Extra‐pair copulation and intraspecific nest intrusions in the Northern Goshawk Accipiter gentilis

Abstract

Detailed descriptions of the behaviour of individual birds engaged in extra-pair copulations (EPCs) are comparatively rare. This might be due to the fact that EPCs occur infrequently and/or that EPC-seeking males and females usually behave in a secretive manner (Birkhead & Moller 1992a). Even in birds of prey, often observed intensively in the field, detailed reports of EPCs are scarce (e.g. Birkhead & Lessells 1988, McGrady 1991, Sodhi 1991, Negro et al. 1992, Arroyo 1999). Observational data are important, however, as they can shed light on aspects of sperm competition (e.g. age and status of intruders, timing of EPC events, behaviour of the sexes), which may pass unnoticed in quantitative DNA-profiling studies. The Northern Goshawk Accipiter gentilis is a socially monogamous, territorial raptor (Squires & Reynolds 1997, Rutz et al . in press) with a distinct division of sex roles during the breeding season. Males provision their mates and, later, their offspring with prey, whereas females incubate the eggs, care for the young and defend the nesting territory (Squires & Reynolds 1997). During the males’ foraging trips, their mates remain unattended at the nest-sites, where they might either seek EPCs or become victim to forced copulation attempts by intruders. The latter case, however, is unlikely in the Goshawk, as females are about a third heavier than males (Squires & Reynolds 1997) and are therefore capable of fighting off unwanted copulation partners. Male birds may reduce the risk of cuckoldry by two main strategies: (i) they can guard their mates by close following; or (ii) copulate with them at a high rate during the fertile period (Birkhead & Moller 1992a). Male Goshawks cannot follow their partners closely while hunting. They can, however, attempt to maximize the time spent near the nest, and hence their mates, when the probability of fertilization is high (i.e. close to and during egg-laying). This strategy is known as ‘mate attendance’ (Birkhead & Moller 1992a). Both mate attendance and frequent copulations have been recorded in the Goshawk (Holstein 1942, Moller 1987). So far, only genetic evidence has existed for the occurrence of EPCs in the Goshawk (Gavin et al . 1998). In this paper the first observation of an EPC in this species is described, and nine cases of intraspecific nest intrusions are reported. Radiotracking data on the spatio-temporal organization and the behaviour of males of three breeding pairs were used to address predictions from the mateattendance and frequent-mating hypotheses (Birkhead & Moller 1992a).