Effects of Food Shortage and Oversupply on Energy Utilization, Histology, and Function of the Gut in Nestling Song Thrushes (Turdus philomelos)

We measured food intake, digestive efficiency, body mass increments, resting metabolic rate (RMR), carcass fat content, size and histological structure of the gut, and the rate of intestinal brush border uptake of l-proline in song thrush (Turdus philomelos) nestlings subjected to food shortage or food surplus under laboratory conditions. We assigned nestlings between 3 and 7 d of age to one of the following treatments: (1) food restriction, which resulted in a slowed growth at the rate found in undernourished, wild nestlings; (2) overfeeding, which totally suppressed begging; and (3) intermediate feeding. Threefold differences in energy consumption caused fivefold differences in body mass increments of the nestlings. Despite this, body mass-corrected RMR and intestinal mass were not affected by the feeding regime. The energy content of fecal output was highest in food-restricted birds, while their carcass fat content was lowest among treatment groups. Intestinal uptake rates of l-proline were low in the overfed and intermediate-fed young but significantly increased in the food-restricted birds, who attempted to maximize their rates of growth and development within the restrictive limits set by feeding regime. We noted a marked decrease of intestinal villi height in overfed birds as compared to intermediate-fed and food-restricted nestlings. We conclude that song thrush nestlings are characterized by a limited plasticity of their developmental program, which prohibits overfed nestlings from significantly up-regulating their gut function to accommodate increased food intake. This suggests that they already grew at a rate close to their physiological maximum. We suggest two interpretations: (1) under natural conditions, song thrush nestlings do not face frequent, unpredictable fluctuations in food abundance that could select for developmental plasticity, or (2) strong selection for uniform adult phenotypes prevents flexible developmental trajectories, which would result in a diversity of adult phenotypes.