Temporary Weight Decrease in Breeding Birds May Result in More Fledged Young

Abstract

The lowered weight in adult birds rearing young should result in energy savings during locomotion. Furthermore, the parents may live partly off the energy released during weight loss. For these reasons the parents could eat a smaller proportion of the food collected themselves and give a larger proportion to the young, potentially leading to more fledged young, faster growth, and/or better condition of the young than if the parents maintained prebreeding weight. I examine this possibility theoretically and estimate the potential number of extra young for various weight reductions and daily flight times in the parents. I estimate energy savings in flight only, since this is the most power-consuming type of locomotion. Because weight reduction in an individual bird lowers not only induced drag but also body drag, the power consumption in flight may vary approximately with the 1.5 power of the weight. Energy savings in flight resulting from weight loss may readily enable some birds to rear on average 1/4-1/2 extra young, whereas the use of energy reserves in the parents may correspond to the rearing of 0.1-1 extra young. These values are additive. I suggest the new view that the temporary weight decrease in adults while rearing young may actually enhance parental inclusive fitness, i.e., it may be adaptive instead of purely detrimental. Although a weight loss is likely to lower parental survival, the potential benefits in terms of more fledged young should shift the optimal weight downward temporarily. This might dictate what proportion of the food collected should be eaten by the parent and what proportion should be given to the young.