Sibling competition affects individual growth strategies in marsh tit, Parus palustris, nestlings

  • Nilsson J
  • Gårdmark A
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Abstract

How do different levels of sibling competition affect nestling growth strategies? We investigated this through the effect of different levels of per capita feeding rate on the average growth rate of nestlings and on individual growth strategies within broods. To manipulate sibling competition, we transferred marsh tit nestlings between nests creating broods with fewer (reduced) or more (enlarged) nestlings than originally, Mean mass decreased as brood size, and thus sibling competition, increased. This was due to a large difference in mass between the smallest young in each brood. The mass of the largest young did not differ significantly between the broods. In reduced broods, the largest and smallest nestlings had equal mass, whereas in enlarged broods the difference was large. In contrast, wing length close to fledging did not differ between the experimental broods. We conclude that the growth of the wing has priority over general increase in size. This depends on the risk of starving for nestlings unable to leave the nest when their most dominant sibling leaves. Thus, low intake rates induce a trade-off in the growth of different structures, leading subdominant nestlings to divert proportionally more resources to wing length growth at the expense of general size. Subdominant nestlings are thus facing both a food constraint, depending on parental feeding rate and number of dominant siblings, and a time constraint, depending on the growth rate of the largest sibling. Hence, to fledge successfully, subdominant nestlings will leave the nest at suboptimal masses with a likely increase in postfledging mortality. © 2001 The Association for the Study of Animal Behaviour.

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