COMPETITIVE AND PREDATORY INTERACTIONS AMONG RAPTORS - AN OBSERVATIONAL AND EXPERIMENTAL-STUDY

We examined the effects of predatory and competitive interactions amon g three owl species on reproductive success and population composition of these raptors both experimentally and observationally. Eagle Owls (Bubo bubo, body mass approximate to 2700 g), Ural Owls (Strix uralens is, 900 g), and Tengmalm's Owls (Aegolius funereus, 130 g) coexist in Fennoscandia and feed mainly on small rodents. Predation may play an i mportant role in interactions among these owl species, because Eagle O wls and Ural Owls can kill Tengmalm's Owls but cannot enter the small entrance hale of Tengmalm's Owl nest boxes. We asked (i) whether preda tion risk and interspecific competition due to Eagle Owls and Ural Owl s reduced breeding density and fitness of Tengmalm's Owls, and (ii) wh ether these interactions increased intraspecific competition for safe nesting sites among Tengmalm's Owls. We manipulated breeding densities of potentially competing owls by erecting nest boxes, the control bei ng boxes in areas where breeding attempts of competing owl species wer e absent. Control nest boxes in areas with no Eagle Owl and Ural Owl t erritories, and nest boxes within Eagle Owl territories, were used by breeding Tengmalm's Owls more than nest boxes within Ural Owl territor ies. Most breeding attempts of Tengmalm's Owls near Ural Owls failed d uring the courtship period. The observational data revealed that breed ing frequency of Tengmalm's Owls was reduced and the mean start of egg laying was delayed by 11 d within 2 km of Ural Owl nests. In addition , male Tengmalm's Owls at these nests were younger and paired more oft en with short-winged (i.e., young and generally subdominant) females t han when farther away from Ural Owl nests. Our results suggest that in experienced male Tengmalm's Owls are forced to establish their territo ries in the vicinity of Ural Owls where they often paired with subdomi nant females. The areas near Ural Owl nests are suboptimal habitats fd r Tengmalm's Owls, whereas those near Eagle Owls are not. We conclude that predatory and competitive interactions from Ural Owls decrease th e breeding population size of Tengmalm's Owls by reducing the suitable habitats. This is the first experimental demonstration that such inte ractions may decrease fitness of raptors.