The Hen Harrier Page 3
Fig. 5 Breeding range of Montagu’s Harrier in Europe
Nevertheless, there may be a more important hunting advantage in the pale grey plumage of male harriers, particularly their white undersides. It is said that the white underparts and generally pale colouring of gulls and other sea-birds make them more difficult to detect from below. Certainly, the vole’s eye view of a hunting harrier must usually be a shape against the lightness of sky; so the paler tones of males may reduce their conspicuousness to such earth-bound prey. To some extent, even the darker females are relieved by pale or variegated tones on their underparts. A light-coloured bird, hunting in sunlight, casts a less dense shadow than a darker one. Since adult males bear the brunt of hunting for several months of the year, it is tempting to look for an advantage in their plumage colour; and it is interesting to learn that the Dutch ornithologist, Nieboer, in his study of the morphology of harriers (1973), concludes that pale colouring in males is indeed an advantage for hunting keen-sighted prey in open country. Support for this view comes from Schipper’s findings that male Hen and Montagu’s Harriers took more free-flying passerine birds than females did. In Montagu’s Harriers, the size difference between the sexes is too slight to explain the males’ greater efficiency in capturing agile prey. Yet light colouring is by no means universal in male harriers, the most obvious exception being the Black Harrier, which is blackest on its underparts.
Fig. 6 Winter and breeding range of Marsh Harrier in Europe
Colour patterns can only be understood as a result of a balance of factors. While the conspicuousness of many male harriers may indeed have a visual importance, as I have suggested, and almost certainly has a hunting advantage,* the range of variation from the extremes of the Pallid to the Black Harrier, appears to be influenced by climatic factors. It is well-known that the intensity of melanistic pigmentation tends to decrease as the mean temperature decreases, and that humidity combined with heat favours dark pigmentation. Clearly, in females and immatures these factors have been much modified for survival, but they seem to provide a neat explanation for the general trends in the colours of adult males which, by taking no part in incubation or brooding the young, have much less need for cryptic colouration, although they are to some extent cryptically coloured as hunters. Certainly the palest male harriers are found in northern climates and the palest of all, the Pallid, breeds in an area of low summer rainfall, while the Pied Harrier is at least partly associated with a monsoon climate. Both the Black Harrier and the dark phase of the Long-winged Harrier, without definite sexual dimorphism, are predominantly dark birds and are found nearest the equator. The Montagu’s Harrier, which ranges over similar latitudes to the Pied, has a melanistic form which occurs not infrequently. A study of its incidence would be interesting.
Montagu’s, Marsh and Pallid Harriers in Britain and Ireland
The recent increase of the Hen Harrier in Britain, Ireland and Holland has occurred over a period when the breeding range of Montagu’s Harrier has suffered a marked retraction. Nicholson estimated the Britain population of Montagu’s Harriers as 40–50 pairs in 1957 and nests had been found as far north as Perthshire in 1952, 1953 and 1955 and in Kirkcudbrightshire in 1953. Parslow (1973) said, however, that a decline had begun in Britain by the mid 1950s, while by 1960 there had been some decrease in almost all the main British breeding areas. This has continued until in 1973 and 1974 there was no evidence of successful breeding at all in this country, although nesting was reported again in 1976.
Montagu’s Harrier has lately been described as ‘probably our rarest diurnal bird of prey’ (Sharrock, 1973). It is a summer migrant (mid-April to October) to Britain where it has bred in a variety of habitats, but especially in young conifer forests and marshes. There have been a very few recent breeding records in Ireland. At the World Conference on Birds of Prey in 1975, a sharp decrease of Montagu’s Harrier in many western European countries caused particular concern, but the reasons for this remain obscure. The possibility that climatic change is involved in the retraction of the Montagu’s Harriers’ breeding range, and the expansion of that of the Hen Harrier, springs to mind in view of the many examples of changes in the breeding ranges of other birds in which climatic change may be involved. Nevertheless I know of nothing to suggest how such changes could affect these harriers.
Adult male Pallid Harrier
At the present time the Marsh Harrier is another very rare breeding bird in Britain. In 1974, breeding was reported from four sites, three of which were in Suffolk (Sharrock, 1975). This represented little change from the immediately preceding years but, earlier, in 1957–58, the British breeding population was apparently higher, at 12–14 pairs in East Anglia and about five pairs elsewhere, than at any time since the middle of the 19th century. In recent times nesting has been confined to low level marshland but prior to about 1840 the Marsh Harrier also nested quite widely on moorland bogs in England, Wales, southern Scotland and Ireland. Its disappearance from these haunts was partly the result of intense persecution by gamekeepers and others, but also it was probably more severely affected than the Hen Harrier by the drainage and reclamation of much bogland. It still, occasionally, appears in summer in this kind of habitat, in upland country, where I have seen it myself in southern Scotland. Since the middle of the 19th century its status as a breeding bird has nearly always been precarious and it became extinct in Britain for some years at the beginning of the 20th century and ceased to breed in Ireland after 1917. That it is a British breeding bird at all today is probably due to careful protection at its main nesting sites. Marsh Harriers are seen in all months of the year but most breeding birds appear to be summer visitors.
The Pallid Harrier is a very rare vagrant to Britain. It breeds mainly from Rumania eastwards but occasional irruptive movements have produced breeding records as far west as Sweden and South Germany.
Origins and structural adaptations
Most authorities recognise ten species (see Table 1) but others (Voous 1960, Vaurie 1965, Nieboer 1973 et al) give the number of species as nine and regard the South American Cinereous Harrier as conspecific with the Hen Harriers (Hen Harrier and Marsh Hawk).* According to Nieboer, the whole group of harriers probably originated in the Palearctic region or at least in the Old World. He suggests that antecedents of the Hen Harrier may have colonised North America rather early in the Pleistocene era and fairly soon reached South America. The Hen Harrier may have originated as a species in North America and recolonised the Palearctic across the Bering Strait about the time of the last interglacial period. After this colonisation, the isolation of the most westerly population of these birds, in one of the glacial maxima, probably led to the specific separation of the Pallid Harrier, which developed a longer wing than the Hen Harrier as an adaptation to life in arid, open habitat with large seasonal fluctuations in temperature (Nieboer, cf. Moreau 1955, Flint 1971). To escape severe winters it developed migratory habits. Nieboer further postulates that the Montagu’s and Pied Harriers speciated in the Western and Eastern Palearctic respectively during one of the earlier glaciations. Their small size has probably resulted from sympatric living with larger species of harrier, especially the Marsh Harrier which hunts similar wet habitats to the Pied Harrier.
The Black Harrier of South Africa, the Spotted Harrier of Australasia and the Long-winged Harrier of South America may all, according to Nieboer, have originated from the common Palearctic stem. Both the Spotted and the Long-winged are presumed to be old forms, long established in their respective regions, and they show no close affinity to each other. Nieboer considers that it is a matter of speculation whether the Marsh Harriers originated in the Palearctic or in Africa.
The sympatric existence of up to four species of harrier of fairly similar size and weight in the Palearctic region has resulted in the development of specialised differences in wing structure, length of tarsus and the structure and size of feet, claws and bill. A detailed account of these morphological differences has b
een given by Nieboer (1973). He has much interesting information on the ways in which structural differences are connected with hunting techniques and preferred hunting habitats. Schipper’s study of sympatric Hen, Montagu’s and Marsh Harriers is referred to later in this book.
Some examples of the operation of specialised structure may be cited from Nieboer; in Western Europe the larger species of harriers have absolutely, and relatively, longer legs than the smaller species and this may be correlated with the height of vegetation preferred for hunting. Thus, the Marsh Harrier hunts tall vegetation, the Montagu’s low and open vegetation, and the Hen Harrier is intermediate in this respect. In Eastern Asia, the rather similar sized Hen and Pied Harriers prefer respectively drier and wetter hunting grounds, and there it is the Pied and Marsh Harriers (both long-legged species) which are sympatric, particularly favouring bogs and marshes. Hunting competition between them appears to be limited by a considerable difference both in body size and in the dimensions of the toes, claws and bill. In the central Palearctic the Hen, Montagu’s and Pallid Harriers live in association and differences in wing structure may be important in enabling them to select different ranges of prey.
Nieboer emphasises the close affinity between the Hen and Pallid Harriers and likens the latter to a specialised Hen Harrier. He speculates on the possibility that hybridisation between these species might occasionally occur. Frank King believes that a male Montagu’s Harrier may have been mated to a female Hen Harrier at a nest in southern Ireland. No proof of this was obtained and it must be said that hybrids between these two species seem very unlikely to occur.
Various explanations have been offered for the general rule that females are usually larger than males in birds of prey (except the vultures). It is certainly true that in the harriers the role of nest defence, in the prolonged absence of the hunting males, is largely assumed by the females. Their size preponderance, as Nieboer believes, might have developed as an adaptation to this role.
Immature Montagu’s Harrier
* Breeding of the Long-winged Harrier in the north of South America may be uncommon.
* Nieboer points out that the larger females (of Palearctic harriers) may be equally effective hunters, taking larger prey, but the pale colour of males may give them some compensatory advantage in hunting small, agile prey, within the limits of their capabilities, set by their smaller size and weaker feet.
* Although the adult male plumage of the Cinereous Harrier differs considerably from the corresponding plumages of the Hen Harrier and Marsh Hawk Nieboer points out that some adult male Marsh Hawks are heavily marked suggesting an intermediate stage of colour pattern between them and the Cinereous Harrier. Possibly the strongest argument for specific separation of these harriers is the large gap between their geographical ranges. No harriers occur in apparently suitable habitats in the intervening area but they may have been there in the past and become extinct. The Cinereous Harrier may well be best regarded as the Hen Harrier of South America.
CHAPTER TWO
Identification and Plumages
Comparison between Hen, Montagu’s and Pallid Harriers
Montagu’s Harrier, female
Pallid Harrier, female
Hen Harrier, female
The very striking difference between the male and female Hen Harrier, in adult plumage, is shown in colour plate 1. The brown female (about 50 centimetres long) is noticeably larger and heavier than the grey male (about 45 centimetres long). Although the latter is pale enough in colour to suggest a gull, in the distance, his broad, black wing-ends are very distinctive. The characteristic outline and wing action, already described, further preclude confusion with gulls. The female’s white rump, contrasting with her much darker plumage, is correspondingly more conspicuous than that of the male.
Field identification of all Hen Harriers is generally only difficult in so far as separation from other species of harrier is concerned. Yet, when seen from below as a silhouette in soaring flight, a Hen Harrier might sometimes be confused with a Goshawk, since both have long broad wings and fairly long tails. There is little difficulty in distinguishing buzzards from harriers at such times, as the former have relatively shorter tails and are more heavily built.
The difference in outline and build between Hen and Montagu’s Harriers has been likened to the difference between gull and tern but this, of course, is an over-simplification. I recall watching a grey cock harrier approaching from a great distance in the foothills of the Auvergne mountains in France. In the bright summer light it looked as pale as the cock Hen Harriers I had lately seen in Scotland and it was only when it came near enough to see the black centre lines on the wing and the red brown streaks on the underparts that I was certain it was a Montagu’s. The absence of a white rump in adult male Montagu’s is not always helpful in very strong light. Nevertheless, there are real differences in shape and flight between Hen and Montagu’s. The wings of the latter are more slender and finely tapered, showing only three distinctly long flight feathers against the Hen Harrier’s four. (Caution is needed here as I have seen a female Hen Harrier in summer moult with only three long primaries on each wing and no obvious gap where the fourth had been moulted).
In males, the wing of the Montagu’s is distinctly longer than that of the Hen Harrier, but in females there is little difference in this respect. In both sexes, however, the wing-tip primaries are distinctly longer in the Montagu’s. The wing movements of the Montagu’s are generally looser, more willowy than those of the Hen—hence the comparison with tern or gull. Family groups of Montagu’s over the tamarisks and reed-fringed creeks of La Vendée, looked to me altogether slighter and more buoyant in flight than Hen Harriers. I also thought their tails seemed relatively longer and narrower. The shape of the head is another useful distinction: Hen Harriers have broader, more owlish heads and shorter necks than Montagu’s and have a much more obvious neck-ruff.
The American Marsh Hawk is regarded as conspecific with the Hen Harrier of the Old World, but in the adult male Marsh Hawk the black wing ends are not so clear cut, the grey upper parts not so clean looking, and red-brown spots are retained on the underparts throughout life, while these are a mark of comparative youth in male Hen Harriers.
Although many female Hen Harriers have a less contrasted facial pattern than the typical female Montagu’s this cannot, in my opinion, be taken as a reliable distinction. There is great variation among individuals of either species, some Hen Harriers having very striking whitish eyebrows and faces, contrasting with dark ear coverts, while Anthony Buxton photographed an old female Montagu’s which had almost no facial pattern.
Adult females of the Marsh Hawk and Hen Harrier are similar, but juveniles differ as described below. The Marsh Hawk is a slightly heavier and longer-legged bird.
Female Pallid Harriers are about the same size and shape as Montagu’s and look very similar in the field. In a discussion of all the possible ways of distinguishing females of these two species, Lars Svensson concluded that the most useful field character was the pattern of head and neck; in the Pallid the dark ear coverts are contrasted with a light ruff which is virtually absent in the Montagu’s. Thus, in females, both Hen and Pallid Harriers show a more prominent neck ruff than Montagu’s. In the Pallid this looks especially noticeable because the neck below the ruff is darker. Svensson also showed that the contrast between the light cheeks and the dark line through the eye was clearest in the female Pallid. The adult male Pallid Harrier is the palest of the three, but may not look obviously paler than an old male Hen Harrier on the back and wings. It is, however, much paler on head and breast which do not contrast markedly with the white underparts and it has noticeably less black on the wing tips than male Hen or Montagu’s. It has no white rump.
Underwings of (l. to r.): Hen Harrier, female; Hen Harrier female with 5th primary moulted; Montagu’s Harrier; Pallid Harrier
Male and female Montagu’s differ little in size, but male Pallids, like male Hen Harri
ers, are distinctly smaller than females. Between Pallid and Hen Harriers there is a noticeable difference in the shape of the wing, caused by the distinctly longer wing-tip primaries of the Pallid (in both sexes), but these are still longer in the Montagu’s Harrier.
Juvenile Hen Harriers of either sex look very much like older females, but the ground colour of their heavily streaked underparts is a deeper tawny shade. For the first few weeks after fledging, they can be recognised at a distance by their shorter wings and tails, their darker upper parts with some fairly distinct pale cinnamon edgings on scapulars and wing coverts, and their more contrasted tail bars of sepia and cinnamon with a definite, narrow pale terminal band. None of these features persist for long and it must be remembered that freshly grown feathers on adult females may show almost as much contrast in the tail.
Nestlings can be sexed by iris colour and leg thickness, usually when about two weeks old (even at ten days, according to Scharf and Balfour, 1971), but in some instances this cannot be done reliably for a further week. Small chicks have very dark brown eyes which become chocolate brown in females and cloudy grey in males. It is usually fairly easy to distinguish the stouter legs of females from the thinner legs of males but special care is required if there is much difference in the ages of a brood. Details of the changes in the appearance of nestlings are given in Chapter 6. At fledging, females may be over 100 grams heavier than males. Young Montagu’s and Pallid Harriers can be distinguished from young Hen Harriers, as soon as they are feathered, by their unstreaked vinous brown underparts. Some European readers may be surprised to learn that the underparts of young Marsh Hawks are the same rich colour, streaked on the upper breast only.*