Dietary Discrimination by Molting White-Crowned Sparrows Given Diets Differing Only in Sulfur Amino Acid Concentration

Abstract

In certain circumstances, wild animals may obtain adequate nutrition merely by random consumption of foods but, in other circumstances, may need to feed selectively for specific nutrients. For instance, it has been argued that requirements for certain amino acids may be so great during periods of production such as egg laying or molt that highly selective feeding is necessary to avert malnutrition. We evaluated the ability of captive molting white-crowned sparrows to choose between two semisynthetic diets (Choice) that were essentially identical, except in sulfur-amino acid (SAA = cyst(e)ine + methionine) concentration. The high-SAA diet was more than nutritionally adequate, and the low-SAA diet was inadequate for molting sparrows. Two parallel experimental groups were fed only the high-SAA diet (Control) or the low-SAA diet (Deficient), respectively. Food was supplied to each bird in two glass food cups, side by side. Choice birds thus had free access to a high-low-SAA-diet pair, while Control (high-high-diet pair) and Deficient (low-low-diet pair) birds had free access to right- or left-hand cups containing identical diets. Daily food intake by Control birds was 50:50 from rightand left-hand cups before, during, and after molt. Their body-mass variation and molt pattern (duration = ca. 54 days) were normal. Daily food intake by Deficient birds was less (25%-30%); body mass was less (ca. 15%); and molt was longer (105 days) than in Control or Choice birds. Many new feathers in Deficient birds were malformed. A subgroup of the Deficient group that was given a high-low-diet choice midway through the experiment quickly regained normal body mass and accelerated molt above the normal rate, so that molt was completed in 75 days. Daily food intake, body mass, and pattern and duration of molt in the Choice birds were indistinguishable from these variables in Control birds. Choice birds consumed a 50:50 ratio of diets before and after molt but during molt preferentially consumed the high-SAA diet in rough proportion to molt intensity, with a mid-molt plateau at about 65% highSAA diet. Tests involving switching of diet (food-cup) locations showed that the birds can sense diet quality and respond to altered diet location within at least 16 h (1 feeding day).