Why Do Thousands of Birds Sleep in One Tree? Information and Warmth

Individuals converge from a wide area in the hour before dark, frequently gathering first at staging sites nearby before moving in together. Numbers can be extraordinary, with starling roosts reaching hundreds of thousands and a small number historically running into millions, and crows, wagtails, swallows and many other species doing the same on smaller scales. The site is used repeatedly for weeks or years, and the accumulation of droppings beneath is substantial enough to kill the vegetation and to cause serious complaints where a roost forms over a town. Departure in the morning is equally sudden, with the whole assembly leaving within minutes and dispersing.

Several advantages have been proposed and each has some support:

• •Warmth, since packed birds lose less heat, which matters on freezing nights
• •Safety in numbers, since a predator attacking a mass takes a small proportion
• •More eyes watching, so an approaching predator is detected sooner
• •Access to good sheltered sites, which are limited and worth travelling to
• •Social information about where food has been found
• •Opportunities to assess and meet potential mates

The most discussed explanation holds that a roost functions as a centre where unsuccessful birds learn from successful ones. The proposal, made in 1973, is that a bird which fed poorly follows one that fed well when the assembly disperses in the morning, identifying it by its behaviour or its condition, and is thereby led to a food source it could not have found alone. That would be valuable where food occurs in rich patches that are hard to locate and are not depleted by extra birds arriving. Evidence is mixed rather than conclusive, with experiments showing that birds do follow others under some conditions and with the mechanism of identifying a successful individual remaining unclear, but the idea has driven a great deal of research.

A large assembly over a town produces genuine conflict and the responses are instructive. Droppings accumulate fast enough to damage buildings and vehicles, make pavements hazardous and kill the trees beneath, and the noise at dusk and dawn is considerable. Health concerns about fungal spores in accumulated droppings are raised frequently and the actual risk is low outdoors. Responses have included scaring with recorded distress calls, pruning or felling the trees involved, lasers, and in several countries mass killing, which is generally ineffective since the birds simply move. Deterrence usually relocates the problem to a neighbouring site rather than solving it. The most successful approaches accept the roost and manage the consequences beneath it.

Gathering in one place is expensive as well as beneficial. Travelling to and from a distant site costs energy daily. Predators learn where roosts are and hunt them reliably, with birds of prey specialising on them. Parasites and disease transmit readily in dense assemblies. Competition for the best positions is real, with dominant birds occupying the sheltered centre and others the exposed edges. The aerial displays performed before settling, in which tens of thousands of starlings wheel in coordinated masses, are among the more spectacular things in nature and are generally interpreted as a response to predators and as a signal that attracts more birds to the site, which makes the display part of the gathering rather than decoration.

Birds scattered by day converge in the hour before dark into assemblies of hundreds of thousands, using the same site for weeks. Warmth, safety in numbers, more eyes watching and access to limited sheltered sites all contribute. The most discussed explanation is that unsuccessful birds follow successful ones out in the morning to find food, which has mixed support. Predators, parasites and daily travel are the costs.