Food supplementation and possible mechanisms underlying early breeding in the Florida Scrub-Jay ( Aphelocoma coerulescens)

Abstract

Food supplementation studies demonstrate the importance of resources in the timing of reproduction. Studies of Florida Scrub-Jays ( Aphelocoma coerulescens) found that supplemented jays bred earlier than unsupplemented jays and that protein may play a critical role. In this study, free-living scrub-jays were provided with supplemental diets high in fat and protein (HFHP) or high in fat and low in protein (HFLP). Jays in both treatments bred earlier than unsupplemented controls (CNT), but HFHP-supplemented jays bred earlier than HFLP jays. To assess possible mechanisms, we measured testosterone (T) in males, estradiol (E 2) in females, and corticosterone (CORT) in both. HFHP males had higher T than HFLP and CNT males, but treatment did not affect E 2 levels of females. Pilot studies of scrub-jays in suburban environments suggest that the spatial and temporal predictability of food may influence corticosterone (CORT) levels. Suburban jays have year-round access to human-provided foods and breed earlier than wildland jays; thus, we compared CORT in all treatments in the natural site ( wildlands) with those of suburban jays. CORT levels of suburban jays were lower than HFLP, HFHP, and CNT jays. HFHP-supplemented jays had lower CORT levels than those of HFLP and CNT jays. The observed differences in the timing of breeding, both between suburban and wildland populations and between experimental groups in the wildlands, may result from differences in the spatial and temporal predictability of food, and the nutritional differences in diets. Because CORT can negatively affect the reproductive axis, we postulate that nutrient availability, the predictability of food, CORT levels, and initiation of reproduction are inextricably linked.