Most birds will endeavour to prevent other birds from occupying their "home" and its surrounding area. They are defending their territory and staking their claim on the nest sites, building materials and food in that territory.
This behaviour is most noticeable during the breeding season and is predominantly targeted at other birds of the same species or species with similar diets, such as is the case with Robins attacking Dunnocks.
Territories are defended at other times of the year though, such as in the winter when food is in short supply. For example, Mistle Thrushes will vigorously defend a bush bearing berries.
Different species have different sized territories, though most garden birds have quite small territories, from a few square metres (e.g. House Sparrows, Rooks) up to say an acre or so (e.g. Blackbird). On the other hand, birds of prey, such as eagles and Buzzards, may have territories covering tens of square miles.
|Blackbird||0.1 ha (0.25 acres)|
|Coot||0.4 ha (1 acres)|
|Song Thrush||4 ha (10 acres)|
|Mistle Thrush||50 ha (125 acres)|
The territories are defended, in most cases, by the male birds singing (see Bird Song) to show their ownership of the territory and their willingness to defend it. If their song is insufficient, the hostilities will escalate, starting with adopting a threatening posture and then chasing the intruder. If that does not work they will fight and some species, like Blackbirds and Robins, will fight to the death.
Occasionally, birds attack their own reflections in windows and mirrors (see Windows).