. 384 Short Communications Vol. 31, No. 4 ]. Raptor Res. 31(4):384-385 © 1997 The Raptor Research Foundation, Inc. Ecology of American Kestrels Wintering on Socorro Island, Mexico Hartmut Walter S. University of California Los Angeles, Box 951524, Los Angeles, CA 90095-1524 U.S.A. Key Words: American Kestrel, Falco sparverius; territori- to a snag to eat it. It would stay for a few min on one ality; foraging behavior, winter diet. snag but would then fly to another, gradually covering the entire 3 ha area. In the afternoon this female was less Although American Kestrels (Falco sparverius) are not active. It rarely foraged in the afternoon but typically known to breed on the Revillagigedo Islands in the Mex- preened or soared above its territory and roost. From 1- ican Pacific, they have been observed there in increasing 4 December, it roosted in dense evergreen foliage at numbers in recent years (Wehtje et al. 1993). I had the 1753, 1752, 1735 and 1722 H. Once (2 December), it H opportunity to observe a wintering population on Socor- roosted in the early afternoon from 1315-1720 when ro Island in December 1992. This note reports on its ter- it began to rain. ritorial and foraging behavior as a contribution to the I observed neighboring kestrels on all sides of this kes- limited literature on the ecology of nonbreeding Ameri- trel. These kestrels behaved similarly and spentverylittle can Kestrels in the Neotropics. time in flight. Mostly, they perched on elevated vegeta- Socorro Island lies 460 km south of Baja California’s tion from dawn to dusk and <10% of their time was Cabo San Lucas. The main study site on Socorro (area spent in flight and foraging. All of the kestrels appeared = 140 km2 highest elevation 1040 m above sea level) to use their own foraging areas and I seldom observed consisted of, a 300 X 100 m grassy field at an elevation intraspecific agonistic interactions with the exception of of about 450 m that was covered with grasses and weedy two birds which occasionally soared, dived and playfully forbs <0.5 m in height and surrounded by low trees. chased each other in the updraft of a precipitous slope. Several snags (up to 2.5 m in height) ofshrubby Psidium I observed another kestrel on 6 December from 0900- trees were dispersed across the open field. Nearby, there 1400 H within the residential Mexican Navy compound were eroded slopes, woodland patches and low scrub. at the southern tip of the island. It perched on metal For 5 d, from the evening of 30 November to the antennas, lampposts and treetops during multiple for- morning of 5 December 1992, I observed the main study aging and resting periods. This bird hunted an area site and the area immediately around it for kestrels dur- about 6-8 ha in size that consisted mostly of grass-cov- ing morning and predusk hours. I observed kestrels from ered, park-like habitat between buildings. It behaved sim- a distance of40-80 m so as not to disturb them. I distin- ilarly spending little time in flight. guished individual birds by their plumage and feather The kestrels were obviously territorial toward other condition and by recording kestrels that repeatedly used species. They aggressively defended their perches against and defended perch sites in hunting areas. Kestrels could resident Red-tailed Hawks (Buteo jamaicensis socorroensis) not be aged with certainty. Kestrels were also seen but and passing Sharp-shinned Hawks (Accipiter striatus) not monitored in other parts of the island. I did not no- Although there were numerous small songbirds and tice any kestrels that flew to and from the island. house mice (Mus musculus) within the kestrel territories I counted at least 14 different kestrels in the southeast and the surrounding areas, I never observed a kestrel quarter of Socorro Island. Only two of them were males. taking a bird or mammal. On several occasions, however, The kestrels that I monitored closely showed a high de- they captured and ate arthropod prey. The most com- gree of site fidelity. The six kestrels regularly monitored mon items were crickets, small grasshoppers, and large near my campsite occupied the same area day after day. locusts which were abundant all over the island. Kestrels One female kestrel that had its night roost in the dense usually spotted arthropods on the ground from perches m foliage of a small Bumelia tree about 8 away appeared or from a low hovering position and pounced on them. to spend the entire observation time in the open, weedy No pellets were found (possibly because of the presence field foraging in an area approximately 3 ha in size. After of large scavenging land crabs, Gecardnus planatus) but I a few days of observations, I could accurately predict the found dismembered legs oflarge grasshoppers or locusts whereabouts of this kestrel. It left its roost each morning at some of the perches. The apparent insectivorous diet before sunrise (0630-0645 H) and flew to one ofthe low of this population is not unexpected since breeding pop- snags where it hunted. Its daily routine appeared to re- ulations andjuveniles have also been reported to be in- quire a minimum of energy. It would hover occasionally, sectivorous (Balgooyen 1976, Varland et al. 1993). then grab a small prey item from the ground and fly up Socorro Island’s tropical latitude makes it a likely mi- December 1997 Short Communications 385 gration and wintering location for American Kestrels, tance and companionship during an unusually wet peri- particularly for female andjuvenile birds (del Hoyo et al. od on Socorro. 1994). The sizes of the two winter territories I observed Literature Cited (3 and 6-8 ha) were small compared to those observed in the northern U.S. (Craighead & Craighead 1969, En- Balgooyen, T.G. 1976. Behavior and ecology of the derson 1960, Mills 1975). In California, Cade (1955) ob- American Kestrel {Falco sparveriusL.) in the SierraNe- served similarly small winter territories (e.g., a vacant lot vada of California. Univ. Calif. Publ. Zool. 103:1—87. 100 X 130 m in size). It may be that Mediterranean-type Cade, TJ- 1955. Experiments on winter territoriality of and tropical winter habitats with their mild or warm cli- the American Kestrel, Falco sparverius. Wilson Bull. 67: mates offer higher densities ofprey biomass forwintering 5-17. kestrels than do habitats in temperate and boreal cli- Craighead, JJ- and F.C. Craighead, Jr. 1969. Hawks, mates. owls and wildlife. Dover Publishers, New York, NY U.S.A. — Resumen. Y observe lo minimo de 14 diferente Falco del Hoyo, A. Elliott and Sargatal [Eds.]. 1994. J., J. sparverius en la isla de Socorro en el pacifico de Mexico Handbook of the birds of the world. Vol. 2. Lynx Ed- en 1992. Los Falco sparveriusparecieron ocupar y defend- icions, Barcelona, Spain. er territorios de presa en areas herbosas en la isla donde Enderson,J.H. 1960. A population study of the sparrow primeramente cazaban grillos, saltamontes y langostas. hawk in east-central Illinois. Wilson Bull. 72:222-231. Estos territorios de cazar varian en tamano de 3-8 ha y Mills, G.S. 1975. Awinter population study oftheAmer- estaban notable mas pequenos que eso antes descubridos ican Kestrel in central Ohio. Wilson Bull. 87:241-247. para Falco sparverius. Varland, D.E., E.E. Klaas and T.M. Loughin. 1993. Use [Traduccion de Raul De La Garza,Jr.] of habitat perches, causes of mortality and time until dispersal in post-fledging American Kestrels. Field Acknowledgments J. Ornithol. 64:169-178. Funding for the stay on Socorro Island was provided Wehtje, W., H.S. Walter, R. Rodriguez-Estrella, Lu- J. by UCLA’s Latin American Center, a UC Mexus research nas and A. Castellanos-Vera. 1993. An annotated grant and by CIBNOR in La Paz through a grant from checklist of the birds of Isla Socorro, Mexico. West. the World Wildlife Fund (USA) for the binational Socor- Birds 24:1-16. ro Island Restoration Project (SIRP). I thank the Mexi- can Navy for logistic support and my research colleagues, particularly Ricardo Rodriguez-Estrella, for their assis- Received 12 December 1996; accepted 14 August 1997 RaptorRes. 31 (4):385-387 J. © 1997 The Raptor Research Foundation, Inc. Diet of the Spectacled Owl (Pulsatrixperspicillata) during the Rainy Season in Northern Oaxaca, Mexico Hector Gomez de Silva Instituto de Ecologia, UNAM, Apartado Postal 70-275 Ciudad Universitaria, UNAM, , C.P. 04510, Mexico, D.F., Mexico Monica Perez-Villafana Calle 1537-3, Col. SanJuan de Aragon, Seccion 6, C.P. 07918, Mexico, D.F., Mexico Jose Antonio Santos-Moreno Departamento de Zoologia, Instituto de Biologia, Apartado Postal 70-153, UNAM, C.P. 04510 Mexico, D.F., Mexico KeyWords: Spectacled Owl; Pulsatrix perspicillata; naked- 750 g in mass (Stiles and Skutch 1989). Based on its size, tailed climbing-rat, Tylomys nudicaudus; diet, Oaxaca, Mex- it is likely that it preys on the largest potential prey spe- ico; niche segregation. cies in tropical forests (Emerson et al. 1994) and it is known take mammals up to the size ofagoutis {Dasyprocta The Spectacled Owl {Pulsatrixperspicillata) is the largest spp.), skunks (Mephitinae) and opossums (Didelphi- owl in humid tropical forests ofthe NewWorld, averaging dae) It also preys on birds as large as oropendolas {Psar- .