Body Mass Variation in Breeding Mountain Bluebirds Sialia currucoides: Evidence of Stress or Adaptation for Flight?

Abstract

Using portable electronic balances, patterns of body mass variation in individual mountain bluebirds (Sialia currucoides Bechstein) were investigated to determine whether mass changes during the breeding season were an adaptation to the energetic demands imposed by young (flight adaptation hypothesis) or a consequence of those demands (stress hypothesis). To distinguish between the hypotheses, a food supplementation experiment was performed in which mountain bluebirds were provided with an estimated 25% of their daily energy needs. Males and females had different patterns of body mass variation. Males maintained mass throughout the reproductive cycle, while females gained mass prior to egg laying, began losing mass during incubation, and lost the most mass during the nestling stage. Females were heavier than males during laying and incubation but reached the same mass as males by the end of the nestling period. In both years, females lost 14% of their initial body mass. Female mass loss during the nestling stage followed a decelerating pattern, supporting the flight adaptation hypothesis. Mass variation patterns during the second breeding attempt were similar to those of the first except that females were lighter at the beginning of the second attempt than they were in the first. Body mass variation patterns in supplemented birds followed the same patterns as those of unsupplemented birds, providing additional support for the flight adaptation hypothesis. Results for females, in particular, support the flight adaptation hypothesis rather than the stress hypothesis. Females put on mass before incubation, perhaps as an energy reserve in case of unfavourable foraging conditions and reduced available foraging time. This extra mass is lost when conditions are more predictable, females are foraging for young, and decreasing flight costs and increasing flight performance is advantageous. Males may maintain a low body mass because their foraging time is not constrained by incubation. Body mass variation thus appears to be an adaptation to the energetic demands of the young and not a consequence of those demands.