100.1 Saturday, Jan. 7 Female Tree Swallows modulate fat and lean mass in anticipation of increased foraging costs of chick rearing BOYLE, W.A.*; WINKLER, D.W.; GUGLIELMO, C.G.; Univ. Western Ontario firstname.lastname@example.org
Birds often lose body mass during nesting. Determining if this mass loss represents an energetic cost of reproduction (energetic stress hypothesis), serves an adaptive function (flight efficiency hypothesis), or results from physiological processes that are neutral with respect to fitness (e.g. gonadal regression hypothesis) is important to interpreting variation in body mass and energy stores in the context of life-history theory. New QMR technology enables precise, repeated measurements of body composition on the same individuals, and we used this to test predictions distinguishing among hypotheses for mass loss in female Tree Swallows (Tachycineta bicolor). Swallows lost mass abruptly in anticipation of the peak foraging demands of feeding chicks. Lean mass and fat mass losses varied independently, with small and gradual losses in lean mass during incubation, and dramatic losses of fat immediately prior to and following hatching. The period of greatest parental foraging costs was not associated with any significant changes in total body mass, lean mass, fat mass, or water. Net change in body mass from early incubation until mid-way through chick rearing was associated strongly with initial body mass and to a lesser degree, brood size. These findings are consistent with females facultatively modulating endogenous energy stores to maximize insurance against bad weather and poor foraging opportunities during incubation, but then lowering body mass to maximize flight efficiency once chicks hatch, thereby reducing their costs of feeding nestlings. This study corroborates results of a growing number of studies suggesting that increases in mass in anticipation of reproduction (and the subsequent loss of that mass) are likely best viewed as part of an adaptive suite of interrelated reproductive decisions made by females each year.