The Hen Harrier Read online

  Clearly, this aggressive trait is a disadvantage where Hen Harriers are in danger of being shot at the nest but, of course, it must have been established long, long ago and it is easy to see that it might deter many potential predators of eggs or chicks. Dalmen and Lorig (1970) found that in polygamous situations, in the persistent absence of the female, nests near a colony of Herring Gulls sustained heavy losses of eggs and young from the gulls; but one nest, which was equally close to the gulls, remained undisturbed on account of the continuous protection given by the female (Nieboer, 1973).

  Is it possible that the recent spread of nesting into conifer forests has, in some way, favoured the survival of less aggressive birds? In a small sample of nests in south-west Scotland (see Table 4), I have found a much higher proportion of relatively mild females at forest sites than on open moor; and David Scott tells me that Irish Hen Harriers, many of which nest in forests, are ‘much less vicious than they were’ in defence of their nests. The numerous examples of successful colonisation of this habitat seem to indicate that any such loss of aggression has not been disadvantageous.

  Apart from man (the Hen Harrier’s greatest enemy), what are the main threats to nests? The evidence is scanty. A well-grown nestling was found in a fox earth in the Southern Highlands (G. Shaw, pers. comm.), but there was no proof that it was taken alive; and a brood was probably killed by a fox in Galloway (Chapter 12). Hagen (1957) said that dogs killed broods in Norway, and Hamerstrom listed nest failures due to predation by skunks, and trampling by deer and cattle, in North America. Dixon (1898) was told by game keepers in Skye that sheep broke many eggs. Crows, and possibly Black-backed Gulls (see Table 15), destroy eggs, though it is doubtful whether they can do so, often, once incubation has properly begun. Weis, however, found that a Jay succeeded in snatching all five eggs of a Montagu’s Harrier while the female was briefly absent for a food pass. I am informed by Dr K. W. Brewster that badgers are a danger to Montagu’s Harriers’ nests. Probably, even the least bold Hen Harriers attack birds such as crows, Ravens, large gulls, Buzzards and Golden Eagles in the vicinity of the nest. It is likely that eagles occasionally kill young harriers and I have presumptive evidence that an eagle killed an adult female (see here). I have found a young Short-eared Owl among prey at a Golden Eagle’s nest. With regard to the possibility that forest nesting sites might be more favourable to unaggressive birds than sites on open moorland, it is worth noting that, in forests, there is no threat to nests from wandering sheep, cattle or dogs. Foxes are usually common in forests but the dense cover which surrounds nests in this habitat must be some safeguard against these animals. Foxes are absent from the important nesting grounds in Orkney and the Outer Hebrides, but very successful nesting has occurred in many regions where foxes are plentiful.

  It may be asked why the remarkable habit of attacking man, and other large mammals, occurs in some harrier species, such as the Hen and Pied, but not, so far as I know, in the Montagu’s or the Marsh Harrier? Possibly this disparity derives from an original preference by Hen and Pied Harriers for drier, more open nest-sites, which would be most at risk from mammalian predators and the trampling feet of herbivores; but if its ancestors invaded North America from the Asian Steppes, why did the Marsh Hawk retain the aggressive habit while becoming a predominantly marshland nester in the absence of other harriers? Or is there some significance, which eludes me, in the reported tendency for the male to be more aggressive than the female in this race of the Hen Harrier?

  The question must be asked whether human visits to nests increase the risks of predation by foxes or other mammals such as badgers. Scharf told Balfour that human visits to Marsh Hawk nests could increase such risks. It is possible that my own visits to nests have occasionally had a similar effect but the high success rate of nests in forests (Table 12), where it is difficult to avoid some tell-tale trampling of surrounding vegetation, suggests that this did not happen. I have, however, generally refrained from disturbing a bird on eggs and, with one exception, have kept an interval of a week or, usually, longer between visits to nests with chicks. The exception was the nest watched from a hide. There, regular visits inevitably left a beaten path but no predation occurred. Nest photography from a close hide sometimes brings unexpected hazards, as Don Macaskill honestly admitted when he discovered that one female reacted to the sound of the camera shutter by flying away with the food which she had just received from the cock. The male landed repeatedly at the nest, but would not leave food, and photography had to be abandoned. Unfortunately, not all photographers are as scrupulous. It is only fair to add that there has been no evidence that nest failures have been significantly increased even by research work involving the capture, by dho-gaza nets, of birds which had chicks, either in Scotland or in North America (Balfour, Hamerstrom).

  Shortly before fledging time, when the nest is littered with prey remains, flies settle increasingly on chicks, especially about their eyes and bills. This is particularly noticeable at forest nests, and on hot days in July I have seen chicks with their eyes totally obscured by flies. Clouds of flies accompany a man to a nest and then transfer to the chicks. I have found some chicks infested with flat flies and fleas. Broods of Pied Harriers in south-east Siberia were sometimes found by Neufeldt to be injured, their nostrils blocked and eardrums perforated, by the larvae of flies of the genus protocalliphora. Hamerstrom found one brood of downy young Marsh Hawks eaten alive by carrion beetles.

  Moult of the adult

  Females begin to moult during the incubation period and continue to do so until after the young are fledged; but Balfour found that females were never missing more than two flight feathers and some tail feathers at one time. Middle and inner primaries with their coverts, the greater wing-coverts, tail feathers and some body feathers are moulted and renewed first. The complete moult may extend over several months but I do not know precisely for how long. Grey-plumaged males begin to show signs of wing and tail moult later, usually not until the chicks are a few weeks old. Though wing and tail moult is gradual, it might be thought that it would reduce hunting efficiency; and it may indeed be a factor in the reduction or cessation of hunting by males, for their broods, late in the season.

  In Marsh and Montagu’s Harriers, as observed by Weis, the females also moulted ahead of the males and the latter then become sole providers for the young. In Pied Harriers, Neufeldt found that females began to lose feathers soon after incubation started and in this species, too, moult began considerably later in the male, which changed only six primaries and half its tail feathers before migrating south. Hen Harriers in their first summer might be at some disadvantage in providing for chicks since males, and probably females also, start to moult wing and tail feathers earlier than older birds. According to the Handbook of British Birds, some first year males have begun to moult as early as March and I have myself seen partly grey-winged males in this month.

  Female Hen Harrier, second summer. Note dark new feathers; inner primaries and their coverts, greater coverts (all newly grown), bastard wing, tertials, lesser coverts, all tail feathers except central one (2 outer, growing); some new feathers on mantle and rump.

  Male Hen Harrier, first summer (July). Note four new grey inner primaries and their coverts; old greater coverts mostly moulted and some new feathers growing; a few new feathers on breast and one in bastard wing.

  The post-fledging period

  After they have made their first flight, young Hen Harriers continue to use the nest or its near vicinity as a base at which to receive food, and to roost, for two or three weeks. During the day they are often well scattered, perching quietly among the heather, or on small conifers, for long periods. Lengthy intervals pass between visits by the adults, chiefly the female, with food. Frequently, the food is dropped in the air to be caught or missed by a flying youngster, which thereby gains some experience at taking a food-pass, though it may be argued that this can only have value as practice for a young female. Young give the squealing food
call as they fly towards their parents for food, but they also make this call at other times—I have heard it from a fledgling as it rose in front of me in alarm. As mentioned earlier, the fledged young of a bold parent are sometimes aggressive to man. Within a week of fledging they become fairly proficient on the wing, improving their skill by diving and playing together in the air. They also dive towards the ground and pounce frequently. As already stated, young Marsh Hawks soon learn to capture insects but the majority of their pounces bring no reward.

  The rich dark colours and shorter tails and wings—the latter are also more rounded and Sparrowhawk-like—distinguish the young at a distance from the old female, and size differences are quite evident between young males and young females. Among the bronzy-green and purple heather clumps the young harriers can be difficult to spot but they become conspicuous against a background of vivid green bracken or conifer trees. I have seen them alight and perch without difficulty on tall bracken tops.

  Lewis O. Shelley (1930), quoted by Bent, related his experience with a family of young Marsh Hawks which he reared from the nest. He said that their sight and hearing were unsurpassed. ‘Any noise, and a good many too slight to be detected by human ears, was noted instantly.’ One flew from 150 metres to take a small cube of meat from his finger tips, seizing it in one foot without slackening its flight.

  To a harrier enthusiast it is always a satisfying sight, though tempered with end of season regret, to watch the young venturing further and further from the nest site. I have watched them for hours as they float just above a long slope of dark green forest, but never drop to make a successful capture.

  At what stage are they totally independent of their parents? I confess I do not know precisely, but unless the family keeps together after leaving the nesting grounds, and such wandering groups are rarely seen, they must fend for themselves by the time they are about eight weeks old. Shreds of evidence which suggest that some young may stay with the female parent for longer periods come from observations by R. C. Dickson and Balfour. Dickson saw a female with a young male and a young female fly in to a winter roost site only days after a similar family had left a moorland nesting ground some 15 kilometres away; and Balfour told me that he had seen a female with her brood of two at a roost that was distant from their nesting ground. Weis said that groups of Montagu’s Harriers consisting of old females and young of the year roosted together before migrating in autumn, the old males roosting separately. The flocks of up to 40 Marsh Hawks seen moving together in autumn over the western prairies, by Audubon (Bent, 1937), may have included family parties.

  From their data on Orkney Hen Harriers, Balfour and Cadbury concluded that only 32% of young Hen Harriers were alive a year later; and Hickey found that 80 of 102 Marsh Hawks which had been shot in their first year were under five months old. To survive these early months of independence the young birds need to become not only expert hunters, but to avoid a whole range of man-made hazards such as guns, traps and overhead wires. They may also run some risk of being killed by a few predators such as foxes, and possibly Golden Eagles or Peregrines.

  Short-eared owl

  * I have recently noticed that the brown upper parts of some young males are slightly lighter than in some young females. See also Chapter 2.

  CHAPTER SEVEN

  Migration and Winter Distribution

  Hen Harriers begin to migrate southward from most of their more northerly breeding grounds in Continental Europe, Asia and North America in late August and early September. In the less severe climate of Western Europe, however, overwintering birds are found as far north as southern Sweden, Denmark and Scotland, where Orkney (Lat. 59° N) is the most northerly, major wintering area in the world. In North America, few remain further north than Lat. 50° N. Mead (1973), writing of European Hen Harriers, gave the normal southern limit of winter range as the Mediterranean but noted that there are records from north-west Africa (Vaurie, 1965).

  In Asia, the winter range extends as far south as Iran, north-west Pakistan, northern India, Upper Assam, Burma, Indo-China and possibly the northern Philippines (Brown and Amadon). Ali and Ripley (1968) described it as ‘probably the commonest harrier in the Sikkim Himalayas’, but Smythies (1953) cited only one positive record for Burma. My own records for the Arakan district of Burma in 1944–45 included only occasional, probable (ringtail) Hen Harriers.

  The winter distribution of the Marsh Hawk extends as far south as Panama in Central America, but it occurs only rarely in the West Indies. There are very occasional records in South America, from Colombia and one in Venezuela (Bent, Brown and Amadon). Most Marsh Hawks winter in the southern states of the USA but several have wandered as far west as Hawaii.

  Families of Hen Harriers have generally left the immediate vicinity of breeding grounds three to five weeks after the young have fledged but, in Britain, most recoveries* of young birds before November of their first winter have been close to where they were hatched. Apart from two possibly exceptional recoveries,† only five out of thirty show movement of more than 30 km in this period and the longest journey by a juvenile, in the period August–October, was only about 140 km, from Orkney, to Shetland, by 4 September. The next longest journey by a juvenile in the same period was also in a northerly direction—82 km from Kincardineshire to Banff by 9 August.

  The record of a ringtail seen crossing the Wicklow coast, on 17 August (Irish Bird Report, 1961), suggests the possibility of an early long distance movement. Autumn movement over the Irish Sea is further indicated by the comments from bird observatories in this area; at Calf of Man, the Hen Harrier has become a regular October visitor since about 1965; and at Copeland Island, off the coast of County Down, it appears especially often in late September (Durman, 1976). There is also a record of one seen flying in over the sea at Clogher Head, County Louth, on 19 September 1973 (Irish Bird Report, 1973). Hen Harriers are much less frequent late autumn migrants at the Isle of May, off the east coast of Scotland, and further south at the Gilbraltar Point observatory they are seen more often in January and February than during the usual periods of passage migration. The only observatory in Britain or Ireland which notes them as most frequent in spring is Walney Island, off the Lancashire coast (Durman, 1976).

  From late September onwards, there is widespread evidence from parts of Britain where few or none are seen in the breeding season, that Hen Harriers are either passing on migration or arriving in winter quarters, but few reach southern England before October. The main period of southward movement at Falsterbo (Rudebeck, 1950) and at Col de Bertolet (Thiollay, 1966) is between late September and late October and the first migrants reach Belgian Lorraine from 25 September (Mois, 1975). There is evidence, in Europe and North America, that some ringtails precede adult males on southward migration. The earliest autumn arrivals in southern England are generally ringtails, which also predominate throughout most winters (Reading Orn. Club Reports, Buxton 1946, Bryant pers. comm.).

  In America, Maurice Broun’s records for 1934, at Kittatinny Ridge, Pennsylvania (Bent, 1937) showed that most of the 51 birds seen up till 19 October were ‘females’ but of 38 observed between 1–12 November, 28 were males. It is interesting to recall that the French name, Busard Saint Martin, which undoubtedly referred originally only to the adult male, arose from the belief that it arrived in France about 11 November (St Martin’s Day). It is possible that the majority of those ringtails which travel south in advance of the adult males are birds of the year.

  According to Brown and Amadon, Hen Harriers are distinctly gregarious on migration, except in America where they usually migrate singly. Audubon (Bent, 1937), however, wrote of the Marsh Hawk in 1840: ‘I have observed it in our western prairies in autumn moving in flocks of 20, 30 or even as many as 40 individuals and appearing to be migrating, as they passed along at a height of 50 or 60 yards, without paying any attention to the objects below; but on all these occasions I could never find that they were bent on any general course more than anothe
r; as some days a flock would be proceeding southward, on the next to the northward or eastward’. Apart from one not wholly convincing description of a flock of ten travelling south in a mountainous part of Gwent, Wales, on 16 September (Gwent Bird Report, 1975), I know of no British records of migrating flocks.

  There is convincing evidence, in Europe, that as winter weather becomes more severe some northern regions are almost entirely deserted by males but a number of females remain behind. No doubt the females’ ability to hunt larger prey, even when ground is under snow, enables them to survive better than males in such conditions. Sorensen, for instance, found that in Denmark females noticeably outnumbered males in the coldest periods of winter (Mois, 1975) and the same is true of north-east Scotland (Picozzi, pers. comm.). At the same time there are strong indications that a higher proportion of males winter in the milder western parts of Britain than in the north or east. F. M. Ogilvie, for instance, remarked that during a number of winter visits to the Outer Hebrides, 1900–14, when he sometimes saw seven or eight Hen Harriers in a day, adult males exceeded ‘females’ by five or more to one, ‘the exact opposite of his experience in East Anglia’. Peter Strang told me that most of the Hen Harriers which he saw in the southern part of Kintyre during the winter were adult males. In Galloway, another western region, the proportion of adult males and ringtails in the winter population is variable but the former sometimes equal the latter or even exceed them in numbers during the early part of the winter. Immigration to this region evidently occurs on a considerable scale, especially in October–November, although the only proof of such immigration comes from a December recovery of a first-winter male, ringed in Kincardineshire in July.*