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Final Activity Report Summary - BREEDING SYNCHRONY (The effects of breeding synchrony on avian reproductive strategies)

Many socially monogamous birds commonly participate in Extra-pair copulations (EPC) that result in Extra-pair offspring (EPO). Why females engage in these EPC remains highly contentious, but several of the leading hypotheses propose that they benefit indirectly by enhancing the genetic quality of their offspring, through either good gene or genetic compatibility effects. Furthermore, the reasons for which the frequency of EPO varies so markedly, both within and between populations, remains poorly understood.

In this study, we investigated these issues in a population of blue tits, cyanistes caeruleus. We firstly assessed if EPO were genetically superior to offspring sired by the social partner and, secondly, if variation in breeding synchrony contributed to explaining variation in the occurrence of EPO within the population.

In order to address the first question, we compared the growth, survival and recruitment of EPO to their Within-pair (WP) half siblings. We also determined whether EPO were produced earlier in the laying order of eggs within a clutch, because earlier eggs usually hatched first, providing these offspring with a competitive advantage over their later hatching siblings. In support to the genetic benefits hypothesis, we found that EPO were larger, heavier and more likely to fledge than their maternal half-sibs. However, we also found that extra-pair paternity declined markedly with laying order, resulting in EPO hatching increasingly earlier than their WP half-sibs as the degree of hatching asynchrony of the clutch increased. After correcting this variation in hatching time, none of the observed disparities between EPO and their WP half-sibs remained significant, even though EPO still tended to be heavier and structurally larger.

These findings suggested that the evidence for genetic benefits from EPC might be less compelling than believed and that all future phenotypic comparisons between maternal half siblings should consider potential hatching order effects. Furthermore, variation in the degree of hatching asynchrony and brood reduction, both within and between populations, might contribute to explaining why the many studies comparing EPO to WP offspring produced inconsistent findings and, in some cases, context dependent differences, such as environmental conditions.

The second question was addressed by manipulating the degree of breeding synchrony within half the population in each of two breeding seasons. Microsatellite genotyping revealed that about 47 % of broods in the population contained at least one EPO. Nevertheless, preliminary analyses suggested that breeding synchrony did not contribute to explaining which females produced EPO. This was the first study to experimentally test whether synchrony affected paternity and should help resolve the on-going controversy.

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