Data from: 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

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 five 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.