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 predicti..., Demographic data were obtained by counting birds in twenty-two c. 2 km2 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. , Office Excel