The northern shrike, which is about the same size as a robin, has a territory of 46 to 98 hectares. The flycatcher, a small insect-eater, defends a territory of only .08 hectares. The size of the territory is determined both by the abundance of food and by the size of foraging areas. For some species of birds, only a nesting territory is defended. The bank swallow nests in colonies and defends only a few centimetres between its nesting cavity and the adjoining nesting cavities.
Swallows eat flying insects.
These insects are both abundant and moving constantly. Since the insects are abundant, it is not necessary for the swallow to defend a feeding territory. Nor would defending a territory be worth the effort, since there is more than enough food for all swallows. The northern shrike catches most of its food within its own territory and therefore defends a large area from other northern shrikes. Large birds defend large territories.