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Summary Evidence is presented to support the hypothesis that communal roosts, breeding colonies and certain other bird assemblages have been evolved primarily for the efficient exploitation of unevenly‐distributed food sources by serving as “information‐centres”. Predation‐pressure is regarded as being the most important factor “shaping” the assemblages. The shaping involves the choice of inaccessible or otherwise safe sites, optimum dispersal, mutual awareness of attack and joint defensive tactics, and serves to minimise the vulnerability to predation which would otherwise result when birds mass together in conspicuous, and often predictable centres.

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Because hosts that accept a parasitic egg laid by the common cuckoo, Cuculus canorus, are unlikely to fledge their own offspring, rejection should be an adaptive response. Evidence that cuckoo host species attain only intermediate rates of rejection are commonly interpreted as resulting from an evolutionary lag. Yet, we found that the acceptance of cuckoo eggs by female great reed warblers, Acrocephalus arundinaceus, occurs mainly among the younger breeders in the host population. We suggest that some level of acceptance can arise in the host population as a result of the need of naive breeders to learn to reliably recognize their own eggs rather than representing evolutionary lag.