DOI: 10.5061/dryad.931zcrjpb
title: Data from: Annual variation in breeding success in boreal forest grouse

description: 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.
description: Demographic data were obtained by counting birds in twenty-two c.
 2 km<sup>2</sup> blocks during August using trained pointing
 dogs.  We tested predictions from each hypothesis
 separately by means of linear multiple regressions for breeding success or
 its components (brood frequency and brood size) as response variable and
 measures of each hypothesised cause as explanatory variables. The two
 grouse species were treated as subjects in a repeated measures analysis of
 deviance via SAS (ver. 9.1) Proc Mixed, specifying “species nested in
 year” in the REPEATED statement and “variance components” as the
 covariance structure. 
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