extract from RSPB
The robin is one of the few birds that hold a territory all year round. In summer a territory is defended by a mated pair, while each bird holds individual winter territories.
Breeding territories average 0.55 ha in size - about six would fit onto an average-sized football pitch while winter territories are around half of this. The exact size depends on the quality of habitat and the density of birds in the area. In some areas, such as Scottish pinewoods with well-spaced, mature trees and few shrubs, breeding densities can be as low as 10 pairs per sq km, while a lowland woodland can support as many as 200-300 pairs per sq km.
Territory boundaries are fluid, and change frequently as circumstances change.
The sole purpose of a robin's red breast is in territory defence: it is not used in courtship. A patch of red triggers territorial behaviour, and robins are known to persistently attack stuffed robins and even tufts of red feathers.
However my two little Robin mates seem to get on OK in the same area, perhaps they are one male and one female !!!