Goshawk Predation Research Articles

Long-term consistency in susceptibility of prey species to predation by an avian predator

Selection by predators affects prey through competition for limiting resources. This not only has consequences for direct mortality but also indirectly affects disturbance. Changes in the intensity of selection on prey by predators may affect the size of prey populations, with consequences for their short- or long-term interactions. We assessed whether predation by northern goshawks Accipiter gentilis modified the composition of prey communities consistently along a temporal gradient, showing long-term consistency in susceptibility of prey species to predation. We followed six populations of the goshawk in two biomes in Denmark and Finland during 1949-2019. Susceptibility to goshawk predation in 2005-2017 in Denmark was only weakly related to susceptibility to goshawk predation in 1977-2004. In Finland, susceptibility of shared prey species to goshawk predation was positively related between periods. The average difference in susceptibility to goshawk predation between periods was considerably higher in Denmark than in Finland. Susceptibility of prey species to predation in goshawks increased with latitude and body mass of prey species, and decreased with period of time and population density of prey species. The changes in susceptibility to predation suggest changes in the characteristics of the local prey pools.

Spatial consistency in susceptibility of prey species to predation by two Accipiter hawks

Predators impose strong selection on their prey, regulate prey populations and engage in coevolutionary interactions with their prey. The intensity of selection and the strength of coevolutionary interactions will depend on how stringent predators are in their choice of prey. We estimated susceptibility of different species of birds to predation by two common raptors, the northern goshawk Accipiter gentilis and the Eurasian sparrowhawk A. nisus, in an agricultural landscape in Denmark and boreal forests in Finland. We estimated susceptibility to predation as the deviation of the log10‐transformed observed frequency of prey of different species from the log10‐transformed expectation based on population density during the breeding season. We found a high degree of consistency in susceptibility to predation by the goshawk in two areas in Finland. More importantly, there was significant consistency in susceptibility to predation between Denmark and Finland, albeit the degree of consistency in the goshawk was higher than in the sparrowhawk. There was considerable overlap in susceptibility to predation between goshawk and sparrowhawk in Denmark, but not in Finland, implying differences in intensity of interspecific competition as reflected by a much higher extent of goshawk predation on sparrowhawks in Denmark than in Finland. Our findings suggest that hawks impose similar selection pressures on their prey populations, and that the degree of consistency has implications for intensity of interspecific killing.

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.

Long-Term Study on Interactions between Tawny OwlsStrix aluco, JackdawsCorvus monedulaand Northern GoshawksAccipiter gentilis

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 letter'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.

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

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

Delayed numerical response of goshawks to population fluctuations of forest grouse

Delayed density‐dependent mortality induced by delayed numerical response of predators can drive prey populations to fluctuate in high‐amplitude cycles. We studied numerical response of goshawksAccipiter gentilisto varying densities of their main prey (forest grouse) in western Finland during 1979–1996. Occupancy rate of goshawk territories tracked grouse numbers with a two year lag. Occupancy rate of goshawk territories and pooled number of adult and young goshawks correlated negatively with a 1–2 year lag to the chick production of grouse. Goshawk to grouse ratio was negatively related to grouse density. This suggests that goshawk predation on grouse is inversely dependent on grouse density. We conclude that in northern Europe with few alternative preys, goshawk predation might contribute to the generation of multiannual cycles of forest grouse. This should be tested experimentally.

The decline of Common KestrelsFalco tinnunculusin a forested area of northern England: the role of predation by Northern GoshawksAccipiter gentilis

We have previously documented the decline of the Common KestrelFalco tinnunculusover a 23‐year period in a large coniferous forest in northern England. Kestrels fed predominantly on Field VolesMicrotus agrestis, which were most abundant in young plantations (1–11 years old). Over the 23 years, voles remained abundant in the study area, but their numbers fluctuated cyclically. Here we consider whether the decline of Kestrels was linked to predation by Northern GoshawkAccipiter gentilis. Goshawks first bred in the study area in 1973 and increased until 1989, after which numbers stabilized. We use a number of approaches to explore the role of Goshawk predation, all of which are correlative, but independent. First, there was a significant negative relationship between Kestrel and Goshawk numbers after controlling for a decline in vole habitat. Short‐eared OwlsAsio flammeus, which also hunt by day, declined over the same period as Kestrels. Second, numbers of Tawny OwlStrix alucoand Long‐eared OwlAsio otusdid not decline as Goshawk numbers increased. These two species are also vole‐dependent, but active by night, and less vulnerable to Goshawk attack. Third, six species of raptor comprised 4.5% of 5445 Goshawk prey items during the breeding season, but more Kestrels were killed than the combined total of all other raptors. Goshawks not only killed many adult Kestrels in early spring, prior to breeding, when it would have most impact on population levels, but there was also a temporal trend for predation on Kestrels to be inversely density‐dependent. Finally, we estimated that Goshawks removed more Kestrels than were recorded each spring in the study area. We interpreted this as indicating that immigrant Kestrels were being removed continually, mostly before they could breed. We conclude that the decline of Kestrels (and possibly Short‐eared Owls) was mainly due to predation by Goshawks. This study provides some of the strongest evidence yet of the role of predation in the hierarchical structuring of raptor communities.

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 predator's 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 goshawk's 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 goshawk's 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.

Goshawk predation during winter, spring and summer in a boreal forest area of central Sweden

Predation by goshawks was studied in a central Swedish boreal forest area. Data were collected in winter (January–February) 1977‐81 by tracking radio‐tagged goshawks, and in the breeding season (April–July) by collecting prey remains at the nest. In the breeding season birds dominated the prey, amounting to 86% of prey number and 91% of prey biomass. Wood pigeon Columba palumbus, black grouse Tetrao tetrix, hooded crow Corvus corone cornix and jay Garrulus glandarius accounted for more than 50% of the prey animals, whereas capercaillie Tetrao urogallus and black grouse accounted for more than 50% of prey biomass. There was no functional response to black grouse density fluctuations. Every year goshawks killed significantly more females than males of both capercaillie and black grouse, due to high vulnerability of the grouse hens while laying and incubating. It was estimated that during spring and early summer goshawk predation removed 25% of the female, and 14% of the male black grouse population. In winter squirrel was the dominating prey, both in terms of number (79%) and weight (56%). The proportion of squirrel in the diet was equally high both in winters of low and high squirrel density. The high proportion of squirrel in the winterdiet, as compared to the breeding season, is believed to be due to squirrels having to accept an increased predation risk in winter, in order to feed efficiently enough.

The Effect of Prey Vulnerability: Goshawk Predation and Population Fluctuations of Small Game

sult of the controlled matings. We were (and said so in our discussion) aware of the possibility that the first male may have had an advantage in siring more offspring than subsequent ones. We therefore decided to continue the study to get a reasonable sample of double mated females. We present here additional data which are given in an extended version of the original table (Tab. 1). The result further confirms that brood diversity is caused by multiple matings within seasons (Tab. 1). With respect to biased broods, the tendency for first male advantage is strenghtened. Note that first male advantage is not correlated with the number of heterozygous offspring since first male genotype was randomly chosen. However, female adders frequently mate with more than two males (mean number of matings/female = 3.8, n = 12, Madsen unpubl.) and an analysis based on a two-male system may be an oversimplification of the real situation. We see no reason to change our previous view that female adders produce multiple broods due to multiple matings within seasons, that first male advantage is not a necessary prerequisite for explaining male combat, and that sperm storage may occur.

Goshawk Predation on Tetraonids: Availability of Prey and Diet of the Predator in the Breeding Season

Goshawk Predation on Tetraonids: Availability of Prey and Diet of the Predator in the Breeding Season