Avoidance of predators has long been regarded as a major benefit in colonial breeding. Nevertheless, field and comparative studies have not shown a clear relationship between predation and coloniality. In the present study, we examine the association between aerial egg predation on the Common Tern (Sterna hirundo), and sub-colony size, nest aggregation and reproductive synchrony. Fieldwork was carried out at the Ebro Delta colony (north-western Mediterranean), where Common Terns breed syntopically with potential predators. Sub-colonies placed in a small area were used instead of distant colonies in an attempt to minimize site effects, e.g., the abundance of predators. We used logistic regression with random effect (i.e., sub-colony) to test simultaneously the effect of the studied factors on the risk of predation. In addition, the random effect allowed us to account for the extra-binomial variability due to the potential non-independence of nests of the same sub-colony (clustered observations). Our results support the contention that both breeding in large colonies and in aggregated territories confers protection against aerial predators. In addition, synchrony in relation to the whole colony had no effect on the risk of egg predation. However, birds breeding asynchronously earlier in the season than the average in their own sub-colony were more likely to suffer egg predation than eggs from late asynchronous nesters.
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Vol. 26 • No. 3