Variation in body condition of breeding Savi’s Warblers Locustella luscinioides: the reproductive stress and flight adaptation hypothesis revisited

Abstract

Current theory suggests that mass change in adult birds while breeding may be adaptive (to reduce wingloading during nestling feeding) or result from physiological stress. To test which might be more important in determining mass loss in breeding Savi's Warblers (Locustella luscinioides), we used a new approach in which the variation in four indices of body condition was described: weight, fat score, muscle score and lean weight (i.e. excluding fat and muscle). We expected weight variations to be adaptive if they involved changes in fat and lean weight, whereas physiological stress should influence the muscle score to a greater extent. As in other species, females showed a greater variation in weight, and carried more fat, than males during the breeding cycle. During incubation, females had greater weight and fat score than males. The weight remained constant and lean weight declined in both sexes, whereas females increased in muscle, which probably reflects the regression of the reproductive organs. During the nestling stage, both sexes declined significantly in all four indices of condition, showing evidence of physiological stress. However, the greater decline in weight in females than in males is consistent with the flight-adaptation hypothesis, as are the cyclic changes in lean weight associated with the various nesting attempts. The fact that both sexes declined significantly in weight, muscle and lean weight with an increasing number of nesting attempts, but not in fat, which was recovered after each nestling period, also indicates that both reproductive stress and adaptive changes occur during breeding. When the whole breeding season was considered, females showed a greater decline in muscle than males, which we interpret to be evidence for a greater reproductive stress in females. We suggest that the small breast muscle size and depleted protein reserves at the end of the breeding period might influence future survival through impaired flight ability and a compromised post-breeding moult.

Breeding limits foraging time: evidence of interrupted foraging response from body mass variation in a tropical environment

“…By brooding nestlings and losing stored body reserves, females may adapt their body mass for better flight performance during nest provisioning. This post‐incubation loss of mass is similar to patterns observed in other species (Moreno , Merkle and Barclay , Neto and Gosler ). Thus, our observations suggest that the reproductive stress and flight adaptation hypotheses are not mutually exclusive and probably explain why there is no clear experimental support for each.…”

“…Body mass variation in breeding birds has received considerable attention in temperate environments compared with tropical ones. In temperate environments, body mass gain before incubation in female birds has been attributed to the increased size of reproductive organs (Vezina and Salvante ), while mass loss after or during incubation is attributed to the effect of reproductive workload or adaptation for better flight performance during nest provisioning (Merkle and Barclay , Neto and Gosler , Boyle et al ). Pre‐incubation body mass gain due to increased size of reproductive organs is largely undisputed (Vezina and Salvante ), but the reproductive stress and flight adaptation hypotheses have been tested using experimental food supplementation to temperate breeding birds (Moreno , Merkle and Barclay ).…”

Environmental and Within-Nest Factors Influencing Nestling-Feeding Patterns of Mediterranean Blue Tits (Cyanistes caeruleus)

“…Nur (1984) suggested the second option as the most plausible hypothesis. The so-called "reproductive stress" hypothesis assumes that reproduction is physiologically costly and that changes in body condition during breeding should be interpreted as the result of the energetic demands of this period (Nagy et al 2007, Neto andGosler 2010). Nevertheless, we cannot explain why the mass of males did not decrease with increasing frequencies of feeding since males apparently worked harder than their mates.…”

Do migrant and resident species differ in the timing of increases in reproductive and thyroid hormone secretion and body mass? A case study in the comparison of pre-breeding physiological rhythms in the Eurasian Skylark and Asian Short-toed Lark

“…Body mass is a comprehensive index that reflects the balance of energy intake, energy expenditure and nutritional status (McNab 2009). The body mass of birds often increases with increasing food abundance in spring (Both and Visser 2005;Cooper 2007;Fischer et al 2010) and females with higher body mass can lay eggs earlier (Neto and Gosler 2010). Our results show that Asian Shorttoed Lark attained its maximum body mass earlier than the Eurasian Skylark in both 2014 and 2015, suggesting that resident species accumulate energy reserves faster than migrants.…”