The Hen Harrier

  Very little human disturbance occurs in young forest plantations during the harriers’ breeding season. Forest rangers kill deer, foxes and crows in harrier nesting areas, but usually avoid unnecessary disturbance of nesting birds. Lack of human persecution, better concealment of nests and, probably, less risk of disturbance by man in the early stages of nesting, seem to be the most likely advantages of nesting in forest rather than on moorland, in this region.

  CHAPTER FIFTEEN

  Breeding Data

  The earliest nest-building was seen on 19 April. In 14 out of 27 nests for which the date was known or could be fairly certainly calculated (from hatching dates), incubation began between 1–15 May; in six nests it began between 16–31 May; in three nests between 27–29 April; and in another three between 3–11 June. In many instances the first egg was probably laid two or more days earlier than these dates.

  At 14 out of 21 nests with known hatching dates, the first chick hatched between 1–15 June; at three nests between 16–31 May, at two between 16–30 June; and at another two nests between 1–15 July.

  The earliest fledging date was 1 July, the latest 17 August. The shortest fledging period was 29 days (possibly due to disturbance) and the longest was 38 days. Most chicks were able to make short flights at 32–34 days.

  Fifty-one nests were known to contain eggs or young. Of these, 25 were on moorland, and 26 in conifer forest. Thirteen of the moorland nests were in Area A, ten in Area B, and one each in Areas F and G. Sixteen forest nests were in Area C, seven in Area E, and one each in Areas H, J and K. There were no nests in Area D. Details of clutch and brood size and fledging success, where known, are given in Table 10. Information on clutch size is very incomplete for forest nests because many nests were left unvisited until after hatching, to minimise disturbance. Total accuracy, in numbers of eggs laid and young hatched, is impossible without more regular nest inspection than was deemed justifiable. However, my figures enable some conclusions to be drawn and comparisons to be made between the success of nests in moorland and forest situations.

  Tables 10–15 do not take account of possible annual differences in breeding success due to changes in food supply, or climatic variations. The main period of nesting on Moorland A covered different, earlier years from the main period of forest nesting; nesting on Moorland B, however, largely coincided with the latter.

  The percentage of clutches which hatched was higher on Moorland B and in the forests, than on Moorland A, although the percentage of nests from which young fledged was similar on the two moorland areas (Table 12). Young were fledged from a significantly higher proportion of nests in forests than on moorland probably due to the lack of human and other disturbance in the forests.

  Complete failure was most common on the grouse moor B, probably due to destruction of nests with young by gamekeepers. (See Tables 13–15.)

  Notes on Table 15

  Unless otherwise stated all nests were found empty after failure. At nests 18 and 19, flank or tibial feathers of the adult were found at the nest, suggesting that they might have been lost during a struggle with a predator, or, possibly, that she might have been killed. Nests 1, 6 and 7 were probably deserted at an early stage, possibly before the start of incubation. They might have been raided by crows while the female was off the nest; and human disturbance may have been a factor at these nests, and possibly at nest 2. There were ten or eleven examples of total loss of a brood. Excluding those on Moorland B, it was very difficult to decide whether man, fox or some other predator was responsible. Total loss of young broods might have been caused by very wet weather affecting food supply, but this possibility could only have occurred at nests 5, 19 and 20. The three clutches which did not hatch raised the possibility of contamination by toxic chemicals, but the analysis of three eggs from nest 4 showed low residues (Table 16). The female at this nest was thought to have been neglected by the only male seen in the area, in a presumed polygynous group, and she may have been forced to hunt for herself during incubation, with possible, consequent chilling of the eggs during wet weather.

  Notes on sex and age ratios in the population

  A minimum of 51 females and 47 males bred during the study period. Two instances of presumed polygyny occurred: in 1964 there was almost certainly only one male to three females with nests on Moorland A; and in 1975 a single male was attending three females with nests in Forest E. In the latter case a second pair had been present on 24 April but only one male was seen in the area thereafter. There was, therefore, no significant excess of females over males, such as occurs in the much larger breeding population of Orkney, where females may be at least twice as numerous as males. A noticeable excess of females in the breeding population of south-west Scotland might have been expected in view of the predominance of females among fledged young, but it may have been prevented by greater mortality of females. There is a little evidence for this, from ringing recoveries. Breeding females were more vulnerable than breeding males to human persecution.

  A total of 59 chicks consisted of 37 females and 22 males. Of these 33 females and 19 males fledged. In 14 nests in which all the fledged young were sexed, there were 25 females and 17 males. Females outnumbered or were equal to males in 14 broods, and males outnumbered females in two broods with young surviving to an age at which they could be sexed. There was therefore some indication that, either females survived better than males, or more females were hatched than males, but the difference was not statistically significant.

  Of the eight recoveries of ringed birds (excluding one male and one female found dead in or near the nest), six were females and two had not been sexed. In only two instances was the cause of death fairly certain: one female was shot and the other was probably killed by a Golden Eagle. There was a strong possibility that shooting or trapping accounted for some of the other recoveries.

  CHAPTER SIXTEEN

  Food and Hunting Grounds in the Breeding Area

  Prey identification in the breeding season was made from: (1) fresh kills or remains, mainly from nests or feeding places used by well-grown young near nests; (2) observations from a hide at one nest in July–August 1975; (3) pellets found at or near nests.

  It is clear that birds were much the most important prey. Field voles were the only mammals of any significance as prey and were a surprisingly small component of the whole (see Tables 17 and 18). Table 18 shows that the results from prey collections and fresh kills, observation from a hide and pellet analyses are strikingly different. The percentage of grouse or other large prey is clearly exaggerated by the figures for remains and fresh kills found at or near nests because most of these were collected when chicks were nearly fledged, or even later, when harriers commonly left only the bones of such large prey at nests, whereas smaller prey was eaten more or less entirely. The hide observations and the pellet analyses each show a high proportion of passerine birds and a much lower proportion of grouse. Beetles occurred quite commonly in pellets but were obviously of minor importance as food. Nearly all pellets contained a fair amount of plant matter, especially Molinia grass. A comparison with the results from analyses of winter pellets (see Table 27), shows that mammals of several species formed a more considerable part of the diet at that season.

  Observations of food passes at all stages and at many nests suggested that most prey brought before the young were three–four weeks old, was small; and as, during this period, remains were rarely left in the nest, very little was identified. I thought that most small prey items were passerine birds. Larger prey was more often seen in food passes just before and after the fledging period. It is quite clear that Red Grouse were by far the commonest species in the range of larger prey, while Meadow Pipits and Skylarks were the small species most often taken. The weight of a four to six week old Red Grouse is about eight times that of a Meadow Pipit (say 120 grams to 15 grams) so, from my prey data, the percentage of grouse by weight would be high, but if many of the grouse were less than four to six weeks old, it would
obviously be much lower. I do not consider that my data permit any positive statement on proportions of prey by weight, but it is obvious, as Tubbs found for the Buzzard, that if harriers can catch larger prey, such as juvenile grouse or Wood Pigeons, the amount of time and energy required to feed their young at the most demanding stage can be correspondingly reduced. Nevertheless, there was evidence from the hide watch that much of the prey, even in the later stages, was still of Meadow Pipit size.

  Table 17 shows that most bird prey consisted of fledglings (or nestlings). Indeed, the six Red Grouse and the single Crossbill were the only positive adult identifications. In 1968, when four out of the six adult grouse were found, there was a high population of Red Grouse in the young forest within easy hunting range of nests. Harriers may have found it easier to catch them by surprise in such habitat than on open moorland.

  When the prey identifications from moorland nests are compared with those from forest nests (Tables 19–21), the most important difference is that none of the former could be assigned to forest haunting species, while some of the latter could. The true proportion of prey taken from forest is impossible to assess accurately as the two commonest prey species, Red Grouse and Meadow Pipit, were plentiful in both young forest and on moorland, but only those items listed in Table 23 were fairly certainly caught in the forest. Even if as many as half the Red Grouse and Meadow Pipit prey was taken from forest, which is unlikely, considerably more than half the prey found at forest nests must have been caught on moorland.

  The most conclusive evidence for the importance of moorland as hunting ground came from watching hunting birds. Our own observations continually showed that adult harriers with nests in forests regularly hunted neighbouring moorland. Jimmy Stewart, the shepherd, informed me that he saw harriers hunting Moorland A almost every day in spring and summer, and quite often recognised two or three different cocks over a period of days; he saw them fly away with prey in the direction of known forest nest areas. I once saw two hunting cocks meet over the moor; they came from moorland nests rather more than a kilometre apart, and there was a brief aerial tussle when they met about half way between their nesting sites. They soon disengaged to hunt in different directions.

  Typically, a cock with a nest of eggs or small young in the forest would set out from the nest area, hunting at first along the edges of forest rides, or sometimes low over the trees, and he would occasionally make a kill in this way. Usually, he continued to the open moorland and proceeded to hunt until lost to sight a kilometre or more away. It was interesting to see that he lost height as soon as he reached the moorland edge and thereafter seemed to hunt more assiduously, mainly very close to the ground, using the contours to assist him in surprise approaches. Because hens do much of their hunting near the nest, those from forest nests often hunted among the trees or along rides. Cocks and hens were each seen to circle to a height of 30 metres, or more, over forest nesting sites and set direct course to moorland hunting grounds, slowly losing height, but not dropping to hunt the forest at all. Young harriers, fledged from forest nests, began their hunting in the forest, especially in the rides within about a kilometre of the nest, but after two or three weeks they ventured out to hunt over moorland, soon dispersing more widely. On 27 July 1974, I watched a juvenile fledged about three weeks, return at dusk from the moorland, evidently to roost in the forest at or near where the nest had been.

  In Tables 24–26, I have listed the bird species which breed commonly on the moorlands and in the 5–15 year old forests, where harriers bred and hunted. I have little quantitative evidence for the relative abundance of different species, but have taken note of counts made by Jeffrey Watson in local forests and have also relied on my own long acquaintance with both habitats in the district. As already mentioned, it is surprising that neither Curlew nor Golden Plover have been found as prey in Areas A, C and E (two young Curlews were noted as prey on Moorland B by R. C. Dickson). At least 15–20 pairs of Golden Plover nested regularly, and Curlews were considerably more numerous, on Moorland A, which was hunted by at least one cock harrier throughout every season. Among forest nesting birds Chaffinches, particularly, might have been expected as more common prey, and there are several notable absentees among the numerous species, as the lists show. Both thrushes habitually made long flights to collect food out on the open, grassy moor and appeared at such times to be vulnerable to predators.

  I have discussed the Hen Harrier’s hunting methods in Chapter 4 and I think the limitations of these may explain some of the apparently surprising absences from the prey list, in so far as these can be taken to represent a valid sample. Forest haunting passerines, if not surprised, can often evade capture by diving into trees where harriers cannot follow. When travelling about the forest, or further, finches and thrushes tend to fly high and I have never seen a Hen Harrier in pursuit of a high-flying bird. Finches and pipits could approach a harrier soaring over forest without provoking any reaction.

  Actual observations of kills or attempted kills in the breeding season have been few, partly because much more time has been given to watching nest areas rather than the main hunting areas, and partly because hunting harriers, by covering so much ground, are very difficult to keep in continuous view for long periods. One female was seen to capture a juvenile Red Grouse in or just above the heather, on moorland, and two different cocks were seen to snatch at adult Red Grouse as they rose from the ground. In one of these instances, a pair of grouse seemed to fly up at the harrier, probably in defence of chicks, and the harrier appeared as surprised as the grouse. Several successful catches of smaller prey, by cock or hen, were seen near or on the ground. A cock and a hen were each seen to capture small birds low among forest trees within a few hundred metres of a nest. On moorland, one spring, Jimmy Stewart reported disturbing a hen while she was feeding on an adult grouse. In summer 1976, R. C. Dickson saw an immature male kill an adult cock Red Grouse on Moorland B.

  In recent years rabbits have become very abundant within easy hunting range of most harrier nests. Surprisingly, none were found as prey in the present study.* Unlike field voles, which may only be detected as prey in pellet analyses, rabbits could be expected to show up among prey remains, and would have been identified from the hide if any had been brought to that nest. Even if a larger sample of pellets might have shown a higher proportion of vole prey, mammalian prey was clearly of much less importance than in many parts of the Hen Harriers’ breeding range. Rabbits were most numerous around the fringes of woodland, in rather enclosed habitat for easy hunting by harriers, though they were commonly taken by Golden Eagles.

  Hunting activity by Hen Harriers in the breeding season was almost entirely confined to moorland, or its fringes, and the plantations of conifers under 15–20 years old. It was extremely rare to see harriers in summer over cultivated fields, lowland pasture or marsh—all hunted in winter—but they would have had to travel a minimum of three kilometres from nests to find much habitat of these types. Harriers were occasionally seen over moorland as high as 300 metres, but they mainly hunted over lower ground. They were very rarely seen hunting the unplanted and ungrazed hill tops, mostly above 300 metres, within the forest areas. The extremes of distances travelled from nests were not known, but observations of cocks moving between the forest and hunting grounds on Moorland A, showed that they commonly hunted at least four or five kilometres from nests. Taking seven kilometres as a possible outer limit of hunting distance from nests, I have tried to assess the comparative suitability of hunting grounds within that distance from central points in the three forest Areas C, D and E. Figure 11 shows that hunting grounds for harriers based in Area D, included rather less moorland than the other two, and strikingly less ground below 250 metres. The puzzling lack of nests in Area D, in spite of a long history of spring and summer sightings of harriers in the area, might therefore be explained by the comparative inaccessibility of good hunting grounds. Although I do not have census figures to prove that prey is scarcer on mo
orland above 250 metres than below, a general knowledge of the birds of the area and the evidence that harriers usually hunted the lower ground are, in my view, strong indications that this was so.* Observations over all the years give strong grounds for believing that breeding success of both moorland and forest nesting harriers was greatly dependent on the accessibility of sizeable areas of low elevation moorland and marginal land. It is likely, as well, that the decreasing extent of such ground is an important factor in keeping the breeding numbers of Hen Harriers so low in the area as a whole. It must also be said that densities of prey species appear to vary considerably, both on moorland and within the young plantations, in different local areas of similar elevation. It may well be true that food for harriers is comparatively scarcer, generally, in the environs of Area D which border on the main granite massif of Galloway. It is certainly my impression that moorland birds such as Red Grouse, Curlew and Golden Plover are scarcer in this area than on the more fertile moorland close to Areas C and E. The results of censuses of breeding birds on selected plots within young forests and on open ground, presently being made by D. Moss, are awaited with interest. Differences in tree growth and survival and in types of ground vegetation within young plantations are probably important factors accounting for local differences in bird communities in plantations of fairly similar age. The distribution of breeding Hen Harriers must, of course, depend on the suitability of the terrain for their style of hunting as well as on the abundance of potential prey.