THE SOCIAL ORGANIZATION AND MATING SYSTEM OF THE STRIATED GRASSWREN Author(s): Jordan Karubian Source: The Condor, 103(2):412-417. 2001. Published By: Cooper Ornithological Society DOI: http://dx.doi.org/10.1650/0010-5422(2001)103[0412:TSOAMS]2.0.CO;2 URL: http://www.bioone.org/doi/full/10.1650/0010-5422%282001%29103%5B0412%3ATSOAMS %5D2.0.CO%3B2 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. SHORT COMMUNICATIONS 405 The Condor 103:405±408 q The Cooper Ornithological Society 2001 SPECIES AND SEX-BIASED PREDATION ON HATCHLING GREEN TURTLES BY FRIGATEBIRDS ON EUROPA ISLAND, WESTERN INDIAN OCEAN1 FRE DE RIC LAGARDE2 DeÂpartement de Biologie, Universite des Sciences et Techniques, Avenue Marcillac, 17000 La Rochelle, France MATTHIEU LE CORRE Laboratoire d'Ecologie Marine, UniversiteÂdeLaReÂunion, 97715 Saint Denis cedex, ReÂunion Island, France HERVE LORMEE Centre d'eÂtudes biologiques de ChizeÂ, C.N.R.S, 79360 Villiers en Bois, France Abstract. Europa Island is a major breeding place dõÂa (35 episodios de eclosioÂn diferentes), 1632 fueron for green turtles (Chelonia mydas; 0.7 to 2.4 million inmediatamente depredadas por machos de F. minor. juvenile turtles hatch there annually), Great Frigate- SoÂlo 10 tortugas fueron consumidas por hembras de birds (Fregata minor; 700 to 1100 breeding pairs), and F. minor, mientras que las 186 restantes fueron cap- Lesser Frigatebirds (Fregata ariel; 1000 to 1200 turadas por individuos juveniles de esta especie. Este pairs). By visual observation, we quanti®ed the pro- comportamiento de alimentacioÂn no fue observado en portions of males, females, and juveniles of both fri- F. ariel. Sugerimos que la depredacioÂn diferencial de gatebird species prospecting over the hatching sites tortugas juveniles es consecuencia de las diferencias and preying on hatchling turtles. Of 1828 juveniles en tamanÄo entre las dos especies de fragatas y entre observed hatching at daytime (35 separate turtle emer- machos y hembras de F. minor. gences), 1632 were immediately eaten by male Great Frigatebirds. Only 10 hatchlings were preyed upon by female Great Frigatebirds; the remaining 186 were The family Fregatidae includes ®ve closely related captured by juvenile Great Frigatebirds. Such feeding tropical seabirds which feed mainly at sea by surface behavior was not observed in Lesser Frigatebirds. We seizing (Nelson 1975). Their main prey are ®sh (main- suggest that the sexual and species differences in ly ¯ying ®sh Exocoetidae) and squid (mainly Ommas- hatchling turtle predation are due to size differences trephidae; Nelson 1983, Schreiber and Clapp 1987, between the two species and between male and female Niethammer et al. 1992), although some authors report Great Frigatebirds. occasional inland predation on seabird chicks and hatchling turtles (Nelson 1975, Megyesi and Grif®n Key words: feeding ecology, frigatebirds, green 1996). Frigatebirds are also well known for their pi- turtle, predation, sexual size dimorphism. ratical habits (Vickery and Brooke 1994). Frigatebirds exhibit reversed sexual dimorphism (fe- Sesgos de Especies y Sexo en la males are larger than males in all ®ve species), that is DepredacioÂn de Tortugas Verdes por more or less pronounced according to the species. For Fragatas en la Isla Europa, OceÂano IÂndico example, male body mass is 85% of female body mass in Great Frigatebirds (Fregata minor; F. Lagarde, un- Occidental publ. data) and 94% in Lesser Frigatebirds (Fregata Resumen. La Isla Europa es un importante lugar ariel; Nelson 1975). Such reversed sexual size dimor- de reproduccioÂn de tortugas verdes (Chelonia mydas; phism is a trait which appears also in raptors (Falco- 0.7 a 2.4 millones de tortugas juveniles eclosionan allõ niformes), owls (Strigiformes), skuas and jaegers anualmente), asõ como de Fregata minor (700 a 1100 (Stercorariidae), and boobies (Sulidae) (Andersson and parejas reproductivas) y F. ariel (1000 a 1200 parejas). Norberg 1981, Mueller 1990). Different hypotheses Por medio de observaciones visuales, cuanti®camos la have been proposed to explain the evolution of this proporcioÂn de machos, hembras y juveniles de ambas trait in birds: resource partitioning, sex role partition- especies de fragatas que investigaban los sitios de eclo- ing in parental care, and sexual selection (Anderson sioÂn y se alimentaban de tortugas juveniles. De un total and Norberg 1981, Mueller 1990). Among these, re- de 1828 tortugas observadas eclosionando durante el source partitioning between members of a given pair assumes differences in feeding behavior, resulting in little overlap in food niches of males and females. Sev- 1 Received 18 May 2000. Accepted 18 January eral studies of kleptoparasitism have shown that male 2001. and female frigatebirds do not chase the same seabird 2 Present address: CEBC-CNRS, 79360 Villiers en species. Females generally chase large species, such as Bois, France, e-mail: [email protected] boobies and Red-tailed Tropicbirds (Phaethon rubri- 406 SHORT COMMUNICATIONS cauda), whereas males do not kleptoparasitize or select of both species (13 counts). We also estimated the ap- smaller host species like terns, noddies, or shearwaters parent sex ratio at the frigatebird colonies by counting (Osorno et al. 1992, Gilardi 1994, Le Corre and Jou- all observed birds along a linear transect on 1 March ventin 1997a). 1997 and 14 April 1997. Although kleptoparasitism is very conspicuous in When an emergence occurred, we counted the num- the vicinity of breeding colonies, it is not the main ber of depredated hatchlings, and the number, species, feeding strategy of frigatebirds (Diamond 1973, Nel- age, and sex of frigatebirds preying upon hatchlings. son 1975, Vickery and Brooke 1994). Indeed, sexual We differentiated males, females, and juveniles of each differences in feeding ecology are still poorly docu- species using Marchant and Higgins (1990) and Orta mented among frigatebirds. Megyesi and Grif®n (1992). (1996) described female-speci®c predation of Brown Noddy (Anous stolidus) and Sooty Tern (Sterna fus- STATISTICAL ANALYSES cata) chicks, suggesting that differences between male The values reported in the results section are means 6 and female feeding strategies do not concern only SD and percentages. Differences were considered sta- kleptoparasitism. Predation of hatchling turtles on tistically signi®cant at P , 0.05. x2 tests for proportion beaches has been reported (Hirth 1971, Nelson 1975, comparisons were performed using Statistica 5.1 (Sta- Stancyk 1982) but not in all studies (Diamond 1973, tsoft 1997). Niethammer et al. 1992), and sexual bias in this be- RESULTS havior has never been examined. The aim of our study was to examine hatchling tur- SPECIES PREYING UPON HATCHLING GREEN tle predation by frigatebirds at Europa Island, in the TURTLES Mozambique Channel. In this place, hatchling turtle Thirty-®ve turtle emergences were recorded during the predation by frigatebirds occurs during daylight (Ser- course of the study, containing 4 to 147 hatchling van 1976), providing opportunity for a detailed study green turtles each (mean 52 6 40). We counted a total of this feeding strategy. of 614 Great Frigatebirds in 13 separate ¯ocks forag- ing over the beach (not merely crossing the beach). METHODS Lesser Frigatebirds were seen ¯ying over the beach STUDY AREA only when commuting between the sea and the colo- nies, and never foraged over the beach. During turtle Europa Island (228209S, 408229E) is one of the best- emergences, all 1828 hatchling turtles that we saw preserved islands of the Malagasy area. It is a ¯at, 30- emerge were preyed upon by Great Frigatebirds. km2 coral island (for a complete description, see Le Corre and Jouventin 1997b). Between 2000 and 11 000 AGE AND SEX RATIO IN FLOCKS OF FRIGATEBIRDS female green turtles breed annually on its sandy beach- AND IN BREEDING COLONIES es (Le Gall et al. 1986, Le Gall 1988), laying approx- Of the 614 Great Frigatebirds we counted foraging imately 3 clutches of 110 eggs each, and about 0.7 to over the beach, 81% were adult males, 19% were ju- 2.4 million juveniles hatch (Le Gall et al. 1985, Miller veniles (sex undeterminable in the ®eld), and less than 1996). Between 700 and 1100 pairs of Great Frigate- 1% were adult females. Most captures were made by birds and 1000 to 1200 pairs of Lesser Frigatebirds males (1632 of 1828 captures, 89%), whereas female breed in the dry forest, dominated by Euphorbia ste- captures were rare (only 10 captures, 0.5%). The re- noclada, that covers the oldest and highest rocky part maining 186 were taken by juveniles (10%). There was of the island (Le Corre and Jouventin 1997b). On Eu- no difference in the proportion of males and females ropa Island, frigatebirds breed throughout the year (Le foraging over the beach compared to those attacking a 2 Corre, in press), while the main hatching period for turtle emergence (x 34 5 29, P 5 0.7 for males and 2 green turtles lasts from January to April (Servan x 34 5 31, P 5 0.6 for females). On the other hand, 1976). there was a greater proportion of juvenile birds for- 2 aging in ¯ocks than preying upon hatchling turtles (x 34 BEHAVIORAL OBSERVATIONS 5 118, P , 0.001).