A comparison between warblers on two islands: testing Lack?s and Skutch?s hypotheses

SOFAER, H. R.; ; SILLETT, T. S.; ; LANGIN, K. M.;; YOON, J.; ; PELUC, S. I.; ; MORRISON, S. A.; ; GHALAMBOR, C. K.

San Diego

Congreso; Cooper Ornithological Society -American Ornithological Union-Society of Canadian Oornithology- Meeting.; 2010

Variation in avian clutch sizes has received considerable study, yet remains poorly understood. Lack hypothesized that food availability drives variation in clutch sizes, while Skutch proposed that visually oriented nest predators limit feeding trips, and thereby limit clutch sizes. We exploit differences in the rainfall regimes and predator communities of Catalina and Santa Cruz Islands to test these hypotheses in Orange-crowned Warblers (Vermivora celata ). Lack?s hypothesis predicts larger clutch sizes on Santa Cruz Island, which receives more rainfall and has higher insect abundance than Catalina Island. In contrast, Skutch?s hypothesis predicts lower nest visitation rates and clutch sizes on Santa Cruz, where the Island Scrub-Jay (Aphelocoma insularis) is endemic. We documented significantly lower nest visitation rates on Santa Cruz, and measured behavioral responses to spatial variation in scrub-jay abundance. Yet while our behavioral results support Skutch, clutch sizes were significantly larger on Santa Cruz, as predicted by Lack. In addition, rainfall and mean clutch sizes were correlated across years, further supporting Lack?s hypothesis. We will compare per-nestling feeding rates between islands, and discuss implications for life history theory.