Multiple regression analyses were made of the effect of food composition upon energetic efficiency and existence energy of House Sparrows, Passer domesticus. Protein content of experimental food is the most important of eleven independent variables tested in the prediction of energetic efficiency. Other variables significantly affecting energetic efficiency are fat content of food, ambient temperature, caloric content per gram of experimental food, weight change, geographic location of the capture site of the bird, and season of capture and measurement. Existence energy, a measurement of energy usage under caged conditions, is significantly affected by ambient temperature, fat content of food, weight change, caloric content per gram of food, season and body weight 0.75.