Bioenergetics and diet in a simple community of shrubsteppe birds.

Simulation model estimates of bioenergetics are coupled with observations of diet selection and arthropod prey abundances to assess (1) the role of bird populations in trophic energy fluxes in a temporally heterogeneous shrubsteppe ecosystem, and (2) the degree to which those populations may be limited by food.The model estimates a total annual energy demand of 2.91 kcal m-2 yr-1 by the entire passerine avifauna during 1974, with daily demands varying from 0.0025 to 0.0260 kcal m-2. Coupling energy requirements with estimates of arthropod availability implies that bird demands on the insect standing crop never exceeded 0.7% per day of that standing crop during the breeding season or summer.Overall, the bioenergetic estimates imply that these birds are unlikely to be important in ecosystem processes and, reciprocally, are unlikely to be limited by food resources even during peak energy demands. As a consequence, I suggest that biological interactions such as competition play a relatively minor role in structuring the bird community in this variable environment.