Goshawk Accipiter Gentilis Research Articles

Impact of climate change and prey abundance on nesting success of a top predator, the goshawk

Contemporary research has documented a large number of shifts in spring phenology and changes in distribution range although the average spring temperatures have increased by only 0.3-0.6 °C over the past 100 years. Generally, earlier breeding birds have larger clutch sizes, and the advancing spring could thus potentially increase breeding success. Shifts in spring phenology can, however, be crucial for bird reproduction, and mistiming the breeding event may even have negative consequences for population development. Our aim was to explore how weather and prey abundance relates to the breeding performance of a north European top predator, the northern goshawk Accipiter gentilis. Our nationwide dataset from Finland, spanning the period 1989-2004, shows that ambient weather has a greater impact on the timing and success of breeding than the density of grouse Tetraonidae, the main prey of goshawks. Higher early spring temperatures were associated with advancing hatching date of goshawks. Correspondingly, grouse density and temperature during laying and brooding were positively associated with brood size, while precipitation showed a negative connection. Applying our models to a future scenario of climate warming, combined with a 50 % reduction in grouse density, suggests that average breeding dates will advance only 2.5 days and average breeding success would remain the same. Notably, breeding success was not spatially equal throughout Finland, as northern and eastern populations suffered most from declining grouse densities. The observed pattern is thus the opposite to what is expected from a population situated at the northern edge of its distribution range, and thus may help to understand why populations may not increase at the northern edge of their thermal distribution due to climate change.

Predator Fitness Increases with Selectivity for Odd Prey

The fundamental currency of normative models of animal decision making is Darwinian fitness. In foraging ecology, empirical studies typically assess foraging strategies by recording energy intake rates rather than realized reproductive performance. This study provides a rare empirical link, in a vertebrate predator-prey system, between a predator's foraging behavior and direct measures of its reproductive fitness. Goshawks Accipiter gentilis selectively kill rare color variants of their principal prey, the feral pigeon Columba livia, presumably because targeting odd-looking birds in large uniform flocks helps them overcome confusion effects and enhances attack success. Reproductive performance of individual hawks increases significantly with their selectivity for odd-colored pigeons, even after controlling for confounding age effects. Older hawks exhibit more pronounced dietary preferences, suggesting that hunting performance improves with experience. Intriguingly, although negative frequency-dependent predation by hawks exerts strong selection against rare pigeon phenotypes, pigeon color polymorphism is maintained through negative assortative mating.

Nest reuse by Goshawks after timber harvesting: Importance of distance to logging, remaining mature forest area and tree species composition

We studied reuse of Goshawk Accipiter gentilis nests after timber harvesting within 50m from the nest in 31 territories in southeastern Norway. The proportion of nests used in the second or third breeding season after logging (55%) did not differ significantly from that of 27 control nests (67%). Both for all nests (N=58), and for nests with logging closer than 50m (N=31), reused nests differed from those not reused by being situated in a larger area of mature forest, and in forests with a higher proportion of Norway Spruce Picea abies, which gives better cover than Scots Pine Pinus silvestris and deciduous trees. However, a higher proportion of nesting territories with the target nest situated in a Spruce-dominated forest stand seemed to lack alternative nests, probably due to the more intensive timber harvesting in Spruce-dominated landscapes. Hence, Goshawks’ tolerance towards logging may be overestimated in Spruce-dominated forests. We recommend that foresters planning timber harvesting near Goshawk nests focus on the remaining mature forest stand and its structure. In order to maximise the chance of nest reuse, a patch of at least 2ha of suitable mature forest around the nest should be retained.

A review of historical management arguments for Northern Goshawk Accipiter gentilis proposed by Norwegian hunters, scientists and conservationists

The Northern Goshawk Accipiter gentilis has influenced the lives of people in rural areas since the beginning of Norwegian civilization. In the first known written sources about the species, during the Viking age, the goshawk symbolized heroism and bravery. Goshawks were historically connected to mysticism and superstition, as evidenced first in old runes and later in fairy tales and common adages. This paper reviews the scientific and ethical argumentation for the management strategy in that took place between 1845 and 1971, when the goshawk was considered a bounty species. At the start of this extermination period, a small number of influential scientists convinced the government to establish a bounty on goshawks that is dictated in hunting legislation from 1845, 1863 and 1899. From 1910 onward, the debate concerning the bounty policy became more diverse, broadening the discussion to include moralistic and conservational issues and finally leading to the protection of goshawks in 1971. Hunting legislation beginning the 1970s refined the utilitarian view of nature by including functional aspects of game species removing the distinction between “vermin” and “utility” animals in 1981. The goshawk has been listed on Norway’s Red List of Threatened Species since 1984. This review illuminates how scientific, aesthetical and ethical arguments were all interwoven in the policy and management for the Northern Goshawk over a relatively short historic timeframe.

2004～2008年に岐阜大学附属野生動物救護センターに搬入されたオオタカの救護事例における回顧的研究

2004～2008年に岐阜大学附属野生動物救護センターに搬入されたオオタカの救護事例における回顧的研究

Goshawk prey have more bacteria than non‐prey

1. Predators often prey on individuals that are sick or otherwise weakened. Although previous studies have shown higher abundance of parasites in prey, whether prey have elevated loads of micro-organisms remains to be determined. 2. We quantified the abundance of bacteria and fungi on feathers of woodpigeons Columba palumbus L., jays Garrulus glandarius L. and blackbirds Turdus merula L. that either fell prey to goshawks Accipiter gentilis L. or were not depredated. 3. We found an almost three-fold increase in bacterial load of prey compared with non-prey, while there was no significant difference between prey and non-prey in level of fungal infection of the plumage. 4. The results were not confounded by differences in size or mass of feathers, date of collection of feathers, or date of analysis of feathers for micro-organisms. 5. These findings suggest a previously unknown contribution of bacteria to risk of predation, with important implications for behaviour, population ecology and community ecology.

Plasma concentration of α-tocopherol in different free-ranging birds of prey

In this study, we investigated the α-tocopherol plasma concentrations in healthy free-ranging nestlings of the white-tailed sea eagle (Haliaeetus albicilla) (n = 32), osprey (Pandion haliaetus) (n = 39), northern goshawk (Accipiter gentilis) (n = 25), common buzzard (Buteo buteo) (n = 31), and honey buzzard (Pernis apivorus) (n = 18) as well as of free-ranging adults of the white-tailed sea eagle (n = 10), osprey (n = 31), and northern goshawk (n = 45). α-Tocopherol plasma concentrations were determined by reverse-phase high-performance liquid chromatography. α-Tocopherol plasma concentrations in nestlings of osprey, white-tailed sea eagle, and northern goshawk did not differ significantly amongst the species, but the common buzzard and honey buzzard nestlings had significantly lower α-tocopherol plasma concentrations than nestlings of the other species (both P < 0.001). Adult male ospreys and white-tailed sea eagles had significantly higher α-tocopherol concentrations compared to adult females (both P < 0.005). Adult ospreys and northern goshawks had significantly higher α-tocopherol plasma concentrations compared to their nestlings (both P < 0.001). In adult female northern goshawks, plasma concentrations of α-tocopherol increased significantly before egg laying (P < 0.001). These results demonstrate α-tocopherol plasma concentrations in birds of prey to be species specific and influenced by age and reproductive status.

Clearcutting forestry and Eurasian boreal forest grouse: Long-term monitoring of sympatric capercaillie Tetrao urogallus and black grouse T. tetrix reveals unexpected effects on their population performances

Along the succession gradient of the boreal forest ecosystem, black grouse Tetrao tetrix inhabits the early and capercaillie Tetrao urogallus the latest stages. When converting old forest to clearcuts and plantations, commercial forestry has therefore been assumed to affect capercaillie negatively and to be favourable to black grouse. During a 30-year period (1979–2008) we monitored sympatric populations of the two species in a forest in southeast Norway based on annual spring and autumn censuses and radio-marked birds. During this period, the proportion of old, semi-natural forest was halved and clearcuts and young plantations increased accordingly. The grouse populations did not change as predicted. While the trend in August numbers of adult black grouse declined, males more than females, abundance of adult capercaillie remained unchanged. Number of males at leks showed similar patterns. Equally surprising, breeding success (number of chicks per female in August) of both species increased, thus indicating that the populations were regulated more by variation in adult survivorship than by recruitment of young birds. No correlations were found with changing climatic factors (precipitation and temperatures in winter and spring, snow depth and time of snow melt), except that year-to-year breeding success was positively correlated with minimum temperatures during 2 weeks posthatch. The results are explained by a combination of more flexible habitat selection than previously assumed and a changing predator regime: In the early period, nearly all capercaillie leks were located in old, semi-natural forest, but as plantations grew older (>30 years), new leks were established there. Similarly, while young capercaillie broods used old semi-natural forest almost exclusively when the study started, they frequently used middle-aged plantations, especially those with a ground cover of bilberry Vaccinium myrtillus, when these became common in later years. The increasing breeding success could largely be explained by more females rearing chicks successfully, presumably due to a marked decline in the main nest predator, the red fox Vulpes vulpes. A practice of thinning of the old, semi-natural forest some years prior to final harvesting probably facilitated predation of black grouse by goshawks Accipiter gentilis. Contrary to many beliefs, our results indicate that both capercaillie and black grouse are quite tolerant to changes in forest management regimes. In our study, numerical and functional responses of predators (mainly red fox and goshawk) apparently played a more important role in regulating grouse numbers than habitat factors per se.

京都府南部におけるオオタカの営巣場所，繁殖期および繁殖成功

京都府南部におけるオオタカの営巣場所，繁殖期および繁殖成功

Sexual signals, risk of predation and escape behavior

Adults of many species display extravagant sexual signals during the reproductive season, apparently evolved as a means of attracting mates or repelling potential competitors, thereby inadvertently also attracting the attention of predators. Many studies have shown predation costs of sexual display. Therefore, we should expect species with the most exaggerated signals to have evolved antipredator behavior that reduces or eliminates predation costs of sexual signaling but also to have evolved behavior that allows for escape from a predator once captured. We quantified 6 aspects of escape behavior in 2105 free-living birds belonging to 80 species when handled after capture for banding. Escape behavior was species specific as demonstrated by significant consistency in behavior among individuals. Escape behavior was significantly related to susceptibility to predation by cats Felis catus and goshawks Accipiter gentilis, showing that escape behavior is under current selection. Escape behavior was related to the ease of feather loss estimated in a previous study but also to the frequency of tailless individuals recorded in the field. Thus, escape behavior reported here was cross-validated against other aspects of antipredator behavior shown to reflect risk of predation. Aspects of escape behavior differed significantly between males and females (biting, fear screams, and feather loss). Sexually dichromatic species differed in escape behavior from monochromatic species by having a reduced frequency of fear screams and increased tonic immobility. These findings suggest that exposure to risk of predation has modified escape behavior in relation to sexual coloration. Key words: alarm calls, escape behavior, fear scream, predation, sex, signaling, tonic immobility. [Behav Ecol]

Parental care in nesting hawks: breeding experience and food availability influence the outcome

Parental food provisioning and sibling rivalry have inspired abundant investigations of evolutionary conflicts within families. Nevertheless, their joint effects have seldom been assessed in relation to parental and environmental state. We investigated state dependency of feeding behaviors through the complete nesting phase in a species whose young both partly beg for food and partly self-feed, the northern goshawk Accipiter gentilis. After hatching, when young relied on being fed beak-to-beak, siblings achieved equal amounts of food irrespective of hatching rank, body condition, and sex. However, mothers new to a territory fed their offspring less than experienced ones independently of food availability. This pattern persisted also after nestlings grew and initiated to self-feed and aggressively monopolize prey. Mothers never interfered with aggressions but stayed with their even feeding strategy paying little attention to begging activity. Although mothers' even feeding strategy is likely to equalize siblings' survival probabilities when food is abundant, the fact that nestlings in good condition monopolize prey in self-feeding situations will boost brood asymmetries when food decreases. Because new mothers feed their offspring less than experienced ones, aggressive sibling rivalry will be particularly crucial among mothers lacking previous local breeding experience. Albeit hitherto overlooked, feeding behaviors constitute important mechanisms explaining experience-related differences in reproductive performance of wild animals.

コガネムシ上科の幼虫を巣上で食べたサシバの観察記録

We observed a young Gray-faced Buzzard Butastur indicus picking up and eating insect larvae from inside the nest. Taxonomic examination of one of the larvae revealed it to be approximately 25 mm in length and characterized by a thick and C-shaped body with obvious legs on the thorax and no urogomphi. These features indicated the insect to belong to the superfamily Scarabaeoidea within the family Coleoptera. Scarabaeoidea may breed in nests of the Gray-faced Buzzard, as shown recently in other carnivorous birds, including the Honey Buzzard Pernis apivorus, Northern Goshawk Accipiter gentilis, Oriental Stork Ciconia boyciana and Great cormorant Phalacrocorax carbo.

Vulnerability of black grouse hens to goshawk predation: result of food supply or predation facilitation?

The plant cycle hypothesis says that poor-quality food affects both herbivorous voles (Microtinae spp.) and grouse (Tetraonidae spp.) in vole decline years, leading to increased foraging effort in female grouse and thus a higher risk of predation by the goshawk Accipiter gentilis. Poor-quality food (mainly the bilberry Vaccinium myrtillus) for these herbivores is induced by seed masting failure in the previous year, when the bilberry is able to allocate resources for chemical defence (the mast depression hypothesis; MDH). The predation facilitation hypothesis (PFH) in turn states that increased searching activity of vole-eating predators during or after the decline year of voles disturbs incubating and brooding grouse females. The behaviours used by grouse to avoid these terrestrial predators make them more vulnerable to predation by goshawks. We tested the main predictions of the MDH and PFH by collecting long-term (21-year) data from black grouse Tetrao tetrix hens and cocks killed by breeding goshawks supplemented with indices of bilberry crop, vole abundance and small carnivores in the vicinity of Oulu, northern Finland. We did not find obvious support for the prediction of the MDH that there is a negative correlation of bilberry crop in year t with vole abundance and with predation index of black grouse hens in year t+1. We did find obvious support for the prediction of the PFH that there is a positive correlation between predator abundance and predation index of grouse hens, because the stoat Mustela erminea abundance index was positively related to the predation index of black grouse hens. We suggest that changes in vulnerability of grouse hens may mainly be caused by the guild of vole-eating predators, who shift to alternative prey in the decline phase of the vole cycle, and thus chase grouse hens and chicks to the talons of goshawks and other avian predators.

Mesopredator Release by an Emergent Superpredator: A Natural Experiment of Predation in a Three Level Guild

BackgroundIntraguild predation (IGP) is widespread but it is often neglected that guilds commonly include many layers of dominance within. This could obscure the effects of IGP making unclear whether the intermediate or the bottom mesopredator will bear higher costs from the emergence of a new top predator.Methodology/Principal FindingsIn one of the most extensive datasets of avian IGP, we analyse the impact of recolonization of a superpredator, the eagle owl Bubo bubo on breeding success, territorial dynamics and population densities of two mesopredators, the northern goshawk Accipiter gentilis and its IG prey, the common buzzard Buteo buteo. The data covers more than two decades and encompass three adjacent plots. Eagle owls only recolonized the central plot during the second decade, thereby providing a natural experiment. Both species showed a decrease in standardized reproductive success and an increase in brood failure within 1.5 km of the superpredator. During the second decade, territory dynamics of goshawks was significantly higher in the central plot compared to both other plots. No such pattern existed in buzzards. Goshawk density in the second decade decreased in the central plot, while it increased in both other plots. Buzzard density in the second decade rapidly increased in the north, remained unchanged in the south and increased moderately in the center in a probable case of mesopredator release.Conclusions/SignificanceOur study finds support for top-down control on the intermediate mesopredator and both top-down and bottom-up control of the bottom mesopredator. In the face of considerable costs of IGP, both species probably compete to breed in predator-free refugia, which get mostly occupied by the dominant raptor. Therefore for mesopredators the outcome of IGP might depend directly on the number of dominance levels which supersede them.

Environmental constraints for plumage melanization in the northern goshawk Accipiter gentilis

Although it is recognized that certain environmental factors are important determinants of the expression of melanin‐based traits, their influence in wild populations of animals is poorly known. One of these factors is the availability of amino acids that serve as precursors of melanins. Here we measured eumelanin and pheomelanin content in feathers of northern goshawk Accipiter gentilis nestlings, hypothesizing that, if the availability of melanin precursors is related to food abundance and habitat quality, plumage melanization should be affected by those variables. Although the eumelanin content increased with food abundance as predicted, the levels of this variable were higher in low‐quality habitats (homogeneous coniferous forests) and in nestlings in poor condition, and the pheomelanin content and eumelanin:pheomelanin ratio were lower and higher, respectively, in subpopulations where nestlings were in poorer condition. Therefore, environmental availability of melanin precursors seems to determine plumage melanization in goshawks, but our findings may also be explained by the differential effects of environmental oxidative stress on both forms of melanin, as eumelanin and pheomelanin production are favoured under high and low levels, respectively, of oxidative stress.

Fear screams and adaptation to avoid imminent death: effects of genetic variation and predation

Fear screams are calls emitted by prey just before or during capture by a predator, and the evolution of such calls has been attributed to the fitness benefits of escape from a predator due to interference by a secondary, kleptoparasitic predator or interference by conspecifics. If fear screams had beneficial effects on individuals carrying the same genes (in terms of learnt avoidance of a predator by related individuals, or predators avoiding future capture attempts of difficult prey), then fear screams should be more prevalent in species with little genetic variation and hence a high degree of shared alleles. In a comparative analysis of 71 species of birds, the proportion of individuals emitting fear screams in different species increased with the degree of genetic similarity among adults in local populations, as reflected by the band-sharing coefficient for neutral minisatellite markers. Furthermore, the proportion of individuals emitting fear screams in different species increased with the degree of preference by a common avian predator, the goshawk Accipiter gentilis, but not by the sparrowhawk Accipiter nisus, showing that individuals belonging to species that suffered disproportionately from predation were more likely to emit fear screams. Finally, the proportion of individuals emitting fear screams in different species increased with the proportion of individuals that were tail-less, thus having successfully escaped a predator. These results suggest that genetic variation has played a role in the evolution of fear screams, and that the frequency of fear screams is related to the risk of predation and the probability of successful escape.

Predators and microorganisms of prey: goshawks prefer prey with small uropygial glands

Summary 1. The uropygial gland of birds produces chemical substances with antimicrobial properties that have been shown to reduce the abundance of feather degrading bacteria and other microorganisms. These microorganisms would affect the flight capabilities of birds and, consequently, a relationship between size of uropygial glands and probability of capture by aerial predators should exist. 2. We tested this hypothesis by estimating the susceptibility of 56 species of prey of the goshawk Accipiter gentilis Linnaeus to predation as the observed abundance of prey relative to the expected abundance from mean population density. 3. In a comparative analysis of the relationship between relative size of the uropygial gland and susceptibility to predation we found a strong negative relationship accounting for 16% of the variance. This relationship was present in analyses that accounted for similarity due to common phylogenetic descent, the fact that prey of intermediate size were preferred, and that larger prey species have larger uropygial glands. 4. These observations are consistent with uropygial glands being under strong selection from aerial predators that are likely mediated by the effect of uropygial glands on feather degrading bacteria and therefore on flight capabilities of birds.

Superpredation patterns in four large European raptors

AbstractPredatory interactions among top predators, like superpredation or intraguild predation (IGP), can influence community structure. Diurnal raptors occupy high trophic levels in terrestrial food webs, and thus can regulate the presence of mesopredators. We studied superpredation (the killing and eating of another predator) in four large European raptors. We gathered 121 dietary studies, totalling 161,456 prey for the Goshawk Accipiter gentilis L., Golden Eagle Aquila chrysaetos L., Bonelli's Eagle Aquila fasciata Vieillot, and Eagle Owl Bubo bubo L. Results showed that superpredation: (1) is a widespread interaction in large raptors, but it can vary according to the top predator species; (2) is not an important energetic resource for large raptors, but rather seems mostly related to diet diversification when the main prey decreases; (3) is spatially clustered reflecting habitat heterogeneity, but shows no temporal or large‐scale spatial trends; and (4) it is associated with lower breeding success of the top predator species. These findings support the food stress hypothesis as the main driving force behind increases in superpredation and IGP in raptors, with the decrease in breeding performance as a side effect. Superpredation by large raptors deserves future research to understand its effects on mesopredators, because on one hand it might contribute to promote biodiversity, while on the other hand, it can sometimes represent an additional risk for small populations of endangered mesopredators.

A comparison of Goshawk summer diet in three areas with different breeding density in western Norway

In Norway, Goshawk Accipiter gentilis populations have declined in continuously forested areas in the inland, possibly because of decreased populations of grouse. The highest breeding densities are now found in landscapes dominated by farmland and urban areas, and in some areas close to the coast. In western Norway, we have compared the summer diet of Goshawks breeding at high densities in one island area and one urban area with that of Goshawks breeding at lower densities in an inland area at higher altitudes, approximately 90 km from the coast. Birds dominated the diet in all areas, but the diet diversity was lower in the inland than in the two other areas. The number of pigeons, Woodcock Scolopax rusticola and ducks/waders found at nest sites decreased with altitude, whereas the number of grouse increased. We conclude that Goshawks in the inland are more dependent on grouse because of lower availability of alternative prey.

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.