When does defending a territory become worth the cost for an animal?

Defending a territory becomes worth the cost for an animal when the resources within that area are "economically defendable"—meaning the benefits of exclusive access outweigh the energetic costs of defense and the potential risks of direct combat.12

The Economic Model of Territoriality

Biologists evaluate this through a cost-benefit framework where animals maximize their net gain.2

• Benefits: These include exclusive access to food, mates, breeding sites, and shelter. The value of these benefits is highest when resources are clumped and of intermediate quality; if resources are too sparse, they are not worth defending, and if they are of extremely high quality, competition may become too intense to maintain exclusivity.1
• Costs: These include the energy expended to patrol and advertise boundaries (e.g., through song or scent) and the physical risks of fighting intruders.132

Territoriality is a fluid system. An animal will typically abandon defense when the density of competitors increases the cost of fighting beyond the value of the resources, or when seasonal changes reduce the resource's quality. Consequently, animals often shift between defending and sharing space based on the immediate, shifting balance of these factors.12