Data from: Annual variation in breeding success in boreal forest grouse

Knowledge of the temporal variation in reproductive success and its key
driving factors is crucial in predicting animal population persistence.
Few studies have examined the effects of a range of explanatory factors
operating simultaneously on the same population over a long period. Based
on 41 years of monitoring (1979–2019), we tested prevailing hypotheses
about drivers of annual variation in breeding success in two sympatric
species of boreal forest grouse – the capercaillie (Tetrao urogallus) and
the black grouse (T. tetrix) – in a 40 km2 boreal forest landscape. From
counts in early August, we measured breeding success (chicks/hen) along
with potential determining factors. We formulated five main hypotheses on
causes of variation (hen condition, chick weather, chick food, predation,
demographic characteristics), and derived 13 associated explanatory
variables for analysis. We first tested the five hypotheses separately and
then used model selection (AICc) to rank the best predictive models
irrespective of hypotheses. Lastly, we used path analysis to illuminate
potential causal relationships.    Barring demographic
characteristics, all hypotheses were supported, most strongly for chick
food and predation. Among predictor variables, chick food (insect larvae
and bilberry fruit crops), vole and fox abundances, the winter-NAO index,
and temperature after hatching, had the strongest effect sizes in both
species. Precipitation after hatching had no detectable effect. Model
selection indicated bottom-up factors to be more important than predation,
but confounding complicated interpretation. Path analysis suggested that
the high explanatory power of bilberry fruiting was due not only to its
direct positive effect on chick food quality but also to an indirect
positive effect on vole abundance, which buffers predation. The two
components of breeding success – proportion of hens with broods and number
of chicks per brood – were uncorrelated, the former having the strongest
effect. The two components had different ecological correlates that often
varied asynchronously, resulting in overall breeding success fluctuating
around low to moderate levels. Our study highlights the complexity of key
explanatory drivers and the importance of considering multiple hypotheses
of breeding success. Although chick food appeared to equal or surpass
predation in explaining the annual variation in breeding success,
predation may still be the overall limiting factor. Comparative and
experimental studies of confounded variables (bilberry fruiting, voles and
larvae) are needed to disentangle causes of variation in breeding success
of boreal forest grouse.