R code from: Hierarchical fear: parental behaviour and corticosterone release mediate nestling growth in response to predation risk
Nestling development, a critical life-stage for
altricial songbirds, is highly vulnerable to predation, particularly for
open-cup nesting species. Since nest predation risk increases cumulatively with
time, rapid growth may be an adaptive response that promotes early fledging. However, greater predation risk can reduce parental
provisioning rate as a risk aversion strategy and subsequently constrain
nestling growth, or directly elicit a physiological response in nestlings with adaptive
or detrimental effects on development rate. Despite extensive theory, evidence
for the relative strength of these effects on nestling development in response
to prevailing predation risk and the underlying mechanisms remain unclear. For
an alpine population of horned lark (Eremophila alpestris), we elevated
perceived predation risk (decoys/playback) during the nestling stage to assess the
influence of predator cues and parental care on nestling wing growth and the
glucocorticoid hormone corticosterone. We used piecewise path analysis to test
a hypothesized causal response structure composed of direct and indirect
pathways. Nestlings under greater perceived predation risk reduced
corticosterone and increased wing growth, resulting in an earlier age at
fledge. This represented both a direct response that was predator-specific, and
an indirect response dependent on parental provisioning rate. Parents that
reduced provisioning rate most severely in response to predator cues had
smaller nestlings with greater corticosterone. Model comparisons indicated the strongest
support for a directed, causal influence of corticosterone on nestling wing
growth, highlighting corticosterone as a potential physiological mediator of
the nestling growth response to predation risk. Finally, cold temperatures prior
to the experiment constrained wing growth closer to fledge, illustrating the importance
of considering the combined influence of weather and predation risk across developmental
stages. We present the first study to separate the direct and indirect effects
of predation risk on nestling development in a causal, hierarchical framework
that incorporates corticosterone as an underlying mechanism and provides experimental
evidence for an adaptive developmental response to predation risk in
ground-nesting songbirds.