Sensitivity of Nesting Great Egrets (Ardea alba) and White Ibises (Eudocimus albus) to Reduced Prey Availability

Abstract

Life-history theory suggests that long-lived bird species will adjust their nesting effort according to current conditions to balance the costs and benefits of current reproduction with their long-term needs for survival and future reproduction. However, responses to the same habitat conditions may differ between species, even within the same ecosystem, to produce different nesting and population patterns. We examined differences in the nesting ecology of two sympatric wading species, Great Egret (Ardea alba) and White Ibis (Eudocimus albus), between years with high (2006) and below-average (2007) prey availability in the Florida Everglades. Clutch size of White Ibises decreased by ∼19% from 2006 to 2007, whereas Great Egret clutch size remained constant. Model selection identified rain, water depth, Julian date, year, and prey biomass as parameters that most influenced daily survival rates (DSR) of White Ibis nests, whereas nest stage, region, Julian date, water depth, and the quadratic form of water recession rate most influenced Great Egret nest DSR. Daily survival for both Great Egret and Whites Ibis nests was higher in 2006 (DSR = 0.992 and 0.999, respectively) than in 2007 (DSR = 0.981 and 0.979). Our results support the hypothesis that prey availability and hydrological factors play crucial roles in regulating populations of wading birds in the Florida Everglades. Results also demonstrated that White Ibis reproduction was more sensitive to changes in hydrological conditions and prey availability than Great Egret reproduction.