Structural equation modeling reveals determinants of fitness in a cooperatively breeding bird

Even in well-studied organisms, it is often challenging to uncover the
social and environmental determinants of fitness. Typically, fitness is
determined by a variety of factors that act in concert, thus forming
complex networks of causal relationships. Moreover, even strong
correlations between social and environmental conditions and fitness
components may not be indicative of direct causal links, as the measured
variables may be driven by unmeasured (or unmeasurable) causal factors.
Standard statistical approaches, like multiple regression analyses, are
not suited for disentangling such complex causal relationships. Here, we
apply structural equation modeling (SEM), a technique that is specifically
designed to reveal causal relationships between variables, and which also
allows to include hypothetical causal factors. Therefore, SEM seems
ideally suited for comparing alternative hypotheses on how fitness
differences arise from differences in social and environmental factors. We
apply SEM to a rich data set collected in a long-term study on the
Seychelles warbler (Acrocephalus seychellensis), a bird species with
facultatively cooperative breeding and a high rate of extra-group
paternity. Our analysis reveals that the presence of helpers has a
positive effect on the reproductive output of both female and male
breeders. In contrast, per capita food availability does not affect
reproductive output. Our analysis does not confirm earlier suggestions on
other species that the presence of helpers has a negative effect on the
reproductive output of male breeders. As such, both female and male
breeders should tolerate helpers in their territories, irrespective of
food availability.