Territorial responses to energy manipulations in the Anna hummingbird.

Territorial activity in the Anna hummingbird (Calypte anna) was measured while energy availability on the territory was varied. On days when energy availability was unlimited, residents defended highly exclusive territories primarily by energetically expensive defense behaviors. As energy availability decreased, exclusiveness declined gradually, relative use of energetically inexpensive defense increased, and owners spent less time on the territory.Territorial behavior also varied with short term depressions in energy availability: A lower percentage of intruders was chased and departures of an owner from its territory were more frequent shortly after feeding.When resource dispersion was increased without changing substantially total rewards per territory, chasing by owners increased.