PIERCE, Barbara/J; MCWILLIAMS, Scott/R: Effects of Food Quality and Digestive Constraints on Recovery of Body Reserves in a Migratory Bird

Small songbirds must regularly rest and refuel at stopover sites during migration, in part because they are unable to carry enough fuel reserves for long-distance, non-stop flights. Length of stay at a given stopover site depends on the nutritional demands of the previous flight, the effect of fasting during flight, and the quality and quantity of available resources at the stopover site. We studied the interactive effects of food limitation and food quality on White-throated Sparrows (Zonotrichia albicollis) captured during migration. We fasted or food-restricted sparrows (n =18) and then fed each group ad libitum on diets that were nutritionally similar to either fruits (16% carbohydrate, 3% protein, 2% fat) or grains (62% carbohydrate, 13% protein, 8% fat). Control birds (n=19) that were fed ad libitum maintained body mass on both diets; birds on the fruit diet ate 4 times more (16.3 � 1.0 g wet mass) than those on the grain diet (4.4 � 0.3 g wet mass). Body and digestive organ mass of sparrows declined during food restriction and especially during fasting. After food limitation, fasted and restricted birds fed the grain diet ate 40% more than control birds on the same diet and regained body mass. Birds fed the fruit diet did not eat more than control birds and did not regain any body mass suggesting a digestive constraint limited food intake. Recovery of body mass was similar between fasted and food-restricted birds within each diet group. Thus, both types of food limitation negatively affected mass of digestive organs whereas only diet quality (and not type of food limitation) affected recovery rates of White-throated Sparrows.