Long-term study on interactions between Tawny Owls Strix aluco, Jackdaws Corvus monedula and Northern Goshawks Accipiter gentilis

Abstract

We monitored the dynamics of owls, raptors and corvids within the 3400 ha of the Amsterdam Water Supply Dunes in the western Netherlands from 1961 through 2007. In the 1930s and 1950s, afforestation projects were initiated. In 1961 the first Tawny Owl nest was found, from whence the population increased steadily to stabilize in the 1970s at c. 24 pairs. Tawny Owl dynamics were affected by competition for nest sites with Jackdaws Corvus monedula and depredation by Northern Goshawks Accipiter gentilis. Jackdaws were able to take over nest boxes from egg laying and incubating Tawny Owls, reducing the latter's breeding success. Tawny Owl numbers and breeding success were negatively correlated with Jackdaw numbers (R2 = 0.556 and R2 = 0.592, respectively). After the first nest in 1993, Goshawk numbers rapidly increased to 10-11 pairs in 1999. The number of Jackdaw breeding pairs correlated negatively with the number of Goshawks (R2 = 0.661). Goshawks apparently limited Jackdaw numbers, which led to an improvement of Tawny Owl breeding success. However, post-fledging mortality of Tawny Owl from Goshawk predation was also significant, reducing local recruitment in the Tawny Owl breeding population.