MCWILLIAMS, S.R.; KARASOV, W.H.; Univ. Rhode Island; Univ. Wisconsin: Spare capacity in the digestive system of a migratory songbird and its ecological significance.

Phenotypic flexibility in the digestive system of migratory birds is critically important in allowing birds to successfully overcome the physiological challenges of migration. Determining when rates of digestion constrain diet choice or re-fattening rates in animals requires understanding the magnitude of spare volumetric or biochemical capacity relative to the magnitude of change in food quantity or quality. We estimated the level of spare capacity and phenotypic flexibility in the digestive system of White-throated Sparrows (Zonotrichia albicollis) in response to changes in feeding rate. Sparrows were either held continuously at +21 C, switched rapidly from +21 C to -20 C, or gradually acclimated to -20 C over 50 days. White-throated sparrows acclimated at -20 C ate 83% more food than birds acclimated at +21 C, and birds at both temperatures maintained constant body mass. When birds were switched rapidly from +21 C to -20 C they increased feeding rate only 45% and this level of food intake was not sufficient to satisfy the extra energy demands given that these birds lost body mass. The digestive adjustments to increased feeding rate that occurred during acclimation to the cold included a 57% increase in small intestine. All birds had similar digestive efficiency and retention times. Given that sparrows acclimated to +21 C had a spare capacity of 45% to start with, adding an increase in gut size of 57% can more than account for the 83% increase in ability to process food for birds acclimated to -20 C. These results imply that immediate spare capacity of sparrows was initially around 45% but it declined (to about 22%) after long-term acclimation to high feeding rates, and that the ultimate capacity of sparrows was about 100 % above “baseline”. Supported by N.S.F. (IBN-9318675 and IBN-9723793 to W.H.K. and IBN-9984920
to S.R.M.).