Defending a territory can allow the territory owner to have exclusive or nearly exclusive access to the territory’s resources, which can include food, a mate that has been attracted to the territory, or a mating site. However, to en joy the full use of these resources, a territory owner must prevent other members of its spe cies from staying within its territorial bound aries. Conspecifics that are likely to attempt to intrude on a territory include individuals that have not yet acquired a territory, individuals that have lost possession of a territory that they formerly defended, and territorial neighbors. All such intruders have the potential to be costly to a territory owner because they seek the same resources as the owner and are likely to use some of the very resources that the territory owner is defending. For example, an intruder on an all-purpose territory may forage, removing some of the territory’s food resources; a male intruder might copulate with the female mem ber of a territorial mated pair, reducing the male territory owner’s probability of paternity and decreasing the reproductive benefit he receives from being territorial.