Territory Area, Flower Density, and Time Budgeting in Hummingbirds: An Experimental and Theoretical Analysis

Abstract

While migrating southward each summer along the Sierra Nevada Mountain Range in California, nectar-feeding rufous hummingbirds (Selasphorus rufus) establish feeding territories within isolated meadows. We investigated the effect of variable food density upon territory area and time budgeting by controlled removals and additions of flowers of Castilleja linariaefolia within the territories of immature birds. The experiments were replicated three times with generally consistent results. Within one day following a 50% decrease in flower density: (1) territory area usually about doubled, resulting in (2) the number of defended flowers remaining similar to the premanipulation value; (3) foraging time increased significantly from about 21% to about 26% of the day as the result of (4) a significant increase in foraging bout duration, with (5) bout frequency remaining unchanged; (6) daily sitting time decreased significantly from about 76% to about 71% of the day; and (7) daily defense time did not change significantly at about 3% of the day. These patterns reversed within 1 day following subsequent experimental increases in flower density. Recent evidence suggests that migrant hummingbirds are energy maximizers that gain weight as rapidly as possible while on their territories. By modifying a simple model of feeding-territory area for such foragers (Hixon 1980) to incorporate previously described aspects of hummingbird foraging, a priori predictions of changes in territory area and time budgeting were generated. The direction of these predictions were independently corroborated by the experimental results. Refining a single factor of the model using our field data produced quantitative a posteriori predictions that very closely approached the experimental results in magnitude as well as direction. These findings tentatively suggest that, in response to short-term variations in food availability, migrant hummingbirds are capable of adjusting territory size and time budgeting in a manner consistent with maximizing daily net energy gain.