Climatic forcing and individual heterogeneity in a resident mountain bird: legacy data reveal effects on reproductive strategies

Optimization of clutch size and timing of reproduction have substantial
effects on lifetime reproductive success in vertebrates, and both
individual quality and environmental variation may impact life history
strategies. We tested hypotheses related to maternal investment and timing
of reproduction, using 17 years (1978-1994) of individual-based life
history data on willow ptarmigan (Lagopus l. lagopus, n=290 breeding
females with n=319 breeding attempts) in central Norway. We analysed
whether climatic variation and individual state variables (age and body
mass) affected number of offspring and timing of reproduction, and
individual repeatability in strategies. The results suggest that willow
ptarmigan share a common optimal clutch size that is largely independent
of measured individual states. While we found no clear direct weather
effects on clutch size, higher spring temperatures advanced onset of
breeding, and early breeding was followed by an increased number of
offspring. Warmer springs were positively related to maternal mass, and
mass interacted with clutch size in production of hatchlings. Finally,
clutch size and timing of reproduction were highly repeatable within
individuals, indicating that individual quality guided trade-offs in
reproductive effort. Our results demonstrate how climatic forcing and
individual heterogeneity in combination influenced life history traits in
a resident montane keystone species.