1. Birds often lose body mass during nesting. Determining whether 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.
2. New quantitative magnetic resonance technology enables precise, repeated measurements of body composition (fat, lean, and water masses) on the same individuals, and we used this method to test a series of predictions to distinguish among competing hypotheses explaining mass loss in female tree swallows (Tachycineta bicolor, Vieillot, 1808).
3. Tree swallows lost mass abruptly prior to 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. Females lost some body water early in incubation, but did not lose more water when brood patches would be expected to atrophy. 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.
4. These findings are consistent with the flight efficiency hypothesis. Females appear to facultatively modulate their endogenous energy stores to maximize insurance against bad weather and poor foraging opportunities during incubation, but then lower their body mass to maximize efficiency once chicks hatch, thereby reducing the costs of feeding nestlings.
5. This study corroborates results of a growing number of studies of small, insectivorous birds (especially cavity-nesting species), suggesting that increases in mass prior to 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.