An experimental study of trade‐offs between brood size and moult in the blue tit (Cyanistes caeruleus)

Abstract

Life‐history theory predicts that whenever expensive activities overlap, there will be a trade‐off between the activities. In a field study of Blue TitsCyanistes caeruleusin Norway, we tested whether parental energy expenditure during reproduction affected timing of moult of primary feathers. Five days after hatching, we manipulated broods to contain either two (n = 10 broods) or 9–12 nestlings (n = 13). In contrast to previous studies, we measured stage of moult at two time points during the nestling period (7 and 14 days after hatching). The onset of moult in the male but not in the female was strongly affected by a reduction in brood size. Stage of moult did not differ between the experimental groups 7 days after hatching, but did after a further week. This difference was also evident when considering only males that had started moulting on day 7. Male moult score was not related to male age (yearling or older), body size (wing length, tarsus length) or hatching time (calendar date) of the brood. However, body condition, expressed as residuals of the regression of body mass on wing length, tended to be greater for parents of both sexes attending small broods than large broods. None of 23 females had initiated moult 7 days after hatching, and only two had done so by day 14. Surprisingly, moult did not seem to affect food provisioning by males relative to females. These results confirm previous studies of passerine birds, namely, that males may start moulting before females and that the moult can quickly be adjusted to the demands of brood‐rearing. The novel result in this case was that such demands affected not only the timing of moult onset but also its speed once it had started.