Food availability is one of the primary limitations to avian populations, particularly during the breeding season (Skutch 1949, Lack 1954, 1966, Ricklefs 1968). Seasonal environmental conditions require most avian species to adjust their physiology, morphology, and behavior throughout the annual cycle (Hahn 1998). When breeding, many avian species are able to regulate their body condition when they are faced with high energy demands (Blem 1990), food scarcity (Stuebe and Ketterson 1982), or food unpredictability (Cuthill et al. 2000). Food may become a limiting factor during the breeding cycle due to increased adult physiological demands for egg production and increased food required to maintain and fledge chicks. While many species have adapted to varying environmental conditions, their ability to respond to natural variability coupled with anthropogenic induced changes are less predictable. Testing the response of avian reproductive efforts to food availability requires an understanding at multiple levels. Measurements of food availability require both data on the density and availability of prey within a defined area (Wiens 1984). Birds can respond to food limitations by increasing foraging rates, selecting lower quality prey items, or traveling further to better quality foraging patches. These altered behaviors can result in adult and chick physiological responses, which can be observed in clutch sizes, hatching success, growth rates, and survival. Measuring the response to changes in food availability at both the adult and nest/chick level allows for a quantitative understanding of their relationship and its implication on populations.
Declines in annual breeding success and populations of colonial nesting wading birds (Ciconiiformes) in the Florida Everglades system are believed to be linked to reduced prey availability as result of historic water management practices (Kahl 1964, Kushlan 1986, Kushlan and Frohring 1986, Frederick and Collopy 1989, Powell et al. 1989, Frederick and Spalding 1994, Ogden 1994, Gawlik 2002). Gawlik (2002) tested the prey availability hypothesis, observing that vulnerability and density of prey did not always result in a similar foraging response or strategy across a suite of wading species. These results suggest that differences in breeding season physiological condition could be linked in part to species specific foraging strategies (searchers versus exploiters; Gawlik 2002) and prey availability across the landscape. Furthermore, not all species of wading birds in the Everglades are declining (Crozier and Gawlik 2003 a ). For instance white ibises ( Eudocinus albus ) have declined approximately 87%, while great egrets ( Ardea alba ) increased 270% in the same period across the Everglades (Crozier and Gawlik 2003 a ).
While wading bird productivity may fluctuate within and between years and timing of reproduction is often variable within and between years (Kushlan 1975, Frederick and Collopy 1989, Ogden 1994), there appears to be no definitive reason why different species nesting and feeding sympatrically may have opposite population trends. Moreover, apparent dissimilarities among the diets of many wading bird species suggest that interspecific competition does not limit prey availability (Ramo and Busto 1993, Smith 1997; but see Miranda and Collazo 1997).
Recent research in the Everglades that examined the breeding physiology of white ibises (Heath et al. 2003), provides a foundation to address how prey availability influences the physiological responses in wading birds and how that influences reproduction. Subsequently, this underused approach to examining physiological condition of breeding wading birds presents a valuable method to begin to understand the constraints of prey availability on wading bird physiology and breeding in the Florida Everglades.
This project will make the link between landscape level prey availability and breeding wading bird physiological condition, and its associated influence on reproductive output. Specifically, measuring the response of plasma triglycerides, glycerol, corticosterone, progesterone, blood sedimentation rates, hematocrit, stress proteins, mass, and pectoral scores to changes in prey availability. The influence of these parameters on adult nesting attempts, nest success, chick survival, and growth will then be considered using an information theoretic modeling framework.
Objectives• Evaluate the relationship between landscape prey availability patterns and physiological condition of adult great egret and white ibis.
• Evaluate the relationship between landscape prey availability patterns and adult great egret, and white ibis physiological condition and productivity.
• Evaluate the relationship between landscape prey availability patterns and adult great egret and white ibis physiological condition, and chick growth and survival.
• Evaluate the effect of prey supplementation on white ibis nest survival, chick growth, chick survival, and validate response variables.
Figure 1. Conceptual model of adult wading bird responses to spatial and temporal patterns of prey availability, and hypothetical responses at the nest level.
