Site-specific variation in partial brood loss in northern goshawks

While many studies have shown that patterns of partial brood loss often differ with offspring sex, food availability or between years, the question of whether family-size adjustment varies between sites has achieved little attention. I investigated this in Finnish northern goshawks Accipiter gentilis, and found that partial brood loss varied considerably between locations. This variation was present both between territories and between two larger regions with different densities of goshawks’ main prey, grouse. Partial brood loss was inversely related to natural main-prey densities, but the relationship was far from straightforward: spatial nonlinearities arising from between-territorial differences influenced the results. Brood-loss patterns also differed between years and between initial clutch-size categories. Potential sex-biased post-hatching mortality was not related to grouse density and partial brood loss did not alter the original population-wide egg sex ratio. Between-territory variance in offspring sex ratios was low. The lack of the spatial dimension in earlier studies might have undermined a correct interpretation of the causes and consequences of brood-loss patterns, irrespective of whether sex ratios were biased.

Byholm, P. 2005, Ann. Zool. Fennici 42: 81-90