Body Mass Reduction During Reproduction in the Pied Flycatcher Ficedula hypoleuca: Physiological Stress or Adaptation for Lowered Costs of Locomotor?

Abstract

An important parameter in life-history theory is reproductive effort, i.e. the proportional amount of resources allocated to reproduction, and its effects on individual animals within a population, including the risk of predation, survival and fecundity in subsequent breeding attempts. Generally, it is believed that reproduction drains energy that otherwise could have been used for survival or subsequent reproduction. There has been a controversy about whether the mass loss in birds represents such an energetic drain in energy store or whether it is an adaptation to lower the flight costs of adults during nestling rearing. Here I present an experiment that aimed to test these two hypotheses, i.e. 'the cost of reproduction hypothesis' and the 'mass adjustment hypothesis', by adding extra food in some territories during incubation and nestling rearing, while others were held as controls. There was no effect of food addition on either final body mass (late nestling) or body mass change (from incubation to late nestling) for females of different treatments. However, in males there was a tendency for a lower final body mass in control broods, compared with males with supplementary food, and also for a larger body mass change of males in control broods compared with males with supplementary food in 1 of the 2 years. There was a positive correlation between number of fledged young and body mass change for females, and a negative correlation existed between fledging success and final body mass for females. Thus, a large number of fledged young led to a greater body mass change and to a lower final body mass for females. However, there were no such correlations in males. Hence this study provides some support for the 'cost of reproduction hypothesis', but also gives support for the 'mass adjustment hypothesis'. A certain amount of decline in body mass during breeding may be of adaptive value, but below some threshold limit, which can vary depending on environmental conditions, a somatic cost probably arises that may reduce future survival and/or fecundity. Thus, the two hypotheses for explaining body mass reduction in birds may not be mutually exclusive. Also, a third alternative 'benefit-cost hypothesis' is proposed, which predicts that parents should decrease their body mass to a level that optimizes parental investment and maximizes life-time reproductive success.