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.

• Protection in numbers - but many roosts are extremely noisy, which would attract the attention of predators. Many species also perform elaborate mass-flight displays, drawing attention to their presence.

• A new theory was raised by Ward and Sahavi in 1972 - in their paper :'The importance of certain assemblages of birds as information centres for food finding' (in: Ibis 115(4):517 - 534) The team proposed that birds in large-scale roosts can pick up valuable information about the availability of local food sources etc. by observing the behaviour of other birds. In other words they are 'reassured' if the flock is large, and so stay in the area where food is plentiful.

This new theory also has hard-to-explain aspects :

“[…] there must be occasions when, perhaps for long periods, all the members of the population are perfectly capable of finding their food unaided, yet they converge regularly upon the roost, colony, or other information-centre.”

Note: The same puzzling questions apply to permanent and semi-permanent large-scale groupings in other animals - see Colonialityplugin-autotooltip__plain plugin-autotooltip_bigColoniality

"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 containi….

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