Passerines are predominantly characterized by social monogamy (Lack, 1968) with posthatching biparental care (Cockburn, 2006), but there are no strict rules of thumb for living according to a certain kind of mating and parental care system (Bennett & Owens, 2002). Hence, there are species where the above systems vary within populations depending on local environmental or social conditions, as in the evergreen case of dunnocks (Prunella modularis, Davies & Lundberg, 1984) or in penduline tits (Remiz pendulinus, Persson & Öhrström, 1989). In spite of this plasticity, cooperative breeding strategies are apparently quite rare among passerines (Brown, 2014;Ligon, 1999). In these cases, a single brood is cared for by at least two individuals belonging to the same
Brood sex ratios (BSRs) have often been found to be nonrandom in respect of parental and environmental quality, and many hypotheses suggest that nonrandom sex ratios can be adaptive. To specifically test the adaptive value of biased BSRs, it is crucial to disentangle the consequences of BSR and maternal effects. In multiparous species, this requires cross‐fostering experiments where foster parents rear offspring originating from multiple broods, and where the interactive effect of original and manipulated BSR on fitness components is tested. To our knowledge, our study on collared flycatchers (Ficedula albicollis) is the first that meets these requirements. In this species, where BSRs had previously been shown to be related to parental characteristics, we altered the original BSR of the parents shortly after hatching by cross‐fostering nestlings among trios of broods and examined the effects on growth, mortality and recruitment of the nestlings. We found that original and experimental BSR, as well as the interaction of the two, were unrelated to the fitness components considered. Nestling growth was related only to background variables, namely brood size and hatching rank. Nestling mortality was related only to hatching asynchrony. Our results therefore do not support that the observed BSRs are adaptive in our study population. However, we cannot exclude the possibility of direct effects of experimentally altered BSRs on parental fitness, which should be evaluated in the future. In addition, studies similar to ours are required on various species to get a clearer picture of the adaptive value of nonrandom BSRs.
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