Animals living in environments that change over space and time must somehow track the environmental change, and the possession of spare capacity and of flexible phenotypes provides two solutions. Flexible phenotypes enable animals to live in changing environments and knowing the limits to and the required timescale for this flexibility provides insights into constraints on energy and nutrient intake, diet diversity, and niche width. Migratory birds are particularly impressive in this regard in that they can fly for days without eating or drinking and they accomplish these impressive travels in part by repeatedly and flexibly changing the size and capacity of their digestive organs. We exposed white throated sparrows (Zonotrichia albicolis), a short-distance migratory bird, to experimentally manipulated ambient temperatures over different timescales, which forces endotherms such as birds to modify their food and energy intake as they maintain a constant body temperature. We then quantified the extent of phenotypic flexibility in the digestive system of this migratory bird (i.e., food intake, digestive efficiency, gut anatomy, retention time of digesta, rates of nutrient absorption) in response to both rapid and gradual increases in energy demand. This is the first published study for any wild vertebrate of both rapid and gradual adjustment of feeding and digestion to high energy demand that simultaneously measured key elements of the digestive system to reveal the mechanism(s) of digestive system adjustment primarily responsible for the demonstrated phenotypic flexibility.
Immediate spare capacity decreased from ca. 50% for birds acclimated to relatively benign temperatures to < 20% as birds approached their maximum sustainable energy intake. Ultimate spare capacity enabled an increase in feeding rate of ca. 126% as measured in birds acclimated for weeks at -29 C compared to +21 C. Increased gut size and not tissue-specific differences in nutrient uptake or changes in digestive efficiency or retention time were primarily responsible for this increase in capacity with energy demand, and this change required > 1-2 days. We conclude that the pace of change in digestive organ size may often constrain energy intake and for birds dictate the pace of their migration.