Nestling carcasses from colonially breeding wading birds: patterns of access and energetic relevance for a vertebrate scavenger community

Wray Gabel,Jabi Zabala,Peter Frederick

doi:10.1038/s41598-019-50986-4

Abstract

Energy transfer is fundamental to ecosystem processes, affecting productivity and community structure. Large aggregations of colonially breeding birds are known as nutrient sources through deposition of feces, but also may deposit large quantities of energy in the form of dead nestlings. The magnitude and ecological relevance of this process to the scavenger community is poorly understood. We used trail cameras to monitor the fates of size-appropriate chicken carcasses in heron colonies in order to quantify the proportion of available fallen nestlings that were consumed by scavengers in the Everglades of Florida, USA. Overall, 85% of 160 carcasses were consumed, with Turkey Vultures (Cathartes aura, 47%) and American Alligators (Alligator mississippiensis, 29%) being the primary consumers. Probability of consumption by alligators or vultures was related to distance from nest to water, local nesting density, and colony type. Consumption probabilities of both scavengers in relation to habitat covariates suggested clear resource partitioning promoting coexistence. We estimate fallen nestlings throughout this ecosystem could support 16% of the alligator population and 147 adult Turkey Vultures during a nesting season. This work indicates that fallen nestlings can serve as an important source of energy for scavengers at colonial breeding aggregations, particularly in oligotrophic systems.

Highlights

Energy transfer underlies many fundamental ecosystem processes[1,2,3,4,5], and nutrient availability and flow are critical to community composition and productivity[6]
Access to nestlings by different scavengers was explained by local environmental covariates
In higher nesting densities, and on Egretta heron islands were more likely to be consumed by alligators, while carcasses farther from water, in lower nesting densities, and on Ardea heron islands were more likely to be consumed by Turkey Vultures

Summary

Introduction

Energy transfer underlies many fundamental ecosystem processes[1,2,3,4,5], and nutrient availability and flow are critical to community composition and productivity[6]. Nell and Frederick (2015)[19] estimated that fallen nestling carcasses of long-legged wading birds (Pelecaniformes and Ciconiiformes, e.g. herons, egrets, ibises, storks, and spoonbills) in the Florida Everglades ecosystem could support hundreds of alligators (Alligator mississippiensis) for periods of several months, assuming all of them were consumed They found that alligators residing within wading bird colonies had improved body condition compared to those not in colonies[20], and because of the ephemeral nature of carrion, alligators and other ectotherms with low maintenance metabolisms would have a physiology that is well suited to scavenging[41]. The relevance of other environmental factors, such as vegetation complexity or distance to water, on scavenger accessibility and the ecological significance of bird carcasses to the scavenger community remains largely unknown This is pertinent in bird breeding colonies because they are a source of dense, pulsed concentrations of carcasses that are widespread in many regions of the globe. The proportion of carcasses that are consumed and their fate in relation to environmental features or trophic position is undescribed

Methods

Results

Conclusion