The bioenergetics of roost site selection for great-tailed grackles

Abstract

A model depicting the flow of energy through a great-tailed grackle (Quiscalus mexicanus) during the winter season in Texas is described. The model is based on information from literature and field observations on distance flown, literature data on food habits, weather data from the roost and surrounding area, and allometric equations for determining existence metabolism; it predicts the daily energy budget of a bird for a typical month. Model simulations with modified conditions of increased flight distance from the roost and decreased roost site protection showed that increased flight had a greater impact on the energy budget than did decreased roost site protection. Great-tails that fly over 63 m begin to neutralize an energetic gain from reduced thermoregulatory costs when roost temperatures are raised 1 C. An energetically sound foraging radius of about 40 km was determined by using estimated feeding rates, stomach capacities, rate of passage of food, and model output. The simulations also show that the greatest energetic advantage in roosting is protection from the wind. The criteria for roost site selection by great-tailed grackles appear to be: (1) ample food supplies need to be within 40 km; (2) the site provides protections from the wind and, secondarily, the temperatures; (3) the site's structural features provide ample perching sites. Management implications relative to the energy budget of this species are discussed. The model's predictability of existence metabolism was assessed by comparing model output to observed values of existence metabolism. The food consumption experiments revealed significant differences between the sexes in existence metabolism, but no significant differences in metabolizable efficiencies (ME = 74.5%) The consequence of the male's larger size is a greater total requirement for energy; however, the female requires more energy per gram than the male.