Biotic and abiotic factors influencing nonbreeding raptor assemblages in southern Texas

Abstract

I investigated factors influencing diversity ami abundance of wintering raptor assemblages in southern Texas. I approached this problem from several spatial and temporal scales, ranging from continental to patch, and 30 years to 30 minutes. I focused mi three main topics of research: (1) Tire impacts of El Nino-Southern Oscillation (ENSO) weather events on wintering raptor distribution and abundance. (2) Tire impacts of land use patterns on species diversity, abundance, and interspecific spacing for wintering raptors, and (3) The impacts of woody structure on patch use by w intering raptors. At the continental scale, ten of forty-five relationships between regional raptor abundance and the southern oscillation index (SOD. an index of ENSO severity, were significant at tire P < 0.05 level. Biologically, tire results implied that changes in tire SOI lead to spatial shifts in abundance for three of fair raptor species examined. At the landscape scale, I investigated the impacts of land use intensity on raptor abundance, diversity and spatial patterns within and between raptor species. Raptor abundance was highest on the roadside transect with intermediate land use intensity', but diversity was greatest along the roadside transect with the least intense land use practices. Both American Kestrels (Falco sparvarius) and Red-tailed Hawks (Buteo jamaicensis) displayed a decrease in abundance in the presence of Harris' Hawks {Parabuteo unicinctus). In addition, Hams' Hawks excluded Red-tailed-Hawks from perches above preferred habitat, forcing them to shift below perch habitat use from woodlots to open fields. Finally. Loggerhead Shrikes (Lamm ludovicumus) displayed spatial clumping in the presence of larger raptors along two of the three roadside transects. At the patch scale. 1 examined the impacts of artificial perches on patch use by American Kestrels. Loggerhead Shrikes, and Northern Hamers. American Kestrels preferred patches with artificial perches to areas with no perches or natural woody vegetation. Northern Harriers displayed no preference for patch type, while Loggerhead Shrikes preferred patches that provided escape cover as well as perching substrate. Patch choice for shrikes was impacted by predation pressure from Northern Hamers, and interference competition from American Kestrels.