safe nesting sites andsafe and food-rich feeding. The former are, in someways, easier to provide; most current large coloniesare designated nature reserves where people are onhigh alert for threats, particularly those from introducedmammalian predators. Even some formerly successfulcolonies, such as Lundy and Canna, where rats havealmost exterminated the birds, have recently beencleared of these predators and may become thrivingcolonies again.

The provision of safe feeding areas at sea is,however, much more difficult. Our findings emphasizethe problems associated with trying to provide theseplaces for such wide-ranging species. Marine NatureReserves provide some protection for the birds whenthey are immediately offshore. In the case of theManx Shearwater, these are mainly beneficial toevening assemblies of birds waiting to come onshore.Our data indicate that the majority of the breedingadults may not join these assemblies, being muchfarther from the colony when dusk falls (Fig. 8),although it is important to consider that birds carrying

trackers may have behaved differently to unmanipu-lated birds and may therefore have reached thevicinity of the colony later on their day of return,simply because they were carrying an additionalencumbrance. Further, as with most other seabirds,the areas used for feeding are much more distantfrom the colony and more diffuse, making it difficultor impossible to provide them with anything nearcomplete protection while they are foraging. Thefuture conservation of vulnerable species with suchwide-ranging habits as the Manx Shearwater willrequire a fuller understanding of the areas of sea theyuse and this will require integrating movement andactivity information from a variety of techniques,including precision GPS tracking. Further, we shouldnot ignore the possibility that resources, and henceresource use, may be changing.

Impacts of tracking on behaviour

The Manx Shearwater is currently one of the small-est seabirds to have been tracked using GPS technol-ogy. We deployed the smallest devices available, usedthe generally agreed lowest impact attachmentmethods (e.g. Phillips et al. 2003), and restricteddeployments to single excursions (occasionally, withone or more subsequent deployments only after aninterval). At approximately 4% body weight, andwith a very low profile, we believe our devicesdeployed in this way had minimal lasting impact. Itis of course possible that behaviour during trackedexcursions was not fully representative of normalbehaviour, and this caveat must always be consideredwhen drawing inferences from this and similar data.Telemetry advances are revolutionizing the study ofwild birds, but improvements must always strive toreduce impact both for ethical reasons and so thatdata remain representative of normal behaviour.

We thank the Wildlife Trust of South and West Wales, theCountryside Council for Wales, Juan Brown (Warden) andmany other staff of Skomer Island National NatureReserve. We also thank Louise Maurice and ChristineNicol for assistance in the field, and Mike Brooke, KerryLeonard, Neville McKee and two anonymous referees forcomments on the manuscript.

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Received 21 September 2007; revision accepted 21 January 2008.

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