Observational and experimental evidence that rapid mass loss is consistent with the flight efficiency hypothesis and not caused by reproductive effort in three passerine species

Abstract Breeding birds often lose mass abruptly around the time that their eggs hatch, but the reasons why and the demographic significance of mass loss are contentious. Energy expenditure during offspring provisioning is one long‐standing explanation that fits with life‐history theory (energetic cost hypothesis). Mass loss may also reflect time constraints that limit foraging and self‐maintenance while warming (i.e. brooding) young (time constraint hypothesis). Alternatively, mass loss may be an adaptive strategy to provide benefits through decreased energetic flight costs while foraging and provisioning young (flight efficiency hypothesis). To test these hypotheses, we measured mass loss across the nesting cycle for three passerine species using an automated weighing system and quantified reproductive effort by filming nests. We also experimentally manipulated reproductive effort using a nest heating experiment to test effects on mass loss. Counter to the energetic cost hypothesis, female mass loss did not match changing parental provisioning effort across the reproductive cycle; mass loss in all three species started about 5 days before eggs hatched and finished well before nestling provisioning effort was greatest. The nest heating experiment reduced female and male provisioning rates but did not influence mass loss in either sex, also rejecting the energetic cost hypothesis. Counter to the time constraint hypothesis, more time spent brooding was associated with less mass loss. Path analysis supported the flight efficiency hypothesis for females, where mass loss was not related to energetic costs or time constraints and was positively associated with fledging more and heavier young, as well as a greater proportion of young. Mass loss also did not influence the chance of double‐brooding or survival to the next breeding season. Males lost mass steadily over the nesting cycle, but our results did not strongly support one hypothesis. Ultimately, results supported mass loss providing adaptive benefits for females, while rejecting reproductive cost hypotheses often expected under life‐history theory. Read the free Plain Language Summary for this article on the Journal blog.