Variation in the timing of breeding between suburban and wildland Florida Scrub-Jays: Do physiologic measures reflect different environments?

Abstract

Evidence exists that access to anthropogenic food by birds in urban areas can result in earlier laying dates and larger clutch sizes, both of which have the potential to increase the relative fecundity of urban bird populations. Several mechanisms exist by which supplemental food can advance laying. Supplemental food may provide additional energy, essential nutrients, or an environmental cue that the resources necessary for breeding are available. We examined several physiologic measures of pre-breeding female Florida Scrub-Jays (Aphelocoma coerulescens) from a suburban and a wildland population to assess these alternative hypotheses. Plasma protein levels were higher in females from the suburban scrub-jay population, but the difference was absent when we controlled for the number of days prior to laying that we sampled the bird. However, plasma protein levels during the earliest sampling period, long before protein mobilization as a result of vitellogenesis should be occurring in either population, were higher in the suburban population. We found no differences in plasma calcium levels or in the total body lipids between suburban and wildland populations. In both populations, total body lipids declined as laying neared, suggesting that Florida Scrub-Jays may use some endogenous resources to fuel reproduction. From other studies, we know that suburban scrub-jays increase food intake rates relative to jays in wildland populations. However, this strategy does not appear to result in improved body condition. We suggest that the readily available proteins in anthropogenic foods may provide essential nutrients and high intake rates may serve as an environmental cue, both resulting in earlier laying in suburban jay populations. However, evidence exists suggesting that this may not increase, but rather decrease fecundity because breeding is out of synch with the seasonally fluctuating foods, such as arthropods, necessary to rear nestlings.