Interspecific offspring killing in owls

In Baranya County (Southern Hungary), tawny owls (Strix aluco) and barn owls (Tyto alba) sequentially use the same nest boxes in a significant number of cases. A total of 460 broods were observed between 1996 and 2003 and, in 12 cases, whole broods of dead tawny owl chicks were registered that had apparently been killed. On investigating the reproductive life characteristics, population sizes, and frequency of killing in these two species, it was concluded that: (1) with growing barn owl population, the number of sequential broods increases but changes in tawny owl population size have no effect on the frequency of sequential broods; (2) the number of killings depends on the number of sequential broods; and (3) with growing barn owl population, the number of killings also grows and this change is unaffected by the size of the tawny owl population. However, no killing occurs as long as 50–60% of the nest boxes are unoccupied. There is no killing either until the percentage of nest boxes occupied by barn owls reaches 40%, although a threshold value like this cannot be shown for the tawny owl. In the cases when a barn owl breeding follows the tawny owl's, the percentage of killing is significantly higher compared to that when barn owls do not breed in the same box. These results indicate that barn owls kill the offspring of tawny owls. By these means, they obtain a breeding place earlier than without killing the chicks of the other species, and this results in higher reproductive success. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 488–494.