Does the Year-to-Year Variation in the Diet of Eagle and Ural Owls Support the Alternative Prey Hypothesis?

Abstract

The alternative prey hypothesis (APH) states that synchronous population fluctuations of small game, such as hare and grouse, are caused by varying predation impact. Voles that show 3-4-yr population cycles in central and northern Fennoscandia, are the staple food of several predators. APH predicts that these predators partly switch their diet from voles to small game as the staple prey decrease. We collected food samples of breeding eagle and Ural owls during 1973-87 in South Ostrobothnia (SO) (63?N, 23?E) and during 1965-80 in Central Ostrobothnia (CO) (64?N, 24.5?E), western Finland. We tested the following four predictions of APH: (1) The yearly abundances of voles in the field should correlate positively with the proportions of voles in the diet. The data from eagle and Ural owls in SO were consistent with this. (2) The proportion of small game in the diet should be negatively related to the abundance of voles in the field. This was true for the Ural owl in SO. (3) The owls should take more small game in poor vole years than in good ones, independently of the proportion of voles in the diet. This was the case for the two owls in CO. (4) The proportion of small game in the diet is nearly independent of its abundance in the field. This held true for the eagle owl in CO and the Ural owl in the two areas. Thus, most of the data supported the four predictions of APH suggesting that eagle and Ural owls behave as APH predicts. However, additional data on their predation impact on small game are needed to better assess how much these two owls are responsible for crashes of hare and grouse populations.