Phenotypic plasticity in mass loss during chick-rearing in the European starling (Sturnus vulgaris)

It has long been recognized that mass loss during breeding could be
adaptive (e.g., by ameliorating the costs of increased parental activity).
However, many studies still commonly interpret mass loss as evidence of
“stress” or a cost of reproduction (a negative effect of high workload
during chick provisioning). Despite several studies reporting evidence in
support of both hypotheses, the ecological and/or life-history contexts
under which mass loss may be viewed as a “cost” or an adaptive strategy
are still unclear. Here, we used a long-term dataset from a breeding
population of European starlings to investigate the natural annual and
individual variation in body mass and mass loss and to test whether mass
loss during chick-rearing represents a phenotypically plastic trait that
varies predictably in relation to ecological context and individual
quality. While there was significant annual variation in incubation mass,
chick-rearing mass, and mass change, there were no systematic
relationships between mass loss and (1) current breeding success or (2)
future fecundity and survival. In addition, we found moderate
repeatability of mass loss (R = 0.59) suggesting there might be additive
genetic variation for this trait, though with considerable residual
environmental variation. However, we found no covariation between this
residual, intra-individual variation and other reproductive or
life-history traits. We therefore found no support for the idea that mass
loss reflects “reproductive stress” in our system: there were no negative
relationships between mass loss and either current or future reproduction
and survival (local return rate). Our results are consistent with mass
loss being an individually plastic trait, potentially with moderate
additive genetic variance, with individuals using mass loss to “level the
playing field” and individually optimize reproductive effort and fitness.