Size matters: shiny cowbirds secure more food than host nestmates thanks to their larger size, not signal exaggeration

BORTOLATO, T.; GLOAG, R.; REBOREDA, J. C.; FIORINI, V.D.

ANIMAL BEHAVIOUR

ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2019 vol. 157 p. 201 - 207

0003-3472

Many hosts of obligate brood parasitic birds invest more in parasitic nestlings than they do in their own young. The shiny cowbird (Molothrus bonariensis), a generalist parasite, is fed at a higher proportion than its host nestmates when it is reared in nests of a smaller-bodied host, the house wren (Troglodytes aedon). We test two hypotheses that could account for this differential allocation of food by host parents. The signal exaggeration hypothesis states that cowbird chicks have visual and/or acoustic begging signals that elicit preferential feeding. The size-advantage hypothesis states that hosts preferentially feed large chicks and/or that larger chicks outcompete host chicks in a scramble competition for food. To gain insight into the relative importance of size vs. species-specific signals on food allocation by house wrens, we performed audio and video recordings in nests with experimental broods of: (i) a two-day old cowbird chick and two-day old wren chick (different species, different size), (ii) a two-day old cowbird chick and eight-day old wren chick (different species, same size), and (iii) a two-day old house wren and eight-day old house wren (same species, different size). When cowbirds shared the nest with a same-size wren chick, both chicks received food in equal proportion. In contrast, larger chicks (both cowbirds or wrens) paired with small wren nestmates always received a higher food share. Cowbird begging behavior and call traits differed from house wrens, but these differences did not always coincide with increased food. We conclude that, at least when cowbird nestlings are young (two days old), their relatively large size accounts for the larger share of food they receive from house wren hosts, rather than some quality of their begging signal.