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The Goshawk in Britain
M. Marquiss and I. Newton
A s a result of deforestation and persecution, the Goshawk Accipiter gentilis was more or less exterminated in Britain by the late 19th century, with only sporadic breeding thereafter (Hollom 1957, Newton 1972). From the late 1960s, however, the situation improved, with breeding pairs becoming established in several areas to form the basis of the present population. This paper reviews the current status of the species in Britain and gives some information on origins, breeding, mortality and diet, in an attempt to assess the main factors influencing numbers and distribution.
Current status Speculation on the numbers of Goshawks in Britain is rife. Optimistic estimates, based largely on hearsay and rumour, put the current breeding population as high as 145 pairs. The main idea giving rise to extravagant speculation is that Goshawks (traditionally thought of as secretive forest birds) must exist in large numbers to be seen even rarely. Thus, every sighting is held to be the tip of an iceberg, with as-yet-unreported populations living in the forest plantations that now clothe the hills in remote areas. This optimism has not been discouraged in previous publications on the status of Goshawks in Britain which have bemoaned the fact that many observers still withhold records (Brown 1976, Sharrock 1980). This paper should go some way to clarify the situation, since it incorporates much information previously unpublished. Moreover, the authors and their associates have systematically searched large areas of suitable woodland and have therefore been able to evaluate sight records as well as to say with some confidence that there are thousands of hectares of British forests unoccupied by Goshawks.
In some places, we have noted the presence of single birds which, despite vigorous display, apparently found no mate and did not breed. On two occasions, single birds built nests, and one of these laid and incubated eggs, though these proved, not unexpectedly, to be infertile. In other places where there were records of displaying pairs, we searched suitable habitat intensively, but produced no evidence of nests. In five such places, the woodland was so restricted that we could be certain that no nest was built. Thus, it seems that pairs have displayed regularly, sometimes in more than one year, in various places where they have not nested. This is not
[Brit. Birds 75: 243-260, June  2 4 3
244 The Goshawk in Britain
unexpected, since non-breeding pairs are frequent among the larger raptors, particularly where food is in short supply or where territory holders include many individuals in immature plumage (Newton 1979).
It was usually obvious when Goshawks were using an area, since even non-breeders left plenty of signs (kills, droppings and moulted feathers) characteristic of the species. Where there were no such signs and nests only of Sparrowhawk Accipiter nisus were found, we suspected that Sparrow- hawks displaying above the canopy had been misidentified as Goshawks. An experienced observer is unlikely to confuse the species because they are so different in shape as well as in size. Inexperienced observers, however, may have been misled by the frequent statements in bird books that male Goshawks can be confused with female Sparrowhawks. Most Goshawks in Britain are of the large northern type (see later), so many females are larger even than Buzzards Buteo buteo, while all males are considerably larger than any Sparrowhawk, and three times as heavy.
In this paper, we are concerned mainly with breeding pairs or potential breeding pairs, so have ignored records of single birds and separated records of proven breeders from records of 'pairs seen'. The latter were likely to have overestimated the numbers of Goshawks present because they could have included some Sparrowhawks and the same individual Gos- hawk displaying over different places.
Another source of error in population estimates is the overlap between records from different observers, who seem to have operated unknown to one another. In one area, for example, we received records of the same nests from four different sources. We have therefore been conservative in our estimates, so as to ensure that the figures given represent the minimum
76. First-year Goshawk Accipiter gentilis mantling dead rabbit Oryctolagus cuniculus. West Germany (Hansgeorg Arndl)
The Goshawk in Britain 245
numbers of pairs. If we had erred in the opposite direction for those records which were possibly of different pairs in the same county, we could have added only six nesting places and eight pairs of displaying birds to the total.
Records fall naturally into 'areas', often straddling county boundaries. The size and shape of these areas (labelled by code letters A-V, table 1) varied according to the number of sites encompassed, but none was larger than 1,650km2. Within areas, most sites were less than 10km apart (occasionally up to 40km), and no two areas were closer than 80km. Areas were widely distributed over mainland Britain, and only northwest Scot- land provided no records. Although more than 90% of the recoveries of ringed Goshawks were within 40km of their birthplace (see later), there may have been some interchange between adjacent areas. For the most part, however, the separate populations appeared to have arisen inde- pendently of one another.
Since 1965, Goshawk nests have been found in at least 60 different places in 14 areas, and pairs have been seen at a further 34 places, including eight additional areas where breeding has not been proved. Far fewer places, however, were occupied in any one year, and, of 56 different places (including all known 'alternatives' for each place) that were checked in 1979 and 1980, nests were found at only 39 (70%): a poor result for what should be an expanding population. Of the 22 areas, nine were represented over the years by only one known site and only three of these were used for nesting in 1979 and/or 1980. Of the remaining 13 areas, only six have had more than three pairs present in any one year, and only five had more than three pairs in 1979-80. Only in two areas (A and B) can it be said that the numbers have continued to increase over the years. Area D had at least five pairs breeding as long ago as 1973 and now has only two known pairs. Taken together, these various observations imply that the future for Gos- hawks breeding in Britain is far from secure.
It is unlikely that we have been informed of every nest found or pair displaying, so the figures we give are minimum values. In all but one of the areas where Goshawks have been proved to breed, however, their presence has been recorded independently by people with different interests. For example, records of birds killed have come from every area where more than one pair has been known to breed, mostly from people different from those who found the nests. We therefore find it hard to believe that there are large, as yet undiscovered, populations of Goshawks breeding in remote parts of Britain. All the evidence indicates the contrary: that as soon as Goshawks become established, they are all-too-obvious to birdwatchers as they display, and to foresters and gamekeepers if they nest and rear young.
Origins Goshawks currently breeding in Britain are apparently not derived from Continental immigrants, but rather from birds which have escaped from falconers or been deliberately released. Both the geographical distribution and the timing of first breeding records are more consistent with the distribution of falconry activities and known releases than with natural colonisation (Kenward et al. in press). The Goshawks now breeding in
246 The Goshawk in Britain
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Britain became established at a time when those in the Netherlands and other nearby parts of Europe were much reduced from pesticide poisoning. As Goshawks had not colonised Britain in the previous 70 years, they would have been even less likely to do so then. Moreover, most established populations are in western, rather than eastern, districts of Britain, farthest from Continental sources.
Goshawks have been released at least in areas B, E, H, M, N, Q and T, and in three of these areas breeding records followed within two years of the releases. Falconers are known to have lost birds in areas C, D, F, L and T and at leas