Breeding success of the Gyrfalcon (Falco rusticolus) in northern Norway in relation to food supply and weather
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- Institutt for biologi 
Food supply and weather are among the main factors limiting bird numbers. In raptors prey availability may be closely linked to the reproduction output while weather conditions most often act indirectly by modifying food accessibility and habitats or influencing birds hunting efficiency. The Gyrfalcon (Falco rusticolus) breeding in northern ecosystems is regarded as a rare case of narrow food specialization depending strongly on ptarmigans. This study is based on a data series from 18 years investigated relationships between Gyrfalcon s breeding performance in northern Norway and potential prey availability (ptarmigans and small rodents) as well as weather conditions (mean temperature, precipitation and snow depth in January March and April May periods). Statistical modeling and correlation coefficients were used to assess the relationships. Although based on relatively small yearly sample sizes, the results showed that the Gyrfalcon was influenced by both potential prey numbers and weather. Ptarmigan densities turned out to be the main factor affecting and explaining variability in the number of breeding attempts and in some degree also the number of young fledged. Microtine rodent densities showed significant positive correlation with the number of Gyrfalcon young fledged. However it might have been due to indirect community interactions. Of all weather parameters only the snow depth in January March correlated significantly positively with one of the three breeding success measures (number of young fledged). It was rather difficult to find a reasonable ecological mechanism that could explain such a correlation. Mean temperature and snow depth in January - March contributed in some models to explain the breeding success. However, they were of less importance. The main findings of this study supported the hypothesis that ptarmigan density is the key factor for the Gyrfalcon during the breeding season.