Behavioral Ecology Vol. 14 No. 1: 16-22
© 2003 International Society for Behavioral Ecology
Gape coloration reliably reflects immunocompetence of barn swallow (Hirundo rustica) nestlings
aDipartimento di Biologia, Sez. Zoologia Scienze Naturali, Università degli Studi di Milano, via Celoria 26, I-20133 Milano, Italy bLaboratoire d'Ecologie, CNRS UMR 7625, Université Pierre et Marie Curie, 7 quai St. Bernard, Case 237, F-75252 Paris Cedex 05, France
Address correspondence to N. Saino. n.saino{at}mailserver.unimi.it.
In some passerines, parents allocate more food to offspring with the brightest red gapes, but the function of parental decisions based on offspring gape coloration is unknown. We hypothesize that gape coloration is part of a communication system where nestlings reveal their condition to attending parents, which may thus base their decisions on reliable signals of offspring reproductive value. We analyze the effects of brood size manipulation, injection with an immunogen and food deprivation, on gape coloration, morphology, and T-cell–mediated immunocompetence of nestling barn swallows (Hirundo rustica). For each gape we measured three components of coloration (hue, saturation, and brightness) and obtained an overall color score by principal component analysis. Enlargement of brood size and injection with an antigen resulted in less red and less saturated and brighter gape color. Nestlings in enlarged broods had smaller body mass and T-cell–mediated immunocompetence compared to those in reduced broods. A positive covariation existed between redness and saturation of gape color and T-cell–mediated immunocompetence. Gape color siblings raised in different nests did not depend on parentage. Thus, condition-dependent gape coloration can reveal different components of nestling state on which parents may base their adaptive decisions about allocation of care to the offspring.
Key words: begging, food provisioning, parental food allocation, parent–offspring conflict, reproductive value.
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