Risto Tornberg

Changes in diet and morphology of Finnish goshawks from 1960s to 1990s

Abstract We studied the morphology of the goshawk in northern Finland by measuring skin and skeletal characters of 258 museum specimens dated between 1961 and 1997. We predicted a decrease in the size of male goshawks from the 1960s because availability of their main prey, grouse, has decreased since then and grouse have been replaced in the diet by smaller prey during the breeding season. Based on the assumption that winter is the most critical period for females, we predicted that female size should have increased because their winter diet consisted of more and more mountain hare, which is a prey generally larger than grouse. Analyses revealed that male size has indeed decreased since the 1960s, while adult females have increased in size. Our data suggest that these morphological shifts were the result of selective pressures due to changes in diet. We also found changes in the (size-independent) shape of the hawks. Relative wing and tail lengths of adult hawks became longer between 1980 and 1990 compared with the 1960–1970 period, while relative juvenile wing and tail lengths tended to decrease. As a result of these morphological changes size dimorphism between the sexes increased from the 1960s to the 1990s.

Pattern of goshawk Accipiter gentilis predation on four forest grouse species in northern Finland

I studied predator-prey relationships between goshawk Accipiter gentilis and four species of forest grouse (Tetraonidae) in northern Finland during 1988–1998. The main purpose of my study was to evaluate the impact of goshawk predation and its possible effect on multiannual cycling patterns in grouse numbers. Theoretically specialist predators should tend to cause stable-limit cycles in prey populations if there is a time-lag in the predators response to prey density and the prey species should be most affected at low densities. Four grouse species, willow grouse Lagopus lagopus, black grouse Tetrao tetrix, capercaillie Tetrao urogallus and hazel grouse Bonasa bonasia, form the main food of the goshawk in boreal forests in northern Finland. Grouse constituted >40% of the goshawks diet during the breeding season. The impact of predation by breeding goshawks on grouse varied depending on grouse species within 7–32% during the breeding season. Losses were highest for willow grouse and lowest for capercaillie. On average, goshawks took 6% of grouse chicks. On an annual basis breeding goshawks took 2–31% of the August grouse population. The goshawks share of the total mortality in grouse was also species related. The most reliable estimates were found for black grouse of which 35% were removed and for hazel grouse of which 40% were removed. Goshawks are relatively specialised on forest grouse in northern boreal forests as was demonstrated by a weak functional response of the hawks to changes in grouse density. Breeding goshawks showed no numerical response to changes in grouse density but the production of young tended to lag one year behind black grouse density. The predation rate of goshawks was inversely density dependent on changes in grouse density, which may have had a destabilising effect on the grouse populations. A positive relationship existed between summer predation on willow grouse and changes in the population the previous year.

Long-term change in territory occupancy pattern of goshawks (Accipiter gentilis)

Abstract: We examined territory occupancy (n = 161; 720 breeding attempts) of northern goshawks (Accipiter gentilis) in western Finland in 1983–1996. Nest sites of goshawks were characterized by old forests. The proportion of longstanding goshawk territories declined steeply from the beginning of the 1990s, while occasionally occupied goshawk territories became common. We suggest that the longstanding occupancy of the same territory, typical for goshawks, has decreased at least partly because of the logging of mature forests, which are preferred nesting habitats of goshawks. The mean occupancy rate of all goshawk territories, however, did not change through the study period, which suggests that the size of the breeding population has been constant.

What Explains Forest Grouse Mortality: Predation Impacts of Raptors, Vole Abundance, or Weather Conditions?

We investigated predation rates of black grouse chicks during 1985–2007 in two localities in western Finland in light of three predation hypothesis: The Alternative Prey Hypothesis (APH) stating that vole-eating generalist predators cause a collapse in grouse reproduction after voles’ decline, the Main Prey Hypothesis (MPH), where grouse specialised predators by a lagged response cause an inversely density dependent predation for prey and the Predation Facilitation Hypothesis (PFH), where generalist and specialist predators act in concert. We also studied the effect of weather on grouse reproduction. We found that buzzard predation alone did not support APH, but did so when combined with goshawk predation. Kill rate by goshawks showed a linear response for black grouse chicks but was not density dependent. It, however, explained the losses of chicks but not their autumn density. Combined density of chicks with adults correlated with vole index in the latter study period (since 1994), thus, giving some support for APH. Weather seemed to have no effect on black grouse reproduction. Although buzzards and goshawks took, on average, only 10% of hatched grouse chicks we conclude that the among-year survival pattern of juvenile forest grouse may largely be determined by raptor predation.

Degradation in landscape matrix has diverse impacts on diversity in protected areas

Introduction A main goal of protected areas is to maintain species diversity and the integrity of biological assemblages. Intensifying land use in the matrix surrounding protected areas creates a challenge for biodiversity conservation. Earlier studies have mainly focused on taxonomic diversity within protected areas. However, functional and especially phylogenetic diversities are less studied phenomena, especially with respect to the impacts of the matrix that surrounds protected areas. Phylogenetic diversity refers to the range of evolutionary lineages, the maintenance of which ensures that future evolutionary potential is safeguarded. Functional diversity refers to the range of ecological roles that members of a community perform. For ecosystem functioning and long-term resilience, they are at least as important as taxonomic diversity. Aim We studied how the characteristics of protected areas and land use intensity in the surrounding matrix affect the diversity of bird communities in protected boreal forests. We used line-transect count and land-cover data from 91 forest reserves in Northern Finland, and land-cover data from buffer zones surrounding these reserves. We studied if habitat diversity and productivity inside protected areas, and intensity of forest management in the matrix have consistent effects on taxonomic, functional and phylogenetic diversities, and community specialization. Results We found that habitat diversity and productivity inside protected areas have strong effects on all diversity metrics, but matrix effects were inconsistent. The proportion of old forest in the matrix, reflecting low intensity forest management, had positive effects on community specialization. Interestingly, functional diversity increased with increasing logging intensity in the matrix. Conclusions Our results indicate that boreal forest reserves are not able to maintain their species composition and abundances if embedded in a severely degraded matrix. Our study also highlights the importance of focusing on different aspects of biodiversity.

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

There are two general effects of habitat loss and fragmentation of mature boreal forests (Schmiegelow & Monkkonen 2002). First, fragmentation by farmland creates stable structures such as permanent edge zones with enrichment of species diversity and density (Andren 1992, Berg & Part 1994). Secondly, modern forestry with clear-cuts creates sharp, unstable boundaries between forest and open areas, usually with less pronounced edge effects (Helle 1983, Schmiegelow & Monkkonen 2002). Considering the vast array of studies on the effects of habitat loss and fragmentation on bird populations, relatively little attention has been paid to the role of predators, other than nest predators, across different landscapes (Lampila et al. 2005). Predators’ searching efficiency may improve due to a diminished area where prey live (Storaas et al. 1999). By killing smaller predators and nest predators, top predators may contribute positively to prey species populations (Petty et al. 2003, Monkkonen et al. 2007). Increased availability of alternative prey as a result of landscape change may deflect predation from the main prey species (Angelstam et al. 1984). The final outcome of these landscape-related predator–prey interactions is likely to depend on direct functional and numerical responses of predators to the variation in the abundance and vulnerability of the main and alternative prey, as well as on the indirect controlling effect of top predators on smaller predators and nest predators. In northern latitudes the Northern Goshawk Accipiter gentilis relies mainly on grouse as a staple food during most of the year (Tornberg 1997, 2001, Tornberg & Colpaert 2001). Breeding season diet, however, contains a large spectrum of alternative prey species, mainly birds (Tornberg 1997). The proportion of grouse in the diet is at the lowest during late nestling phase when fledglings of alternative prey such as larger passerines and waterfowl are readily available (Linden & Wikman 1983, Tornberg 1997). Goshawks mainly use mature forests for nesting (Penteriani 2002), but they are more flexible in their choice of hunting habitats (Kenward & Widen 1989, Tornberg & Colpaert 2001). Even though the diet and habitat associations of the Goshawk are relatively well known, we do not have a clear picture of how these vary with landscape structure. In this study, we examined Goshawk predation on grouse (Willow Grouse Lagopus lagopus, Black Grouse Tetrao tetrix, Capercaillie Tetrao urogallus, Hazel Grouse Bonasa bonasia) along a landscape gradient. We also examined whether predation on alternative prey was dependent on the same landscape gradient.

Landscape and season effects on the diet of the Goshawk.

There are two general effects of habitat loss and fragmentation of mature boreal forests (Schmiegelow & Monkkonen 2002). First, fragmentation by farmland creates stable structures such as permanent edge zones with enrichment of species diversity and density (Andren 1992, Berg & Part 1994). Secondly, modern forestry with clear-cuts creates sharp, unstable boundaries between forest and open areas, usually with less pronounced edge effects (Helle 1983, Schmiegelow & Monkkonen 2002). Considering the vast array of studies on the effects of habitat loss and fragmentation on bird populations, relatively little attention has been paid to the role of predators, other than nest predators, across different landscapes (Lampila et al. 2005). Predators’ searching efficiency may improve due to a diminished area where prey live (Storaas et al. 1999). By killing smaller predators and nest predators, top predators may contribute positively to prey species populations (Petty et al. 2003, Monkkonen et al. 2007). Increased availability of alternative prey as a result of landscape change may deflect predation from the main prey species (Angelstam et al. 1984). The final outcome of these landscape-related predator–prey interactions is likely to depend on direct functional and numerical responses of predators to the variation in the abundance and vulnerability of the main and alternative prey, as well as on the indirect controlling effect of top predators on smaller predators and nest predators. In northern latitudes the Northern Goshawk Accipiter gentilis relies mainly on grouse as a staple food during most of the year (Tornberg 1997, 2001, Tornberg & Colpaert 2001). Breeding season diet, however, contains a large spectrum of alternative prey species, mainly birds (Tornberg 1997). The proportion of grouse in the diet is at the lowest during late nestling phase when fledglings of alternative prey such as larger passerines and waterfowl are readily available (Linden & Wikman 1983, Tornberg 1997). Goshawks mainly use mature forests for nesting (Penteriani 2002), but they are more flexible in their choice of hunting habitats (Kenward & Widen 1989, Tornberg & Colpaert 2001). Even though the diet and habitat associations of the Goshawk are relatively well known, we do not have a clear picture of how these vary with landscape structure. In this study, we examined Goshawk predation on grouse (Willow Grouse Lagopus lagopus, Black Grouse Tetrao tetrix, Capercaillie Tetrao urogallus, Hazel Grouse Bonasa bonasia) along a landscape gradient. We also examined whether predation on alternative prey was dependent on the same landscape gradient.

Diet shift induced rapid evolution of size and function in a predatory bird

A predator’s body size correlates with its prey size. Change in the diet may call for changes in the hunting mode and traits determining hunting success. We explored long-term trends in sternum size and shape in the northern goshawk by applying geometric morphometrics. Tetraonids, the primary prey of the goshawk, have decreased and been replaced by smaller birds in the diet. We expected that the size of the goshawk has decreased accordingly more in males than females based on earlier observations of outer morphology. We also expected changes in sternum shape as a function of changes in hunting mode. Size of both sexes has decreased during the preceding decades (1962−2008), seemingly reflecting a shift in prey size and hunting mode. Female goshawks hunting also mammalian prey tend to have a pronouncedly “Buteo-type” sternum compared to males preying upon birds. Interestingly, the shrinkage of body size resulted in an increasingly “Buteo-type” sternum in both sexes. In addition, the sternum shape in birds that died accidentally (i.e., fit individuals) was more Buteo-type than in starved ones, hinting that selection was towards a Buteo-type sternum shape. We conclude that these observed patterns are likely due to directional selection driven by changes in the diet towards smaller and more agile prey. On the other hand, global warming is predicted to also cause a decrease in size, thus these two scenarios are inseparable. Because of difficulties in studying fitness-related phenotypic changes of large raptors in the field, time series of museum exemplars collected over a wide geographical area may give answers to this conundrum.

Apparent survival, territory turnover and site fidelity rates in Northern Goshawk Accipiter gentilis populations close to the northern range limit

ABSTRACT Capsule: Mark–recapture data suggest low apparent survival and sex- and population-specific site fidelity and territory turnover in adult Northern Goshawks Accipiter gentilis breeding in northern Europe. Aims: To understand how species cope with global environmental change requires knowledge of variation in population demographic rates, especially from populations close to the species’ northern range limit and from keystone species such as raptors. We analyse apparent survival and breeding dispersal propensity of adult Northern Goshawks breeding in northern Europe. Methods: We used long-term mark–recapture data from two populations in Finland, northern Europe, and Cormack–Jolly–Seber models and binomial generalized linear models to investigate sex- and population-specific variation in apparent survival, territory turnover and site fidelity. Results: We report low apparent survival (53–72%) of breeding adult Goshawks. Breeding dispersal propensity was higher in females than males, especially in northern Finland, contrasting with previous studies that suggest high site fidelity in both sexes. Conclusion: Low apparent survival in females may be mainly due to permanent emigration outside the study areas, whereas in males the survival rate may truly be low. Both demographic aspects may be driven by the combination of sex-specific roles related to breeding and difficult environmental conditions prevailing in northern latitudes during the non-breeding season.

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

There are two general effects of habitat loss and fragmentation of mature boreal forests (Schmiegelow & Monkkonen 2002). First, fragmentation by farmland creates stable structures such as permanent edge zones with enrichment of species diversity and density (Andren 1992, Berg & Part 1994). Secondly, modern forestry with clear-cuts creates sharp, unstable boundaries between forest and open areas, usually with less pronounced edge effects (Helle 1983, Schmiegelow & Monkkonen 2002). Considering the vast array of studies on the effects of habitat loss and fragmentation on bird populations, relatively little attention has been paid to the role of predators, other than nest predators, across different landscapes (Lampila et al. 2005). Predators’ searching efficiency may improve due to a diminished area where prey live (Storaas et al. 1999). By killing smaller predators and nest predators, top predators may contribute positively to prey species populations (Petty et al. 2003, Monkkonen et al. 2007). Increased availability of alternative prey as a result of landscape change may deflect predation from the main prey species (Angelstam et al. 1984). The final outcome of these landscape-related predator–prey interactions is likely to depend on direct functional and numerical responses of predators to the variation in the abundance and vulnerability of the main and alternative prey, as well as on the indirect controlling effect of top predators on smaller predators and nest predators. In northern latitudes the Northern Goshawk Accipiter gentilis relies mainly on grouse as a staple food during most of the year (Tornberg 1997, 2001, Tornberg & Colpaert 2001). Breeding season diet, however, contains a large spectrum of alternative prey species, mainly birds (Tornberg 1997). The proportion of grouse in the diet is at the lowest during late nestling phase when fledglings of alternative prey such as larger passerines and waterfowl are readily available (Linden & Wikman 1983, Tornberg 1997). Goshawks mainly use mature forests for nesting (Penteriani 2002), but they are more flexible in their choice of hunting habitats (Kenward & Widen 1989, Tornberg & Colpaert 2001). Even though the diet and habitat associations of the Goshawk are relatively well known, we do not have a clear picture of how these vary with landscape structure. In this study, we examined Goshawk predation on grouse (Willow Grouse Lagopus lagopus, Black Grouse Tetrao tetrix, Capercaillie Tetrao urogallus, Hazel Grouse Bonasa bonasia) along a landscape gradient. We also examined whether predation on alternative prey was dependent on the same landscape gradient.