Nesting failure in Finnish Northern Goshawks Accipiter gentilis: incidence and cause

Habitat composition is an important factor influencing nesting failure probability in birds. However, although various habitat effects such as fragmentation and edge density are known to have clear negative effects on the breeding success of passerines, the role that habitat composition plays in shaping nesting failure patterns among other avian groups is less well known. We studied nesting failure in a large forest raptor, the Northern Goshawk Accipiter gentilis , during the period 1999–2003 in Finland. Illegal human persecution was found to constitute a major cause of failure among nests where cause could be determined accurately (comprising c . 60% of all failed nesting attempts). Egg predation by corvids and nestling predation by Eagle Owls Bubo bubo were also common. However, the exact cause of nesting failure could not be inferred in every case. Overall, the degree of initial parental investment (clutch size and egg volume) was significantly smaller in nests that failed than in nesting attempts that were successful. This did not apply to nests that were destroyed by humans, in which investment was at a level equal to nests that were successful. Although the probability that a nesting attempt would fail was also related to small-scale nest concealment, nesting failure probability was not associated with main prey density or several measures describing territorial habitat composition at larger scales. Small initial parental investment, not habitat composition, is thus the major correlate of nesting failure in Goshawks.     

The hunting habitats of Goshawks Accipiter gentilis in boreal forests of central Sweden

Radio-tracking was used to study the habitat utilization of hunting Goshawks Accipiter gentilis in central Swedish boreal forest. Data were collected during September-June, in 1977–1981. There was no major difference between autumn, winter or spring in any aspect of habitat preference, despite seasonal differences in prey composition. Despite the strong reversed sexual size-dimorphism, no habitat segregation between the sexes was found. It is argued that this is due to the lack of prey segregation between the sexes. Goshawks showed a strong preference for mature forest, which was chosen twice as often as would be expected from its prevalence in the environment, whereas the younger successional stages of the forest were under-used. Goshawks preferred large habitat patches, and showed no major preference with respect to tree species composition of the forest. Kills made by radio-tagged Goshawks showed the same habitat- and patch-size distribution as did Goshawk locations in general. It is argued that the profitability of different hunting habitats, and thus habitat preferences, is not only determined by prey density in the habitats, but also by other habitat features influencing the Goshawk's ability to hunt there. Winter home-ranges averaged 5700 ha, considerably larger than Goshawk ranges recorded in areas with richer prey supply.     

Increased lifetime reproductive success of first‐hatched siblings in Common Kestrels Falco tinnunculus

Order of birth has profound consequences on offspring across taxa during development and can have effects on individuals later in life. In birds, differential maternal allocation and investment in their progeny lead to variance in the environmental conditions that offspring experience during growth within the brood. In particular, laying and hatching order have been proposed to influence individual quality during the growing period, but little is known about the fitness consequences that these two factors have for offspring from a lifetime perspective. We explored the effect of laying and hatching order on post‐fledgling survival (measured as recruitment probability) and lifetime reproductive success (LRS) in Common Kestrels Falco tinnunculus, using a long‐term and individual‐based dataset. First‐hatched chicks showed higher survival probability and LRS than their siblings. This effect was not due to body condition of the individuals at adulthood, the quality of their mates or the reproductive outcome compared with later‐hatched individuals. Instead, first‐hatched chicks had a higher recruitment probability. This could be explained by the higher body condition attained by first‐hatched chicks at the end of the nesting period, perhaps due to an enhanced competitive advantage for food over their siblings at the time of hatching. Laying order, in contrast to hatching order, appeared to have little or no effect on LRS. Our results suggest that hatching order within siblings predicts fitness, and that better early‐life conditions during growth experienced by first‐hatched chicks improve first survival and then recruitment, resulting in an enhanced LRS.     

Starvation mortality and body condition of Goshawks Accipiter gentilis along a latitudinal gradient in Norway

Relative starvation risk and body condition were investigated in 599 Goshawks that had died in collision accidents or of starvation. Specimens were collected by the public along a 1300-km north–south (58°N–71°N) gradient in Norway, representing the northernmost geographical range of the species. The probability of a Goshawk's death being caused by starvation as opposed to by a collision accident increased with latitude with juvenile males at a disproportionately higher risk than others. Of birds killed in accidents, females generally were in better condition than males, and adults in better condition than juveniles. A season-by-latitude interaction indicated that males from northern latitudes were in poorer condition during winter and spring than males from southern parts of the country. This could also be modelled as a curvilinear relationship with daylength. There were no significant relationships between weather factors in the weeks prior to the deaths of the birds and the relative starvation probability or the condition of trauma victims. The results suggest that food limitation plays a relatively higher role in northern populations, affecting young males especially. This was also supported by the fact that the sex ratio of accidentally killed birds was increasingly female biased with increasing latitudes. It is suggested that the relatively higher mortality risk of males is due to their smaller average body size, and that selection for starvation resistance during winter is the reason behind the clinal increase of body size in Goshawks towards the northern and eastern parts of Europe.     

The relationship of spring and summer weather with density and breeding performance of the Buzzard Buteo buteo, Goshawk Accipiter gentilis and Kestrel Falco tinnunculus

The density of territorial pairs, laying pairs and the breeding performance of three raptor species, Buzzard Buteo buteo , Goshawk Accipiter gentilis and Kestrel Falco tinnunculus , were investigated in relation to rainfall and temperature during the breeding cycle. In all species the density of territorial pairs was not related to spring rainfall and temperature. In the Buzzard the percentage of pairs that bred unsuccessfully was significantly related to May rainfall. The number of young fledged was negatively correlated with rainfall in April and May. In the Goshawk the percentage of laying pairs was negatively correlated to rainfall in March and April. Success in terms of number of young fledged was related mostly to May temperature. In the Kestrel the density of territorial pairs was dependent on March-April rainfall. The number of young fledged per successful pair was highly correlated with May and June temperature. All these relationships showed that the hatching period and the subsequent two weeks were most important for chick survival.     

Foraging area and hunting technique selection of Common Kestrel (Falco tinnunculus) in winter: the role of perch sites

In winter, Common Kestrel minimizes energy expenditure by using the low-cost, low-profit technique of perch hunting. Existence of the perch sites is the precondition of perch hunting. Therefore, one can predict that the kestrels would prefer a habitat with more perch sites, and perch sites should have an important role in the kestrels' hunting technique use, habitat selection and habitat use in winter. To test this prediction, the authors manipulated two areas in a grassland. They increased the potential perch sites in one area with bamboo poles (hereafter test area) and kept another as control (hereafter control area). They observed and compared the kestrels' use and their behaviors in these two areas. Far more kestrels appearing in the test area with increasing perch sites than in the control area were recorded. The kestrels stayed in the test area with more perch sites significantly longer than in the control area. And in the test area with more perch sites, kestrels hunted 77.24% of the total hunting with the technique of perch hunting. In the control area, kestrels hunted only with the technique of flight-hunting. There was a significant correlation between the technique used by kestrels and the areas with or without perch sites. In the test area with increasing perch sites kestrels spent 51.8% of their time in perching and 12.1% in air, which were 30.1% and 34.8%, respectively, in the control area. There was no significant difference in hunting profit between areas. The results suggest that perch sites play an important role in the selection of hunting technique and foraging habitat for kestrels in winter, and kestrels appear to prefer the habitat with suitable perch sites in winter.     

Honey Buzzard Pernis apivorus nest‐site selection in relation to habitat and the distribution of Goshawks Accipiter gentilis

The selection of a suitable nest‐site is critical for successful reproduction. Species' preferences for nest‐sites have presumably evolved in relation to local habitat resources and/or interactions with other species. The importance of these two components in the nest‐site selection of the Eurasian Honey Buzzard Pernis apivorus was assessed in two study areas in eastern Austria. There was almost no difference in macro‐ and micro‐habitat features between nest‐sites and random plots, suggesting that Honey Buzzards did not base their choice of nest‐site on habitat characteristics. However, nests were placed significantly further from nests of Northern Goshawk Accipiter gentilis than would be expected if nest‐sites had been chosen at random. Furthermore, in one study area Honey Buzzards appeared to favour areas close to human settlements, perhaps indicating a mechanism to avoid Goshawks, which tend to avoid the proximity of humans. No habitat variable was significantly associated with the loss of Honey Buzzard young, but predation was higher in territories closer to breeding pairs of Goshawks at both study sites. Although Honey Buzzards are restricted to nesting in forests, their choice of nest‐site therefore appears to be largely dictated by the distribution of predators. Studies of habitat association may yield misleading results if the effects of predation risk on distribution are not considered.     

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

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.     

Survival, ranging, habitat choice and diet of the Northern Goshawk Accipiter gentilis during winter in Northern Finland

The winter ecology of the Northern Goshawk Accipiter gentilis was studied in northern Finland in 1991-95. The data consist of 26 radiomarked hawks, of which 331 day and 299 night positions were located. Average range size was 9894 ha for males (minimum convex polygon, n = 4) and 6282 ha for females ( n = 11). Ranges calculated by the 75% harmonic mean contour gave range areas of 3283 ha for males ( n = 3) and 2753 ha for females ( n = 11). Harmonic mean centres concentrated near human settlements, implying better food availability there compared with woodland areas. Some Goshawks preyed upon animals living in the city dump in late winter. Goshawks preferred deciduous and mature conifer forests and avoided open areas such as fields and bogs. They also avoided very heterogeneous sites. The winter diet consisted mostly of Mountain Hares Lepus timidus (30.9%), Red Squirrels Sciurus vulgaris (23.6%), Brown Rats Rattus norvegicus (12.7%) and forest grouse Tetraonidae (27.1%), of which Black Grouse Tetrao tetrix was the most important (14.5%). By weight, hares constituted 70% of the biomass consumed.     

Prey preferences and recent changes in diet of a breeding population of the Northern Goshawk Accipiter gentilis in Southwestern Europe

Capsule: Northern Goshawk Accipiter gentilis diet has changed significantly since the 1980s, probably due to changes in populations of preferred prey species.

Aims and methods: To assess changes to the breeding season diet of the Northern Goshawk in southwest Europe over three decades. We examined prey remains at and around nests and assessed avian prey availability using point count and line transect surveys.

Results: During 2008–11, Goshawks mainly ate birds, with Feral Pigeons Columba livia f. domestica being the most important prey species. Goshawks preferred prey of 100?400?g and forest prey species to non-forest species. Goshawk diet has changed significantly over recent decades: 22% of current prey items belong to species that were not part of the diet in the 1980s. We suggest that these dietary changes reflect changes in the abundance of prey species of the preferred size caused by changes in land use leading to an increase in forest cover, new prey species colonization and changes in the abundance and management of domestic prey.

Conclusion: This study emphasizes that major transformations occurring in agroforestry systems are affecting the main preferred prey of important forest predators, which may have consequences for conservation of both the predators and their prey.     

Density‐dependent increase in superpredation linked to food limitation in a recovering population of northern goshawks Accipiter gentilis

A better understanding of the mechanisms driving superpredation, the killing of smaller mesopredators by larger apex predators, is important because of the crucial role superpredation can play in structuring communities and because it often involves species of conservation concern. Here we document how the extent of superpredation has changed over time, and assessed the impact of such temporal variation on local mesopredator populations using 40 yr of dietary data collected from a recovering population of northern goshawks Accipiter gentilis, an archetypical avian superpredator. We then assessed which mechanisms were driving variation in superpredation, e.g. was it opportunistic, a response to food becoming limited (due to declines in preferred prey) or to reduce competition. Raptors comprised 8% of goshawk diet on average in years when goshawk abundance was high, which is higher than reported elsewhere. Additionally, there was a per capita increase in superpredation as goshawks recovered, with the proportion of goshawk diet comprising raptors increasing from 2 to 8% as the number of goshawk home‐ranges increased from ≤ 14 to ≥ 25. This increase in superpredation coincided with a population decline in the most commonly killed mesopredator, the Eurasian kestrel Falco tinnunculus, which may represent the reversal of the ‘mesopredator release’ process (i.e. mesopredator suppression) which occurred after goshawks and other large raptors declined or were extirpated. Food limitation was the most likely driver of superpredation in this system given: 1) the substantial decline of two main prey groups in goshawk diet, the increase in diet diversity and decrease in goshawk reproductive success are all consistent with the goshawk population becoming food‐limited; 2) it's unlikely to be purely opportunistic as the increase in superpredation did not reflect changes in the availability of mesopredator species; and 3) the majority of mesopredators killed by goshawks do not compete with goshawks for food or nest sites.     

Ranging behaviour and foraging habitats of breeding Sparrowhawks Accipiter nisus in a continuous forested area in Norway

The ranging behaviour and foraging habitats of Sparrowhawks Accipiter nisus were studied in a continuous forest area in southern Norway by use of radiotelemetry in 1995 and 1996. The mean size of the home ranges was 9.2 km² for males (sd ± 3.7, n = 6) and 12.3 km² for females (sd ± 6.4, n = 6), but the difference was not significant. Females ranged farther away from nests (mean 1824 m) than did males (mean 1240 m). None of the Sparrowhawks were located outside forest habitats. For the three pairs where both mates were radiotracked in 1996, habitat use did not differ between the sexes. Habitat composition in the home ranges differed from that of the study area. The most important difference was a higher proportion of medium-aged forest and a lower proportion of old forest in home ranges than in the study area. The selection for medium-aged forests was probably a response to high food supply and good hunting opportunities. Mixed regeneration and old forests were used more than clear-fell areas, which were seldom used. Mixed regeneration was also used more than coniferous replanting. The large home ranges in this study compared with those in studies in Great Britain is probably due to lower land productivity and associated lower densities of prey species in the present study. The study indicates that the Sparrowhawk benefits from modern forestry, which has created an increased proportion of medium-aged stands in the forest landscape.     

The role of predation in the decline and extirpation of woodland caribou

To select appropriate recovery strategies for endangered populations, we must understand the dynamics of small populations and distinguish between the possible causes that drive such populations to low numbers. It has been suggested that the pattern of population decline may be inversely density-dependent with population growth rates decreasing as populations become very small; however, empirical evidence of such accelerated declines at low densities is rare. Here we analyzed the pattern of decline of a threatened population of woodland caribou (Rangifer tarandus caribou) in British Columbia, Canada. Using information on the instantaneous rate of increase relative to caribou density in suitable winter foraging habitat, as well as on pregnancy rates and on causes and temporal distribution of mortalities from a sample of 349 radiocollared animals from 15 subpopulations, we tested 3 hypothesized causes of decline: (a) food regulation caused by loss of suitable winter foraging habitat, (b) predation-sensitive foraging caused by loss of suitable winter foraging habitat and (c) predation with caribou being secondary prey. Population sizes of caribou subpopulations ranged from <5 to >500 individuals. Our results showed that the rates of increase of these subpopulations varied from −0.1871 to 0.0496 with smaller subpopulations declining faster than larger subpopulations. Rates of increase were positively related to the density of caribou in suitable winter foraging habitat. Pregnancy rates averaged 92.4% ±2.24 and did not differ among subpopulations. In addition, we found predation to be the primary cause of mortality in 11 of 13 subpopulations with known causes of mortality and predation predominantly occurred during summer. These results are consistent with predictions that caribou subpopulations are declining as a consequence of increased predation. Recovery of these woodland caribou will thus require a multispecies perspective and an appreciation for the influence of inverse density dependence on population trajectories.     

Physiological response to stress in fledgling Lesser Kestrels Falco naumanni: the role of physical condition, sex and individual genetic diversity

Exposure to chronic stress early on during development has important deleterious consequences later in life, reducing important components of individual fitness such as survival and future reproduction. In this study, we evaluate the factors associated with physiological response to stress in fledgling Lesser Kestrels Falco naumanni , paying particular attention to the potential role of individual genetic diversity. For this purpose, we used heterophil/lymphocyte ratios (H/L ratio) as a haematological stress indicator and typed the analysed individuals at 11 highly polymorphic microsatellite loci, which allowed us to estimate their genetic diversity. We found that the H/L ratio decreases with fledgling physical condition, suggesting that this parameter is a good indicator of nutritionally based physiological stress. Physiological response to stress was higher in males than in females and this effect was independent of physical condition, suggesting that the observed pattern is due to inherent sexual differences in the factors influencing H/L ratios. Finally, the H/L ratio was positively associated with the genetic diversity of offspring. Previous experimental studies have found that individuals with higher genetic diversity show increased levels of circulating glucocorticoids, which in turn are directly responsible for increasing H/L ratios. On this basis, we suggest that a positive effect of genetic diversity on corticosterone levels may explain the observed association between H/L ratios and individual heterozygosity. Overall, this study highlights the utility of leucocyte profiles to study stress in wild bird populations and poses an interesting question about the effects of individual genetic diversity on haematological response to stress.     

Energetic limitation of avian parental effort: Field experiments in the kestrel (Falco tinnunculus)

We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring parental effort in natural broods of different size and parental response to manipulation of food satiation of the brood. Parental effort was quantified as total daily time spent in flight, and total daily energy expenditure, from all-day observations. During nestling care males with different natural brood sizes (4 to 7 chicks), spent an average of 4.75 h · d^?1 in flight independent of brood size, and expended an average total daily energy of 382 kJ · d^?1. Due to a higher flight-hunting yield (mammal-prey caught per hour hunting), males with larger natural broods were able to provision their broods with the same amount of food (mainly Microtus arvalis) per chick (62.6 g · d^?1), with the same effort as males with smaller broods. This provisioning rate was close to the mean feeding rate of hand-raised chicks in the laboratory, that were fed ad libitum, (66.8 g · d^?1 · chick^?1). Our food deprivation experiments revealed that male kestrels strongly respond to food shortage in the nest. In the older nestling phase males on average increased their daily rate of food delivery to the nest as a response to experimental food deprivation by almost three times to 646.4 g · d^?1, by increasing their flight activity level from 4.46 to 8.41 h · d^?1. This increased energy expenditure was sustained, for as long as eleven days, by increasing the metabolizable energy intake up to what is presumed to be the maximum rate. Even under considerable experimental food stress (chicks not being satiated due to continuous removal of delivered food by the observers) about half of the available daylight time remained unused for foraging. We conclude 1) that the mean daily energy expenditure of males during nestling care — to which clutch size is apparently initially adjusted — is well below the maximum they are able to sustain and 2) that the energy expenditure they can sustain under extremely high nestling demand is not set by the available time for foraging or the available energy in the environment. Thus the birds normally operate well below their presumed maximum, and only during food shortage, e.g., as caused by our experiments, do they increase activity up to this maximum. Therefore we conclude that the kestrels have costs other than energy expenditure, such as parental survival, that are involved in the increased “cost” of parental effort. We discuss possible generalisations about existing energetic limitations during parental care in altricial birds. From published estimates of daily energy expenditure during parental care (DEE_par) in 30 different bird species we derived the equation: DEE_par = 14.26 kg^0.65 Watt. This relationship differs significantly in slope (T = 2.49; p > 0.02) from the allometric equation for the maximum rate of energy assimilation (DME_max) as provided by Kirkwood (1983): DME_max = 19.82 kg^0.72 Watt. In smaller species (ca. 25 g) DEE_par about equals DME_max, while in the larger species (ca. 10 kg) DEE_par represents only about 60% of the predicted DME_max. This suggests that limitations in parental effort are more frequently set by the maximum sustainable energy intake in the smaller species than in larger species. Our allometric equations for DEE_par suggests that the relation between BMR, estimated using the equations of Aschoff and Pohl (1970), and the observed parental energy expenditure, is such that on average bird parents work at a daily level somewhere between 3 and 4 times BMR.     

The predation of predispersed Juncus squarrosus seeds by Coleophora alticolella (Lepidoptera) larvae over a range of altitudes in northern England

Seed production by the rush Juncus squarrosus and its subsequent predation by the larvae of the moth Coleophora alticolella were measured at a series of sites at different altitudes in northern England. The density of seeds produced is progressively reduced with increasing altitude. The density of larvae was greatest in the middle region of the transect. Seed predation was therefore highest at about 300 m. Predation was reduced to nil above 520 m, where the larvae were absent, but remained between 40 and 50% below 250 m. The density of predispersed seed surviving was greatest at the lowest site where the highest density of seeds was produced and predation was moderate. The lowest densities of surviving seeds were found around 300 m and also above 600 m where there was no predation but few seeds were produced. A similar density survived around 520 m as at 245 m, although the density of seed produced at 520 m was less than 30% of that at 245 m. The results are discussed in relation to the population dynamics of the moth in different parts of its altitudinal distribution.