Do digestive contents confound body mass as a measure of relative condition in nestling songbirds?

Wildlife Society Bulletin
By: , and 



Relative nestling condition, typically measured as nestling mass or as an index including nestling mass, is commonly purported to correlate with fledgling songbird survival. However, most studies directly investigating fledgling survival have found no such relationship. We weighed feces and stomach contents of nestling golden-winged warblers (Vermivora chrysoptera) to investigate the potential contribution of variation in digestive contents to differences in nestling mass. We estimated that the mass of a seventh-day (near fledging) nestling golden-winged warbler varies by 0.65 g (approx. 9% of mean nestling mass) depending on the contents of the nestling's digestive system at the time of weighing, and that digestive contents are dissimilar among nestlings at any moment the brood is removed from the nest for weighing. Our conservative estimate of within-individual variation in digestive contents equals 72% and 24% of the mean within-brood and population-wide range in nestling mass, respectively. Based on our results, a substantive but typically unknown amount of the variation in body mass among nestlings is confounded by differences in digestive contents. We conclude that short-term variation in digestive contents likely precludes the use of body mass, and therefore any mass-dependent index, as a measure of relative nestling condition or as a predictor of survival in golden-winged warblers and likely in many other songbirds of similar size.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Do digestive contents confound body mass as a measure of relative condition in nestling songbirds?
Series title Wildlife Society Bulletin
DOI 10.1002/wsb.406
Volume 38
Issue 2
Year Published 2014
Language English
Publisher Wildlife Society
Contributing office(s) Coop Res Unit Leetown
Description 6 p.
First page 305
Last page 310
Online Only (Y/N) N
Additional Online Files (Y/N) N