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

Publisher Website
Google Scholar
Abstract
While migrating southward each summer along the Sierra Nevada Mountain Range in California [USA], nectar-feeding rufous hummingbirds (Selasphorus rufus) establish feeding territories within isolated meadows. The effect was investigated of variable food density on 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 3 times with generally consistent results. Within 1 day following a 50% decrease in flower density: territory area usually almost doubled, resulting in the number of defended flowers remaining similar to the premanipulation value; foraging time increased significantly from .apprx. 21-26% of the day as a result of a significant increase in foraging bout duration, with bout frequency remaining unchanged; daily sitting time decreased significantly from .apprx. 76-71% of the day; and 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. 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 (Hixo 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 the field data produced quantitative a posteriori predictions that very closely approached the experimental results in magnitude and direction. 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.