Pigeons have their territories just as do non-gregarious birds, the only difference being that the territory is more restricted. Young unmated birds usually treat all others as enemies. Constant fighting causes the size of individual territories to fluctuate. The size of each territory is greatly affected by the space available and the density of population. Birds whose territories adjoin tolerate each other whereas they fiercely drive away less familiar individuals. Roughly speaking, each bird claims territory with an emphasis inversely proportional to the distance from his nest. If only one pair of birds occupies a pen it will claim the entire territory and treat any newcomers as intruders. A more or less clearly delimited neutral zone usually exists between adjacent territories.