Abstract
Mountain bluebirds (Sialia currucoides) were supplemented with food during the nestling period to assess the trade-off between allocation of energy to parental self-maintenance and investment in offspring. Three treatment groups were established, with pairs of birds receiving 0 (control), 9 (small), or 18% (large) of the estimated daily energy requirements of both parents and their brood. Unsupplemented adult females lost an average of 3 g during the nestling period, while both supplemented groups maintained their body mass at approximately 33 g. Adult males in all three groups maintained their mass at approximately 30.5 g. Nestlings in both supplemented groups fledged with heavier masses than did those in the control group. Those receiving the large supplement also grew faster. Food appears to limit the lifetime reproductive success of mountain bluebirds. Adult females allocated additional energy to self-maintenance rather than to increased investment in current offspring. This result was not seen in males. We conclude that our results demonstrate a trade-off between investment in current versus future components of reproductive success.
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