The Hen Harrier

by Donald Watson

  I often wondered how Hen Harriers which were newcomers to the district discovered the roost. Although hunting ranges were widely dispersed and it was unusual to see two or more harriers hunting within sight of each other, a newcomer was perhaps able to make visual contact with birds familiar with the roost and to follow their flight direction. A second year male was once seen apparently following an older male into the roosting area, and on many occasions the arrival of two or more birds within sight of each other was consistent with the above suggestion. It was not known whether the same birds ever occupied the same ‘beds’ on successive nights.

  Most of the birds arrived at the roost with full crops. This was easy to detect in females (if they flew close enough) because a full crop bulged like a pouch, but it was much less obvious in males. The difference presumably reflects the difference in average weights of prey taken by the sexes. Further observations would be necessary to find out if a significantly higher proportion of birds regularly arrived with empty crops after days of bad hunting weather, but on 24 November 1974, a very wet day, I noted that of those clearly seen none had full crops.

  Harrier relationships at the roost

  Birds which had found a roosting spot were frequently displaced by others. As might be expected, this was most common when the total number of birds was high. Usually, ringtails drove out males; and I had the impression, from relative size, that if a ringtail was displaced by a male it was always a first year male. A female often flew deliberately to a spot where a male had settled and drove him out by diving with lowered feet. On one extraordinary occasion, a large female dived at a spot where we had seen a male drop and, as if by a conjurer’s trick, two males sprang up from the same spot. There was also an indication of a hierarchy according to age, because some birds would be driven out more than once on the same night and whenever these were clearly seen they were recognisable as first or second year males. When the roost was most crowded, one or two of these ‘junior’ birds sometimes flew to and fro for a long time and only found an undisturbed spot after all the others had settled.

  Gurr (1968) has suggested that in the case of the Australasian Harrier, communal roosting may facilitate pairing. At roosts of Hen Harriers in Belgian Lorraine, behaviour which suggested that birds were paired or pairing was observed several times in March or April. A male was repeatedly seen to leave a group of harriers already at the roost and fly to meet a distant incoming female. The male and female then flew into the roosting area together but subsequently separated. When, in April 1969, only one male and one female were continuing to use a roost, whichever arrived first would wait for the other and fly out to meet it as soon as it appeared in sight. The two birds then settled beside each other (Mois, 1975).

  The following observations at Roost 1 may indicate paired or pairing behaviour:

  1On 17 March 1970, two adult males—one a noticeably dark, blue-grey bird, suggesting some melanism—had alighted conspicuously on tussocks of yellow grass, prior to going to roost. The lighter male was approached by a female which alighted within a few metres without aggression and so they remained for several minutes. Shortly afterwards, the same or a different female had settled beside the darker male and these two had also stayed together amicably for some minutes. Later they dispersed to roost separately.

  2On 19 December 1971, a female circling over forest near the roost was approached by two incoming males. They all circled together for a minute or so; then one male closely followed the female into the roosting area and settled beside her at a ‘pre-roosting’ spot on the ground. The second male approached again and both males rose and flew low over the roosting ground, one just below the other in the manner described by Balfour as the ‘escorting’ flight of a bird ‘seeing off’ an intruder on the nesting ground.

  3On 4 February 1973 (cf 17 March 1970), two adult males alighted close together, conspicuously, on tussocks in the roosting area. I observed a female rise from where she had been perched in the open and fly to a roosting site among rushes. The lighter-coloured of the two males immediately flew to where she had pitched and almost stalled above her. He then swung away and dropped into a roosting site a little distance away. There was no conflict between the two birds. Shortly after the second, darker male, evidently a fairly young bird, flew and attempted unsuccessfully to displace the light male, then settled elsewhere. The light male rose again and was displaced from another roosting site by a different female. He settled to roost a third time in a new site and a female, from the rushy site, almost certainty the bird he had ‘visited’ earlier, rose and flew directly for at least 50 metres, to drop close beside him without any attempt at displacement.

  4On 6 December 1973, an adult male and a ringtail (? female) circled together over the roost for five minutes, the male flying briefly with loosely flapping wings, suggestive of display flight. They then dropped, apparently to roost, in the same area but the ringtail twice put up the male and thereafter, for another seven minutes, the two birds circled close together, much like a pair in spring, and finally settled to roost not far apart.

  5On 24 December 1975, in the morning, an adult male and a female, after rising, alighted and perched for several minutes within a metre or two of each other on a hillock in the roosting area.

  Even as late as 1 April a female was seen to displace a male in an apparently aggressive manner, but the female of a pair on their nesting ground dives at her mate in much the same way. The obvious tendency for females to displace males at the roost should perhaps be regarded as an aspect of sexual behaviour, rather than simple dominance by the stronger over the weaker. It is perhaps worth noting that the roosting ‘beds’ of harriers are rather similar to their actual nests.

  As already mentioned, high soaring flight before settling to roost was often seen on fine windy nights. Mois recorded similar aerial manoeuvres over a roost in Belgian Lorraine, in the same weather conditions. There, however, females rarely shared in these flights, while in Galloway both adult males and ringtails, which certainly included some females, took part. Up to eight harriers were seen behaving in this manner together, sometimes diving at and buffeting one another, apparently playfully. On these occasions they looked not unlike kites. In common with the habit of perching conspicuously before roosting, these flights might advertise the presence of harriers at the roost and attract others. Newcomers, and birds which had hunted unsuccessfully, could recognise birds which arrived with full crops and might profit from observing their flight-lines from hunting grounds.

  Hunting at the roost

  In the early days of roost-watching, I thought that the attraction of the marshy site might be partly for crepuscular feeding. Certainly it looked an excellent habitat for water voles and there was some possible bird prey, such as Snipe, Reed Buntings, Meadow Pipits, Skylarks, Wrens and occasional Stonechats. Larger birds might also have been taken, since a few Red and Black Grouse regularly flew to roost on the same ground as the harriers. Yet the harriers showed little interest in prey at the roost, quite often disturbing small birds or grouse which they made no attempt to capture. On a few occasions, early arrivals did appear to be hunting the margins of the area; for instance, on 21 October 1970, three males were hunting and one was seen to rise from the grass with small prey, and on 15 February 1974, a ringtail dropped from a post and flew up with a small capture. On another night, a ringtail made a rather casual dive at a Snipe which had risen ahead of it but did not press the attack. Remains of water voles and beetles, including the great water beetle, were found in harrier pellets at Roost 1, and it is possible that such prey was taken after the birds had settled on the ground. The overall impression, however, is that the roosting ground had little significance as a hunting area. Red and Black Grouse were alarmed and rose only when a harrier flew close above them and they continued to share the roost with the harriers throughout the whole period. A pair of Stonechats frequented the harriers’ favourite roosting patch for several weeks during October–November of one year. They
disappeared later and it is possible that they were taken, but the habitat was only marginally suitable for them and it is equally possible that they moved away. Dickson records regular pre-roost hunting around the rhododendron bushes on the perimeter of Roost 2. Black Grouse also roosted among the harriers there.

  Relations with other birds and mammals including man

  Apart from possible prey species, the only other birds seen at the roost which ever caused any reaction from the harriers were crows, owls and other birds of prey. Harriers sometimes dived at Carrion Crows or the latter chased and mobbed harriers. A Barn Owl frequently hunted over the roost, though mostly around the perimeter. Both it and the occasional Short-eared Owl became involved in brief aerial tussles with harriers, but a Barn Owl hunting the perimeter was usually ignored. At Roost 2, aerial conflicts between harriers, Short-eared Owls and Merlins were common and I have described in Chapter 4 how a Merlin and a harrier competed for prey there. At Roost 1, Merlins, Sparrowhawks and Peregrines were all irregular visitors; of these only a Peregrine, which perched on a hillock in the roosting ground, received a sustained diving onslaught from a male harrier, which failed to dislodge it. A Sparrowhawk once joined two harriers in a high circling flight, and all three birds stooped playfully at each other.

  Foxes were occasionally seen at or near all three major roosts but the tall vegetation made it impossible to follow their progress. They were never positively seen among the harriers, but Dickson saw twelve harriers at Roost 3 circling, diving at and following an unknown predator which was probably a fox. On other occasions, a number of harriers were seen to rise from their roosting places for no apparent reason, occasionally chattering in alarm, and the presence of a fox or other ground predator was suspected. Otters haunted the margin of Roost 1 in some winters and once I saw a ringtail dive at the spot where two otters had just disappeared into tall vegetation. On 8 December 1969, I experienced a communal reaction to my own presence while I was watching the roost from a slightly exposed viewpoint. A ringtail approached overhead, landed briefly on the stream bank some 50 metres away and began to chatter. She then flew towards me, hesitated, but returned to make a low level attack with arched wings, exactly like a bird in defence of its nest. She climbed again, without striking, then repeated the attack. Her chattering cries were taken up by at least five more birds which left the roosting ground and approached me. Two more ringtails and a male circled close above my head without attacking. As soon as I gained some concealment the chattering ceased and the birds drifted back and began to settle again. It had been a remarkable and slightly eerie episode and I could not help wondering if the ringtail which attacked me was a local breeding bird, which might have recognised me as the particular villain who had infuriated her by visiting her nest (see Balfour’s comment on recognition of himself in Chapter 6). Dickson had a rather similar experience on the night when he discovered Roost 3; first a ringtail rose, chattering, after which other harriers began to call and circle overhead. This behaviour was repeated on later occasions when the birds were disturbed at this roost.

  The influence of weather on numbers of harriers at the roost

  Although no other roost has been found in the neighbourhood of Roost 1, at times some or even most of the harriers in the district evidently roosted elsewhere. On a number of occasions birds flew into the roost area and passed out of view without alighting, but it was impossible to be certain that the same birds did not return later. Sometimes birds rose and left the roost after they had settled; for example, on 27 December 1970, a ringtail settled and rose again three times before spiralling high into the sky where it was joined by three other ringtails; all four then flew off, losing height as they went, as if about to land, distantly, in the forest.

  Throughout all the winters, the numbers of harriers at Roost 1 showed marked fluctuations in different weather conditions. The highest counts occurred in windy and, usually, fairly mild weather, and the lowest on calm and generally cold nights. The roost was always sparsely attended in hard frost. Diurnal sightings (see Table 30a) do not indicate any marked change in the numbers of harriers hunting the area, within 24 kilometres of the roost, between October and February, thus supporting the view that a proportion must sometimes have roosted elsewhere, probably within this area. The following observations may throw some light on this question:

  1Just before sunset on 20 November 1976, I saw two ringtails drop as if about to roost in tall grass at the edge of a young forest plantation about 16 kilometres north-east of Roost 1. They had been hunting the forest area in company. Weather was cold with light wind. None roosted in this area on 21 or 22 November, although there was no marked change in the weather.

  2About sunrise on 2 December 1976, in very cold, calm weather, Philip Coxon flushed a ringtail from a marshy islet close to a river bank, in a valley about 8 kilometres from Roost 1. It seemed highly probable that the harrier had roosted on the islet.

  I can only speculate on how and why the weather influenced the numbers attending Roost 1 whereas it did not do so, significantly, at Roost 2. It seemed unlikely that a large alternative roost remained undiscovered near Roost 1. It may have been important that the best hunting grounds appeared to be more widely dispersed from Roost 1 than from Roost 2. This may have been particularly true when hard frost or snow reduced the effective hunting grounds in the near neighbourhood of Roost 1; the harriers might then conserve energy by remaining to roost on or near the more distant hunting grounds, either singly or in groups. In contrast birds at Roost 2 could nearly always find good hunting grounds in the surrounding low-lying coastal area which escaped the worst severities of the weather. My own guess is that harriers find travelling flight much easier when the wind is fresh or strong, from whatever direction, and that lengthy journeys consume most time and energy in windless conditions (they might then remain to roost, either singly or in small groups, on or near the more distant hunting grounds). In late winter some might roost on local breeding grounds, although none were proved to do so before mid-March.

  The shelter provided by the communal roosting sites could obviously be a special attraction in wet and windy weather. On particularly stormy nights the birds tended to crowd into a small area; this would be an advantage in mutual warning of danger, especially through alarm calls. Although the marshy roosting grounds were obviously more accessible to foxes in frosty weather, no other site was likely to be safer, unless it was in trees well clear of the ground or, perhaps, on an island as suggested by Philip Coxon’s observation. On the very cold night of 26 December 1976, I noticed that the single harrier (a male), which came to Roost 1, dropped into the only part of the roosting ground which was not white with frost.

  Seasonal fluctuations at the roosts; sex and age structure of the roosting population

  Counts at Roost 1 in all the winters since 1967–68 are shown in Fig. 13. Monthly aggregates of peak counts over eight winters, at Roosts 1 and 2, are compared in Figs 14 and 15. The highest nightly totals tended to occur in November at Roost 1, and in December and January at Roost 2. These probably reflect winter immigration into the region and onward movement to lower ground as winter advances.

  At both roosts adult males often made up a high proportion of all the birds early in the winter, ringtails usually predominating from November onwards. At Roost 1, adult males regularly outnumbered ringtails in October. (It may be recalled that Frank King found up to ten adult males, with no ringtails, at a roost in south-west Ireland in October.) It is possible that this early winter predominance of adult males consists largely of local breeding birds remaining behind after many females and young have migrated or dispersed (see Chapter 7).

  Fig. 13 Numbers of Hen Harriers at Roost 1, Sept.–March, 1967–76

  Some of the greatest fluctuations in numbers at Roost 1 appear to be correlated with changes in weather conditions, as already explained; for example an exceptionally high January count of at least 30 birds, with adult males and ringtails about equal, occurred after m
ore than a week of westerly gales and torrential rain. It is not known whether such mid-winter peaks were caused by fairly local changes in distribution, or by longer distance immigration from regions which might have been evacuated owing to poor hunting conditions or a reduction of available prey, in unfavourable weather. No immediate correlation between changes in numbers at Roosts 1 and 2 was evident. It seems likely, however, that Hen Harriers wintering over a very large area form a dynamic population between different gathering points, at roosts, as suggested by Mois for Luxembourg, the Eifel district of Germany and north-east France.

  Small peaks in March, at Roost 1, may have been caused by migrants returning to the vicinity of breeding grounds; these did not occur at Roost 2 which was more distant from any breeding locality. The strongest suggestion, that some birds which bred near Roost 1 sometimes used this roost, comes from the following observations. On 30 March 1975, I saw no sign of harriers during a day spent largely in a breeding area. On 1 April, six harriers, including at least four adult males, came to the roost. On 6 April the roost was deserted, and on the following day a pair were over the breeding area all day and roosted there.

  It may also be noted (see Figs 13 and 14–15) that the highest numbers occurred at Roost 1, in the winter of 1969–70, and that in the following summer there was a small peak in the local breeding population.

  It was impossible to judge whether female harriers at the roost were first year or older birds. First year males could be distinguished from females, by size and shape, if clearly seen. They were never numerous but one or two were generally present among every ten to fifteen ringtails. Among the birds classed as adult males, a remarkably high proportion (usually more than 75% at Roost 1) were considered to be second winter birds, judging from their very dark (dusky or dusky-brown) mantles. Dickson commented that a high proportion of males at Roost 2 were likewise. This might be an indication that few males survive long enough to attain the full grey plumage; or possibly that, for some reason, older males were disinclined to join the communal roosts.