Microtine Density and Weather as Predictors of Chick Production in Willow Ptarmigan, Lagopus l. lagopus

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Abstract
We have evaluated to what extent biotic (predation) and abiotic (weather) factors influence annual chick prodution in willow ptarmigan (Lagopus lagopus lagopus). An index of predation was derived from data on microtine density; weather conditions from daily records of temperature and precipitation. Stepwise regression analysis between observed chick production on two Scandinavian study sites (Tranoy and Lovhogen) for periods of 23 and 19 yr, and independent variables characterizing predation and weather conditions suggested which variables were the most important in determining chick production. The resulting model explains between 56% and 25% of the annual variation in chick production; the calculated values deviating on average between 1.14 and 0.95 chicks/2 adults from the observed ones. Predation was found to be the dominant factor, accounting for 41% and 25% of the annual variation in chick production in the two areas. We suggest that year-to-year variation in reproductive success is, in general, influenced to a greater degree by biotic than by abiotic factors, possibly because the predators "co-adapt" to the ptarmigan.