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."

One possible influencing factor has recently been investigated by a research team from the Department of Organismic and Evolutionary Biology at Harvard, who have studied energy conservation in fish schooling. Their research demonstrated, for the first time, that in the case of large-scale schools of fish, there can be a very significant energy economy in group swimming - in some cases showing more than a 50% energy saving. Presumably, there could be similar energy saving benefits in bird flocks - though this has yet to be scientifically investigated and/or demonstrated.

In summary, our experiments on giant danio have demonstrated substantial energy conservation resulting from schooling dynamics across a wide range of speeds in fish. Direct measurement of both aerobic and non-aerobic energy use is critical for understanding the rapid collective movement of animals. Fish schooling in the high-drag viscous aquatic medium serves as a model for understanding how group movement by animals can be a more energy-efficient biological collective than movement by isolated individuals."

See Energy conservation by collective movement in schooling fish Journal of Experimental Biology. 764. 226, jeb245617 (2023)

Note: Many of 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 containing more than a million individuals breeding in close contact..