Short communication: Landscape and season effects on the diet of the Goshawk

Abstract

There are two general effects of habitat loss and frag-mentation of mature boreal forests (Schmiegelow &Monkkonen 2002). First, fragmentation by farmlandcreates stable structures such as permanent edge zoneswith enrichment of species diversity and density(Andren 1992, Berg & Part 1994). Secondly, modernforestry with clear-cuts creates sharp, unstable bound-aries between forest and open areas, usually with lesspronounced edge effects (Helle 1983, Schmiegelow &Monkkonen 2002). Considering the vast array of stud-ies on the effects of habitat loss and fragmentation onbird populations, relatively little attention has beenpaid to the role of predators, other than nest preda-tors, across different landscapes (Lampila et al. 2005).Predators’ searching efficiency may improve due to adiminished area where prey live (Storaas et al. 1999).By killing smaller predators and nest predators, toppredators may contribute positively to prey speciespopulations (Petty et al. 2003, Monkkonen et al.2007). Increased availability of alternative prey as aresult of landscape change may deflect predation fromthe main prey species (Angelstam et al. 1984). Thefinal outcome of these landscape-related predator–preyinteractions is likely to depend on direct functionaland numerical responses of predators to the variationin the abundance and vulnerability of the main andalternative prey, as well as on the indirect controllingeffect of top predators on smaller predators and nestpredators.In northern latitudes the Northern Goshawk Accipi-ter gentilis relies mainly on grouse as a staple food dur-ing most of the year (Tornberg 1997, 2001, Tornberg& Colpaert 2001). Breeding season diet, however, con-tains a large spectrum of alternative prey species,mainly birds (Tornberg 1997). The proportion ofgrouse in the diet is at the lowest during late nestlingphase when fledglings of alternative prey such as largerpasserines and waterfowl are readily available (Linden& Wikman 1983, Tornberg 1997). Goshawks mainlyuse mature forests for nesting (Penteriani 2002), butthey are more flexible in their choice of hunting habi-tats (Kenward & Widen 1989, Tornberg & Colpaert2001). Even though the diet and habitat associationsof the Goshawk are relatively well known, we do nothave a clear picture of how these vary with landscapestructure.In this study, we examined Goshawk predation ongrouse (Willow Grouse Lagopus lagopus, Black GrouseTetrao tetrix, Capercaillie Tetrao urogallus, Hazel GrouseBonasa bonasia) along a landscape gradient. We alsoexamined whether predation on alternative prey wasdependent on the same landscape gradient.METHODSStudy areaThe study area comprised roughly 1700 km