Testing optimal body mass theory: Evidence for cost of fat in wintering birds

Copyright: © 2015 Rogers. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://creativecommons.org/licenses/by/3.0/Optimal body mass models for small wintering birds are central to animal ecology, and offer insights into maximizing individual fitness in a complex environment. Such models assume both costs and benefits of fat deposition, and consider how they affect winter survival probability. Hypothesized mass-dependent costs of elevated fat include increased wing load and subsequent reduced ability to avoid predators, as well as increased predator exposure while feeding to fatten. A likely benefit of winter fat is increased fasting capacity during resource shortages. Here I test optimal body mass theory by searching for both cost and benefits of winter fattening, utilizing interspecific variation in winter fat in natural populations. If increased predation risk is a mass-dependent cost of fattening, wintering birds occupying dense (closed) winter habitat offering low exposure to predators should show (1) higher fat reserves, and (2) higher wing load, than wintering birds occupying less dense (open) habitat offering less protection from predators. This prediction was tested in the two winters in south-central Kansas, a north temperate region with winter precipitation falling as snow and low ambient winter temperatures. The predicted patterns in wing load and winter fat reserve were observed: with phylogeny controlled, both were significantly lower in two open cover species (Dark-eyed Junco Junco hyemalis, American Tree Sparrow Spizella arborea) than in eight closed cover species (seven sparrow species in the genera Melospiza, Passerella, and Zonotrichia, and the Spotted Towhee Pipilo maculatus). Body mass increased with wing area at a greater rate in the closed than in the open cover group, indicating lower body-size specific wing load in the latter group. With phylogeny controlled, fat level varied inversely with resource predictability among three winter foraging guilds occupying a vertical gradient of snowfall probability, indicating both costs and benefits exist in the study system.This research was supported by NSF grant EPS-9874732 to C.M. Rogers, by NIH NCRR grant (Kansas Biomedical Research Infrastructure Network [KBRIN] P20 RR16475) to C. M. Rogers and M. Schneegurt, and by four NIH KBRIN undergraduate student fellowships. WSU field station facilities were developed with NSF grant DBI 0626817 to C. M. Rogers. The Department of Biological Sciences and College of Liberal Arts and Sciences at WSU provided additional financial and logistical support. This research was approved by the WSU IACUC