Meeting Abstract
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. We exposed white throated sparrows (Zonotrichia albicolis), yellow-rumped warblers (Setophaga coronata), and cedar waxwings (Bombycilla cedrorum) 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. 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. Supported by NSF (IOS-0748349 to S.R.M.) and NSC Poland (2015/19/B/NZ8/01394 to U.B.)
