The evolution of group living remains an outstanding question in evolutionary ecology. Among the most striking forms of group living are the enormous assemblages of breeders that occur in many colonial marine birds and mammals, with some colonies containing more than a million individuals breeding in close contact. Coloniality is an evolutionary puzzle because individuals pay fitness costs to breed in high densities. Despite numerous potential benefits proposed to overcome these costs, we still lack a general framework to explain coloniality.

Source : The evolution of coloniality: the emergence of new perspectives [^paywalled ] Trends in Ecology and Evolution, Volume 12, Issue 9, pp. 342-347

Many species of mammals, birds, insects, fish etc. etc. live and/or breed in extremely large colonies - sometimes with many millions of individuals. Up until the end of the 1980s, there were two major theories regarding how and why the adoption of the large colony strategy came about - with the underlying assumption that there must be some kind of evolutionary advantage(s) to it. The first was that it might make it easier to find food. The second that it made predation less likely.

Since then, scientific opinion has moved towards the idea that there are probably multiple complex and interacting reasons (e.g. heat conservation, food-availability information-exchange etc.) for the emergence of large colonies, and that the reasons probably vary according to the species involved.

Many hypotheses have been proposed to explain how colonial breeding may benefit the individual. but there is still little support for most of them and none appears compelling."

[source as above]

For further info specific to birds. see Bird flockingplugin-autotooltip__plain plugin-autotooltip_bigBird flocking

Many theories have been put forward to explain large-scale flocking and roosting behaviour in birds. But from an evolutionary point of view, all the theories have clear drawbacks.

Ideas include:

• Heat conservation - but in many roosts the birds maintain a significant distance from each other, allowing most of the accrued heat to radiate away.