Annual survival of Sparrowhawks Accipiter nisus breeding in three areas of Britain

Annual survival of breeding female Sparrowhawks Accipiter nisus was estimated by a capture-recapture procedure in three different areas, Eskdale and Annandale in southern Scotland and Rockingham Forest in east-central England.
In Eskdale, where the breeding population remained fairly stable during a 19-year study, annual survival averaged 59% (s.e. = 4%). In Annandale, where the breeding population declined during a 10-year study, annual survival averaged 66% (s.e. = 4%). In Rockingham, where the breeding population increased during an 11-year study, annual survival averaged 72% (s.e. = 4%). In this area, survival declined during the study as numbers rose. The apparent density-dependence in survival was therefore confounded with a time trend.
In all three areas, annual variations in survival were negatively related to the number of rain days during October-April, but the relationship was significant only in Eskdale. This area had the longest run of data and the greatest variation in the number of winter rain days. The relationship with rain days could partly account for the differences in mean survival between areas, with lowest survival in the wettest area and highest survival in the driest.     

Age-related survival in female Sparrowhawks Accipiter nisus

Long-term capture-recapture of female Sparrowhawks Accipiter nisus nesting in three different areas enabled age-related survival rates to be calculated, after correcting for age-related and annual variations in recapture frequency. The data provided strong evidence for an improvement in survival rates in the first 3 years of life and for a decline in the last 5–6 years, during a maximum life span of around 10 years. The decline in survival among older birds was attributed to senescence. At all ages, survival rates were higher in the English area, where breeding numbers were rapidly increasing, than in two Scottish areas, where breeding numbers were stable or declining. The precise pattern of change in survival rates with age may vary between areas, according to prevailing circumstances.     

Intraspecific brood-parasitism and dispersal in fledgling Sparrowhawks Accipiter nisus

The progeny of early-nesting Sparrowhawks Accipiter nisus have a higher survival rate than those of late nesters. To ascertain the reasons for greater survival in early-season Sparrowhawk fledglings, I studied post-fledging dispersal behaviour in this species by direct observations and radio-tracking during 3 years in Rockingham Forest, Northamptonshire, U.K.
Post-fledging brood-parasitism was found among early-dispersed young of both sexes.
Early-dispersed young spent up to 6 days being fed by the parents of other fledged, but still dependent, broods, as far as 6 km from their own nests.
Three broods were provided with supplementary food for 4 weeks, starting 1 week before expected dispersal. These young dispersed when significantly older than young from control broods. In both groups, males dispersed, on average, 3–4 days earlier than females. The ultimate dispersal age of young in control broods was negatively correlated with their rate of mass gain during the nestling period. Unlike the young of the control broods, fledglings in broods with augmented food were usually silent.
These findings offer an explanation for why Sparrowhawk young that disperse early in the season survive better than those which disperse late.     

Post-fledging recovery and dispersal of ringed Eurasian Sparrowhawks Accipiter nisus

Ring recoveries were used to explore the effect of early experience on the subsequent survival and dispersal of young Sparrowhawks in two areas in south Scotland. Young raised on high grade territories were recovered in greater proportion than young on low grade territories, implying that they had survived better after leaving the nest. Recovery rate tended to decline with increasing elevation of the nesting territory, implying that young from the high-ground territories survived less well than those from low-ground territories. Recovery rate showed no relationship with brood-size or sex composition, or with maternal age (yearling or older).
After the post-fledging period, dispersal distances were greater in females than in males and showed significant increases with increasing elevation of nesting territory (one area only) and with lateness of laying (or fledging) date. Regardless of laying date, young males from yearling mothers also dispersed further than young from older mothers, and young males from high elevation territories further than those from lower territories. No significant relationships emerged between dispersal distances and grade of natal territory, brood size or sex composition.     

Eye colour,age and breeding performance in Sparrowhawks Accipiter nisus.

In the British habitats studied, most species and pairs were seen within 10 minutes. Hence longer point counts are a poor investment of time.     

The effect of parent sex on prey deliveries to fledgling Eurasian Sparrowhawks Accipiter nisus

The relative contribution of each parent when providing for the fledglings has been recorded in only a few raptor species. We studied prey deliveries by Eurasian Sparrowhawk Accipiter nisus parents to fledglings at seven nests in southern Norway. Parents and young were fitted with radio-transmitters. Males delivered a larger number of prey to the young than did females throughout the post-fledging period (on average c.  80% of the deliveries). Two females were never observed to deliver food to the offspring, and their mates apparently raised the young to independence alone. The duration of the post-fledging period was positively related to per-capita delivery rate in the late stage.     

Sparrowhawks Accipiter nisus affect the spacing behaviour of wintering Turnstone Arenaria interpres and Redshank Tringa tetanus

The presence of a Sparrowhawk induces short-term abandonment of territories by wintering Redshank and increases both the density within flocks and the ranging behaviour of wintering Turnstone. The latter effect is thought to be due to the disturbance caused by hawk attacks, and the results suggest benefits for flocking beyond predator detection.     

Density-dependent mortality of wintering Dunlins Calidris alpina through predation by Eurasian Sparrowhawks Accipiter nisus

Density-dependent mortality in wintering shorebirds is critical to shorebird population dynamics, but is difficult to study and demonstrate. Most winter mortality of small shorebirds probably results from predation by raptors. Over 13 winters, I studied Eurasian Sparrowhawk Accipiter nisus predation on wintering Dunlins Calidris alpina at a rocky shore in south-east Scotland, a site apparently of marginal suitability for wintering Dunlins. Most Dunlins on the study site were juveniles (first winter birds), and winter juvenile mortality rate due to Sparrowhawk predation was strongly and positively related to Dunlin density (maximum winter count). There was no indication of any marked effect of weather on Dunlin winter mortality rate, except that mortality was slightly greater in winters with lower temperatures. Dunlin mortality was unrelated to the mortality through Sparrowhawk predation of a preferred prey, the Redshank Tringa totanus , or to sighting rates of hawks, suggesting that increased hunting of Redshanks by hawks or a numerical response by hawks were not involved in density-dependent mortality of Dunlins.     

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.     

Post-fledging survival of Sparrowhawks Accipiter nisus in relation to mass, brood size and brood composition at fledging

Using ring recoveries, the post-fledging survival of Sparrowhawks was examined in relation to the growth rates and fledging masses of young and in relation to the size and sex composition of broods, which contained up to six young. On most aspects, no significant relationships were found: light young survived as well as heavy ones, both in the population as a whole and in individual broods; and young in large broods survived as well as young in small broods. This was attributed to relative food abundance in the post-fledging dependency period and to the fact that most food-related mortality among young occurs at an earlier stage in the breeding cycle. However, males in all-male broods and females in all-female broods survived slightly better after fledging than others of their sex in two-sex broods. The reason is unknown.     

Surprise as a winter hunting strategy in Sparrowhawks Accipiter nisus,Peregrines Falco peregrinus and Merlins F. columbarius

Sparrowhawks Accipiter nisus, Peregrines Falco peregrinus and Merlins Falco columbarius were studied hunting Redshanks Tringa totanus, Dunlins Calidris alpina and Skylarks Alauda arvensis over three winters on a small Scottish estuary. Most Sparrowhawk and Merlin hunts consisted of a single attack (mean = 1.0 and 1.1, respectively), whereas Peregrine hunts often consisted of several attacks (mean = 1.8). Most hunts were short (<1 min), but Peregrine and Merlin hunts occasionally lasted over 5 min. In general, all three raptor species attacked by surprise, although Peregrines were more likely to make nonsurprise attacks. Prey attacked were usually initially very close to the raptor (<100 m); Peregrines attacked prey most often at long distances (>500 m). Chase lengths were mostly <5 second in length, although Peregrines, and particularly Merlins hunting Skylarks, often chased for several minutes. Peregrines attacked most prey in flight from flight, while Merlins and Sparrowhawks attacked birds on the ground with a flight from a perch. All three raptor species preferentially attacked larger Dunlin flocks, but Peregrines also favoured single birds. Capture rates of Redshanks and Dunlins were similar for the three raptor species (C. 10%), but for Skylarks, capture rate by Merlins was much higher (12%) than by Sparrowhawks (3%) or Peregrines (0%). Capture rates were highest when raptors attacked by surprise, particularly for a Peregrine hunting in the first minute of arrival on the study site if no Peregrines had been hunting there for the previous hour (16% success for the first minute compared with 2% in subsequent minutes). Sparrowhawks were more successful when attacking small rather than large Dunlin flocks. The use of short surprise attacks interspersed with long periods of inactivity was common to the three raptor species and was interpreted as a strategy to minimize the amount of energy and risk involved in hunting during the nonbreeding season.     

Growth of nestling sparrowhawks (Accipiter nisus)

Growth rates, mortality and parental care of nestling sparrowhawks were studied in southwest Scotland. Ae Forest was a conifer plantation 200–400 metres above sea-level, while the Annan valley consisted of farmland, woods, and small plantations on low ground.
Nestling sparrowhawks were measured daily from hatching for 21–24 days. Weight, tarsus length and outermost primary feather length were recorded. Nestlings could be sexed by the age of 16 days from the larger size of females, which were significantly heavier than males at one day, had longer tarsi at nine days, and longer primaries at 18 days. Growth rates were calculated using linear regression over standardized periods of about 10 days.
Growth rates were independent of brood size, and were negatively correlated with hatching date in one area. Hatching order and growth rate were correlated within broods. The greatest differences in growth rates were found between zones of Ae Forest, and between forest and valley.
Twenty-one per cent of nestlings over two days old died. Causes of mortality were starvation, wet weather, predation and desertion. Most of the mortality occurred in parts of Ae Forest remote from valley woodlands.
The presence or absence of the adult female was noted on nest visits. When habitually brooding, until the young were 11 days old, the hen was present on about 85% of visits. By fledging this figure fell to 66% in the valley, and to 32% in the forest.
The development of sexual dimorphism is discussed; females gained weight and body size faster than males, which developed various skills sooner.
It is suggested that differences in growth rates between parts of the study area were related to food supply. Poor growth rates, high mortality, and lack of parental care all occurred in areas which were remote from sources of abundant prey, as measured by song-bird censuses.     

Clinal variation in the morph ratio of Black Sparrowhawks Accipiter melanoleucus in South Africa and its correlation with environmental variables

The morph ratio distribution in polymorphic species often varies clinally, with a gradual change in morph ratios across the distributional range of the species. In polymorphic bird populations, clinal variation is rarely quantified. We describe a cline in the morph ratios of Black Sparrowhawks across South Africa, which is principally driven by a higher ratio of dark morph birds in the newly colonized southwest of the country. Across the 1400 km of our cline, the probability of a bird being a dark morph declined from over 80% close to the Cape Peninsula to under 20% in the northeast. Higher frequencies of dark morphs were associated with a higher proportion of rainfall falling during the winter breeding months. Further investigation revealed relationships between the proportion of dark morphs and altitude, amount of rainfall during the breeding months, and an interaction between this variable and temperature. These results provide some support for the suggestion that the higher frequency of dark morphs in the southwest is an adaptive response, rather than the result of a founder effect or genetic drift. These findings also suggest that, in theory, polymorphic species may be better adapted to cope with the challenges of climate change or may be able to expand their ranges more quickly into novel climatic areas, since selection pressure can act on a pre‐existing trait that may be beneficial in new conditions.     

Life history events of the Eurasian sparrowhawk Accipiter nisus in a changing climate

Current climate change has been found to advance spring arrival and breeding dates of birds, but the effects on autumn migration and possible responses in the distribution of wintering individuals are poorly known. To thoroughly understand the consequences of climate change for animal life histories and populations, exploration of whole annual cycles are needed. We studied timing of migration (years 1979–2007), breeding phenology (1979–2007) and breeding success (1973–2007) of Eurasian sparrowhawks Accipiter nisus in Finland. We also investigated whether the migration distance of Finnish sparrowhawks has changed since the 1960s, using ringing recovery records. Since the late 1970s Finnish sparrowhawks have advanced their spring arrival, breeding and autumn departure considerably, but the migration distance has not changed. Early migrants, who are the ones with the highest reproductive success, show the strongest advance in the timing of spring migration. In autumn, advanced departure concerns young sparrowhawks. Late autumn migrants, who are mainly adults, have not advanced their migration significantly. The sparrowhawk is the most common bird of prey and the main predator of most passerines in Finland. Therefore, changes in sparrowhawk migration phenology may affect the migration behaviour of many prey species. The breeding success of sparrowhawks has increased significantly over the study period. This is however more likely caused by other factors than climate change, such as reduced exposure to organochlorine pollutants.     

Multiple-brooding in birds of prey: South African Black Sparrowhawks Accipiter melanoleucus extend the boundaries

Multiple-brooding (raising more than one brood of young in quick succession) occurs infrequently in raptors and is generally restricted to either smaller species with shorter nesting periods, co-operative breeders or species capable of capitalizing on conditions of prolonged food abundance whenever they occur. This paper presents the first recorded cases of multiple-brooding in the Black Sparrowhawk Accipiter melanoleucus from two distinct locales in South Africa. In the Western Cape (Cape Peninsula), four attempts to multiple-brood were recorded in four different years, involving three distinct pairs of birds, and in KwaZulu–Natal (Eshowe), three distinct pairs of Sparrowhawks successfully multiple-brooded on several occasions over a 5-year study period. These results establish the Black Sparrowhawk as one of only two relatively large, monogamous raptor species, and the only specialist bird-eating raptor, in which multiple-brooding has been recorded with any frequency . The species' capacity to thrive in human-modified environments (i.e. alien plantations) and particularly to exploit associated foraging opportunities (e.g. high densities of doves and pigeons in suburban areas) may, at least partly, account for the instances of multiple-brooding reported here. We suggest that biologists be more vigilant for cases of multiple-brooding in raptors, as it is possible that this trait is more common than originally thought and has previously been overlooked.