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BREEDING BIOLOGY OF WHITE-EARED GROUND-SPARROWS (MELOZONE LEUCOTIS), WITH A DESCRIPTION OF A NEW

NEST TYPE

Luis Sandoval & Daniel J. Mennill

Department of Biological Sciences, University of Windsor, Windsor, Ontario, N9B3P4, Canada. E-mail: biosandoval@hotmail.com

Resumen. – Biología reproductiva del Cuatro-ojos de Cabeza Negra (Melozone leucotis), con ladescripción de un nuevo tipo de nido. – Proveemos la primera descripción detallada de la biologíareproductiva del Cuatro-ojos de Cabeza-negra (Melozone leucotis), una especie de paseriforme Neotro-pical que habita tacotales en Centroamérica muy poco estudiada. Basados en ocho años de datoscolectados de varias poblaciones del Valle Central de Costa Rica, describimos el nido, huevos, parasi-tismo, cuido parental y fenología reproductiva de M. leucotis. Los nidos pertenecen a uno de dos tipos; elprimer tipo es una estructura gruesa de fibras vegetales en la capa externa rugosa y una interna fina, yel segundo tipo es una estructure más simple y pequeña en forma de plataforma de palitos. El primertipo de nido se encuentra localizado entre la vegetación cera al suelo, y el segundo directamente sobreel suelo. Esta es la primera descripción del segundo tipo de nido. Los huevos son de color blanco en elfondo con manchas cafés, y la nidada va de dos a tres huevos. El parasitismo por parte de los Piuces(Molothrus aeneus) fue severo; cinco de los diez nidos encontrados fueron parasitados, incluyendo dosnidos hiperparasitados con 6 y 7 huevos de M. aeneus. Las hembras incuban los huevos y alimentan alos pichones, mientras que el macho asiste localizando el alimento para los jóvenes. Esta especie poseeuna época reproductiva larga de Marzo (a finales de la época seca) hasta setiembre (durante la épocalluviosa). Esta investigación es la primera descripción formal del comportamiento reproductiva de estaespecie, para un género de aves donde el comportamiento es poco conocido en los trópicos.

Abstract. – We provide the first detailed description of the breeding biology of White-eared Ground-sparrows (Melozone leucotis), a little-studied Neotropical songbird that inhabits thickets in Central Amer-ica. Based on eight years of data collected from populations in the Central Valley of Costa Rica, wedescribe White-eared Ground-sparrows’ nests, eggs, nest parasitism, parental behavior, and breedingphenology. Nests conformed to one of two general types: (1) a bulky structure of plant fibres with acoarse outer layer and a fine inner layer; and (2) a smaller, simpler structure made up of a platform of thinplant fibres placed on top of rocks. Nest of the former type were located in vegetation near the ground,whereas the latter type were constructed directly on the ground. This is the first description of the secondnest type. Eggs had a white background with variable brown spotting. Eggs were laid in clutches of twoor three. Parasitism by Bronzed Cowbirds (Molothrus aeneus) was severe; five of ten nests were parasit-ized, including two that were multiply parasitized with 6 and 7 cowbird eggs. Only females were observedincubating the eggs, but both sexes provisioned nestlings; only females were observed provisioningfledglings while males assisted in locating food. This species has a long breeding season, begins inMarch (late dry season), and continues until September (late rainy season). We provide the first formaldescription of the breeding behaviour of White-eared Ground-sparrows, from a genus of birds whosebehaviour is poorly known in the tropics. Accepted 11 June 2012.

Key words: White-eared Ground-sparrow, Melozone leucotis, breeding biology, nest architecture, nesttype description, thicket habitat.

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INTRODUCTION

The breeding biology of many species oftropical birds is poorly described and manyspecies have never been studied in detail(Stutchbury & Morton 1995, Macedo et al.2008). This gap in our knowledge is especiallyproblematic for birds whose populations aredeclining or whose habitats are being increas-ingly threatened. Information about breedingbiology of these species is valuable in its ownright, but may also help to guide managementand conservation decisions (Greeney & Rom-bough 2005, Greeney et al. 2008, Sandoval &Gallo 2009).

Tropical Melozone species live in thickethabitats (Stiles & Skutch 1989) and thesehabitats are presently afforded no specialconservation attention. Thicket habitats aremade up of early successional vegetationthat, without management, changes quicklyover time. The lack of attention to the biolog-ical importance of thicket habitats isexpected to result in an overall decrease inthis type of habitat, which will havenegative conservation implications for birdspecies that inhabit thickets (Sánchez et al.2009).

White-eared Ground-sparrows (Melozoneleucotis) are one of three Mesoamerican speciesin the genus Melozone (AOU 1998, DaCosta etal. 2009). They are distributed from Chiapas(Mexico) to Costa Rica’s Central Valley at ele-vations of 500–2000 m a.s.l. (Stiles & Skutch1989, Howell & Webb 1995, AOU 1998, Gar-rigues & Dean 2007). This ground-sparrowlives year-round in territorial pairs in youngsuccessional vegetation stages that vary fromsecondary forest edges to shade coffee plan-tations and thickets (Stiles & Skutch 1989,Howell & Webb 1995). Little is known aboutthis species’ breeding biology; publishedknowledge is restricted to anecdotal accountsthat include one described nest, the estimatedgrowth rate from a single chick, and the

weight and size of two eggs (Winnett-Murray1985).

In this study, we describe the breedingbiology of White-eared Ground-sparrowsbased on observations collected during eightyears of field study including two years ofresearch on a color-banded population in theCentral Valley of Costa Rica. We presentobservations on nests, eggs, brood parasitism,nest location, and parental care. We describevariation in nest architecture and include twoaccounts of a previously undescribed nesttype.

METHODS

We collected data on the breeding biology ofWhite-eared Ground-sparrows from 2003 to2011 in six locations in Costa Rica, spanningthe entire species range in the country. Wediscovered nests by following adult birds asthey delivered nesting material, as they carriedfood to the nestlings, or when an incubatingbird flushed from the nest. When a nest wasdiscovered we recorded its height (m), thesubstrate on which it was placed, the amountof cover above the nest, and the nest’s con-tents (number of nestlings or eggs). To quan-tify the nest cover at any height directly abovethe nest, we used an ordinal scale from 0 (anest with no vegetation above it) to 5 (nestcompletely covered with vegetation; Sandoval& Barrantes 2006). We found a total of 10nests over the eight-year study.

To describe nest architecture, we mea-sured three features: nest height, diameter ofthe inner cup (an average of two perpendicu-lar measurements), and the depth of the innercup (measured from the bottom of the centerof the nest cup to the rim). We measuredlength and width at the widest point of eggsto the nearest 0.1 mm using dial callipers.Over eight years, we collected five eggs fromthree nests. Three of the five eggs wereunhatched eggs from normal nests (two from

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one nest and one from another nest), whereasthe last two eggs came from a parasitized nest.We collected nine nests after breeding wascomplete. These specimens were deposited inthe Museo de Zoología at Universidad deCosta Rica (accession numbers providedbelow).

We collected data on parental care byobserving pairs as they provisioned nestlingsor juvenile birds. Over eight years, we gath-ered observations of 32 different pairs,including 21 pairs where one or more birdswere banded. Males and females are mono-morphic. We distinguished males fromfemales because females have an incubationpatch and shorter tarsi, whereas males lack anincubation patch, have longer tarsi, anddevelop a cloacal protuberance during thebreeding season (LS unpub. data). We charac-terized the presence of incubation patchesbased on 21 pairs where at least one memberof the pair was banded.

RESULTS

Nests. Over an eight year period of studyingWhite-eared Ground-sparrows in the CentralValley of Costa Rica, we found ten nests.Nests were very difficult to find because thisspecies inhabits the dense vegetation of thick-ets or shade coffee plantations, and birds arechallenging to observe for extended periods.The ten nests were 0 to 2 m in height (Table1). Four nests were found in coffee plants(Coffea arabica) between the main stems; twowere on the ground under an overhangingrock; one was among exposed tree roots atopa clay cliff along the edge of a trail; one wasbelow a group of dead cypress branches(Cupressus luscitanica); one was placed below anAraceae leaf on a dense cluster of grasses; andthe last was found between the leaves of aDracaena sp. bush (Dracaenaceae). Six of theten nests were in shade coffee plantations andfour were in unmanaged areas of thicket habi-

tat. Most nests were well concealed with anextensive covering of vegetation; nine of theten nests had cover scores = 3, whereas theremaining nest was completely uncovered(Table 1).

Eight of the nests matched the existingdescription of the nest of this species(Winnett-Murray 1985). These nests consistedof a dense cup of loosely woven plant fibreswith an inner layer of more tightly-wovenplant fibers. Nest size was highly variable,leading to variation in nest measurements:64.8 ± 16.1 mm in the outside cup height,36.8 ± 11.4 mm in the inner cup depth, andfrom 69.3 ± 13.9 mm in the inner cup diame-ter (Table 1).

We found two active nests with differentarchitecture, which appear to represent a pre-viously undescribed type of nest for this spe-cies (Fig. 1). Instead of consisting of a densecup of dead leaves with a tightly-woven innernest, these two nests consisted solely of a plat-form of dead leaf petioles and thin plantfibres. Both nests were placed on the groundin rocky areas. Both nests were found in thesame population of birds in Getsemaní, Here-dia province (10°02’N, 84°06’W, 1400 m a.s.l).The first nest was located in a hollow areacovered by rocks in a secondary forest on theedge of a river (found 16 May 2004). The sec-ond nest was located under rocks between adense young secondary forest growth and aplantation of Mora sp. (Moraceae; found 21April 2011). These nests were markedly dif-ferent from the other nests and from writtendescriptions of this species’ nest, with a shal-lower outside cup height 38.7 ± 0.6 mm, innercup depth 10.8 ± 2.6 mm, and an overall flatstructure (Fig. 1, Table 1; values are given asmeans ± SD here and throughout). Thesenests also were wider than the previousdescription, from 76.75 ± 16.0 mm to 86.9 ±21.2 mm. Nests of both forms were depositedin the Museo de Zoología at Universidad deCosta Rica (accession numbers: UCR1,

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UCR20, UCR29, UCR30, UCR40, UCR41and UCR52-54).

Eggs. We measured nine eggs from five nests,including five eggs that were collected anddeposited at the Museo de Zoología at Uni-versidad de Costa Rica. All eggs had aglossy white base with irregular dark brownspots concentrated near the base of the egg(Fig. 2a). Eggs where elliptical in shape. Egglength ranged from 23-27 mm with a meanof 25.6 ± 1.4 mm. Egg width ranged from17.2–19.9 mm with a mean of 17.8 ± 1.0 mm.The average clutch size of the ground-sparrow at nine nests was 2.2 ± 0.5, ranging

from a minimum of two eggs to a maximumof three eggs.

Nest parasitism. We found five nests parasitizedby Bronzed Cowbirds (Molothrus aenneus). Thecowbird eggs were readily distinguished bytheir white to clear blue color, lack of mark-ings, and more circular shape (Fig. 2b). Twonests were multiply parasitized, with six andseven cowbird eggs in addition to the twoground-sparrow eggs. After we removed cow-bird eggs, one of the two host eggs hatched inthe first nest and neither of the host eggshatched in the second nest. The remainingparasitized nests had one or two cowbird

TABLE 1. Measurements of ten White-eared Ground-sparrow nests in Costa Rica. Nest type indicatesone of two general types: type 1 is a nest with a bulky outer layer of coarse plant material and an inner layerof fine plant material; type 2 is a previously undescribed nest type consisting of a single, smaller layer ofplant fibres. Nest cover is a measure of the amount of vegetative cover was scored according to a five pointscale (details given in Methods). For one nest (indicated by *) we were unable to collect all nest measure-ments for one of the nest.

Location Date Nest type

Cupheight (mm)

Cup diameter

(mm)

Cup depth(mm)

Nest heigth (mm)

Nest cover

Nest contents(plus cowbirdinformation)

Getsemaní, Heredia provinceGetsemaní, Heredia provinceGetsemaní, Heredia provinceGetsemaní, Heredia province*Concepción, Heredia provinceConcepción, Heredia provinceCalle Hernandéz, Heredia provinceAserrí, San José provinceGetsemaní, Heredia provinceGetsemaní, Heredia province

25 May 2004

07 June 2005

07 May 2006

31 August 2008

18 April 2010

28 April 2010

12 May 2011

30 May 2011

16 March 2004

21 April 2011

1

1

1

1

1

1

1

1

2

2

47.35

45.3

76.25

-

88.8

62.5

57.75

75.55

39.15

38.35

48.3

73.2

54.8

-

81.4

62.1

68.1

72.5

68.7

95

33.4

20.5

36.1

-

55.2

30.8

33.5

47.8

12.6

9

2

1

1.3

1

1.5

1.6

2

1

0

0

0

3

5

5

4

4

5

3

5

5

2 eggs

2 eggs (plus 1 cowbird hatchling) 2 eggs (plus 1 cowbird egg) 2 hatchlings and 1 egg

2 eggs (plus 6 cowbird eggs)0 eggs (plus 4 cowbird eggs)2 eggs (plus 7 cowbird eggs)0 eggs

2 eggs

2 egg (plus 1 cowbird egg)

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eggs. The mean clutch size for nests that wereparasitized by cowbirds was 5.4 ± 2.9 (includ-ing both host eggs and cowbird eggs; rangingfrom 3–9 eggs per nest) (Table 1). In addition,we observed a pair of adult ground-sparrowsfeeding a fledged cowbird chick (observed on19 April 2010).

Parental behavior. We banded both parents in 15pairs and only one of the parents in 6 pairs(N = 38 birds banded over the eight year

period). Based on the pairs where both indi-vidual were banded only one of the individu-als had an incubation patch (the individualwithout cloacal protuberance), and for threenests where we observed incubation, only thebird with incubation patch (female) was seento incubate. We observed both parents provi-sion food to the nestlings at three nestsobserved over approximately 6 h. By contrast,only females provisioned food to young oncethey had fledged (N = 8 banded pairs and

FIG. 1. Photographs of nests of White-eared Ground-sparrows (Melozone leucotis) from the Central Valleyof Costa Rica. (a) Top view of a nest which lacks the outer layer of loosely woven plant fibres, consistingonly of inner layer of more tightly-woven plant fibres (Type 2). (b) Side view comparing the more commonnest type (Type 1; left) and the less common, previously undescribed nest type (Type 2; right).

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approximately 28 h of observations). In thesepairs, the male moved far from the female andjuveniles, searching for food on the ground orin bushes. Although ongoing playback studiessuggest that duets are important in territorydefense (LS unpub. data), during the breedingseason duets also play a role in revealing thelocation of food; on multiple occasions, whenthe male found food, he sang, and the femaleresponded by vocalizing, producing a duet.

Females contributed to the duet either from adistance or after flying to perch close to themale. Juveniles often followed the female tothe male’s position. Females were observedfeeding both insects (e.g., moths) and fruits(e.g., Ficus sp., Acnistus arborecens, and a Mela-stomataceae berry) to juveniles.

On three occasions while observing anest, the female was the first to approach thenest or fledglings. Each time, the female

FIG. 2. Photographs of eggs from nests of White-eared Ground-sparrows (Melozone leucotis) in the CentralValley of Costa Rica. (a) Eggs had a white base and were marked with variable brown spots. (b) Six eggs ofShiny Cowbirds (M. aeneus), and two eggs of White-eared Ground-sparrows from a multiply parasitizednest in Concepción, Heredia province.

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attempted to distract the observer by produc-ing a broken leg display together with harshnoisy calls. During these displays, the malestayed 2 to 10 m from the nest, until thefemale approached him at which point theyduetted and produce solo calls.

Breeding phenology. Our observations suggestthat White-eared Ground-sparrows have longbreeding seasons. We found nests betweenMarch and September, with the maximumnumber in April and May (N = 3 and 4,respectively). We found only one nest duringthe other months, except during July whennone were found (Table 1). We have observedjuvenile birds following adults from March toSeptember. In the Central Valley of CostaRica, the dry season ends in March or April,and the rainy season lasts from May to Sep-tember (Sandoval 2011). Therefore, White-eared Ground-sparrows appear to nest fromthe middle-to-late dry season and throughoutthe rainy season, with an apparent peak in theearly rainy season.

DISCUSSION

This report substantially advances our under-standing of the breeding biology of this little-studied Neotropical species. Results of recentmolecular studies have revealed seven speciesin the genus Melozone (DaCosta et al. 2009,Chesser et al. 2010). Four of these speciesinhabit the Neotropics (M. albicollis, M. kieneri,M. biarcuata, and M. leucotis) and three speciesinhabit the North Temperate Zone (M. fusca,M. crissalis, and M. aberti; AOU 1998). Acrossthe genus, the breeding biology is betterknown for the temperate species (Tweit &Finch 1994, Johnson & Haight 1996, Benedictet al. 2011) than for the Neotropical species(Winnett-Murray 1985, Stiles & Skutch 1989,Howell & Webb 1995).

The nest of the White-eared Ground-sparrow was first described 27 years ago

(Winnett-Murray 1985). This descriptionmatched most of the nests we observed,which consisted of a bulky outer layer of deadleaves and dry grasses, and an inner layer oftightly woven plant materials (Winnett-Mur-ray 1985). We twice found an alternate formof nest, which lacked the bulky external layer.In both cases the nest was placed on theground in a rocky area. The rocks surround-ing the nest likely provided support to thenest cup that is normally provided by theouter layer of plant material. These two nestforms may be distinct types of nests, or theymay form a gradient from a platform of finethin plant fibres (i.e., the rarer, newly-described nest) to a bulky cup of dead leaves(i.e., the more common, previously-describednest). These alternatives are difficult to distin-guish with the sample size (1 nest describedby Winnett-Murray, 1985; 10 described in thecurrent study) and we encourage others tocollect data about nest substrate and nest typefor this species and also for other species inthis genus. In any case, this variation in nestarchitecture may represent an adaptiveresponse to nest microhabitat. The bulkiestnests we measured were placed betweenbranches in coffee plants, where the nestmaterial may provide more support for theeggs and nestlings, or thermal advantages forthe developing young. The less substantialform of nest was only found on the ground oron flat substrates, and may be sufficient sup-port for the eggs and nestlings, while mini-mizing conspicuousness to predators.

Across the genus Melozone, all nests sharesimilar characteristics, with somewhat bulkycups of dead leaves and grasses (Baicich &Harrison 2005, Benedict et al. 2011). Theheight at which nests are built appears to besimilarly low throughout the genus (Winnett-Murray 1985, Stiles & Skutch 1989, Howell &Webb 1995, Baicich & Harrison 2005). Thevariable nest types that we describe here forWhite-eared Ground-sparrows appear to be

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unique for the genus (Baicich & Harrison2005, Stiles & Skutch 1989, Howell & Webb1995). The egg color and patterning, whichconsists of a light background with darkbrown spotting, is similar to previous anec-dotal reports (Winnett-Murray 1985, Stiles &Skutch 1989, Tweit & Finch 1994, Johnson &Haight 1996, Baicich & Harrison 2005, Bene-dict et al. 2011). Clutch size reported here issimilar to that found in other tropical speciesof Melozone (Winnett-Murray 1985, Stiles &Skutch 1989, Howell & Webb 1995) but it issmaller than in temperate Melozone species,where the average clutch size is four eggs(Tweit & Finch 1994, Johnson & Haight1996, Benedict et al. 2011).

Nest parasitism by the Bronzed Cowbirdappears to be high in White-eared Ground-sparrows in Costa Rica, with more that 50%of the nests that we found containing para-sitic eggs or chicks. In a previous reviewpaper, based on studies of many differentcowbirds hosts in Costa Rica, parasitism byBronzed Cowbirds was reported for twoWhite-eared Ground-sparrow nests (Sealy etal. 1997). We found two nests that were multi-ply parasitized by Bronzed Cowbirds, with sixand seven parasitic eggs. The reproductivesuccess of the host parents was very low inthese nests; in the first nest none of the hosteggs hatched, whereas in the second nestonly one of the two host eggs hatched (inboth cases, after we removed the parasiticeggs). The low reproductive success may bethe result of inadequate incubation due to thelarge number of eggs in the nest. Thisground-sparrow appears to be unable to dis-tinguish its own eggs from the cowbirds’eggs, as has been reported in other species(Fraga 1985, Sealy et al. 1997). Given the highparasitism frequency we record, and theapparent lack of egg recognition, we expectthat White-eared Ground-sparrows are heav-ily negatively affected by cowbird parasitism(Friedmann & Kiff 1985), especially because

host and parasite are sympatric throughouttheir distribution in Costa Rica (Stiles &Skutch 1989, Garrigues & Dean 2007).

Our observations suggest that only femaleWhite-eared Ground-sparrows incubate, aswas suggested previously from observationsof a single nest by Winnett-Murray (1985),and as is common in the temperate Melozonespecies (Tweit & Finch 1994, Johnson &Haight 1996, Benedict et al. 2011). We foundthat both parents provision hatchlings, butafter fledging only the female feeds the fledg-lings while the male searches for food. Inter-estingly, we found that males reveal thelocation of food to the female, and subse-quently the fledgling birds, through an inter-esting use of duet song; upon discovering afood source, males sing a song, and thefemale responds to complete the duet as shemoves towards his location. Females defendthe nest and the young against potentialpredators through a distraction display,whereas males do not approach potentialpredators as closely. The differences betweenmale and female nest defense behaviour maybe the result of differences in their investmentin reproductive activities, or in security inpaternity, as has been proposed in other spe-cies with different defense behavior in malesand females (Dickinson 2003, Sandoval2009).

This is the first detailed study on thebreeding biology of any Neotropical speciesin the genus Melozone. Information on naturalhistory of this group is critical for under-standing the biology of these species and willbe important in setting future conservationplans. All of the nests we observed werefound in wild thicket habitat (i.e. areas of veg-etation left to grow without management) orin managed shade coffee plantations that aresimilar in structure to thicket habitat. Thisspeaks to the importance of maintainingstands of vegetation that include early succes-sional stages, and the importance of shade

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coffee plantations in Neotropical conserva-tion efforts.

ACKNOWLEDGMENTS

We thank Olman Sandoval and Carolina Men-dez for help with field research. We thank theReserva Biologica Monteverde for all of thesupport they provided to conduct research inthe reserve. We thank Lauryn Benedict, Spen-cer Sealy, and André Weller for commentsthat improved the manuscript. This researchwas supported by grants from the Ministeriode Ciencia y Tecnología (MICIT) and theConsejo Nacional para Investigaciones Cien-tíficas y Tecnológicas (CONICIT) of CostaRica, and by the Natural Sciences and Engi-neering Research Council of Canada(NSERC) and the Government of Ontario,Canada.

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