The Hen Harrier Read online

  The work of building a nest to the stage when it is ready for the first egg occupies a hen for ‘fairly short spells’, over a period of ‘a few days, even a few hours in the case of a replacement nest, others taking a fortnight or more’ (Balfour).

  Male Hen Harriers sometimes build ‘nests’, which they do not line, although Balfour in his long experience never met the habit among Orkney harriers. Blake (1956) wrote that he had seen over twenty ‘cock’s nests’ and had watched several being built, while Dickson has found several nests, which he attributed to cocks, on a moor in south-west Scotland, but he has not observed any being built. He described these as ‘platforms, two feet or so in diameter, of dried grey heather stalks’. I have only once seen a male, which had apparently lost its mate, building a similar structure (Chapter 12). David Whitaker watched a cock adding material to an old nest over a period of an hour. At the time he had no mate but was displaying frequently. Later a female joined him but took no interest in the nest he had been building. I have found several platforms, hardly more than bundles of long heather twigs carelessly thrown together, which were close to nests with young, and I observed that well-grown chicks received and ate prey there. In the latter instances I had no proof that the ‘nests’ were the work of the cock. P. H. Bahr (1907) found a chick, only fourteen days old, using a ‘second nest, of a few twigs of heather, 30 yards from the original nest’. Dickson has found feathers and faeces at ‘cock-nests’, suggesting that they were used for roosting and this might possibly be their primary purpose. Yet, since Hen Harriers choose extremely wet situations for roosting in winter, I cannot imagine why they should build up a platform for roosting on relatively dry moors in summer unless, perhaps, to gain a better field of observation. Weis (1923) said that, in Denmark, male Marsh Harriers regularly built ‘cock-nests’ without lining, and used them as feeding and resting bases. He found no sign of this habit in male Montagu’s and claimed that they had no need of it because they could find dry ground to sit on. It seems to me most likely that ‘cock-nests’ are only built where cocks outnumber hens or when a cock has lost his mate. In Orkney, of course, there are only about half as many cocks as hens in the breeding population. Generally, it may be no more than a ‘frustration’ activity, but the platforms used by chicks as feeding bases, whether built by cock or hen, are possibly due to excessive human disturbance at the nest. Balfour, at any rate, considered that a photographer had upset the birds at one Orkney nest where such a platform was built.

  Within some of the nesting habitats which I have already described, some preferences in the choice of the nest-site may be cited.* Balfour, for instance, found that the more favoured nesting areas in Orkney always contained enough damp vegetation to escape damage by fire. About two-thirds of a large sample of nests found were in mixed heather and rushes, while more of the rest were in pure rushes than in pure heather. In North America, Charles A. Urner (1925) found that some nests of the Marsh Hawk built in very marshy ground were exceptionally built-up (to 37.5–40 centimetres), so escaping floods which washed out the nests of Short-eared Owls in the same marsh. Balfour said that in Orkney, nests were built up higher in tall or wet vegetation. Dixon (1898) described a nest site in Skye ‘in an almost impenetrable heather thicket’, and this is still a good description of some sites in the Outer Hebrides where the surroundings of the nest are sometimes a dense mixture of tall, chest-high heather and willow. Some of these sites are on ungrazed islands in lochs. I noticed that rushes were growing among the heather around a nest in North Uist and, according to Hopkins, one site in South Uist is in a damp low-lying area of tall grass. Sites in ‘beds of rushes’ in the Border Hills were known to Bolam in the nineteenth century and I have seen a nest in Galloway, at a height of 420 metres, placed in a big tract of rushes with no heather in the vicinity. Bolam said that his father, long ago, knew of nests ‘in the thickest hawthorn and gorse glens’ on Alnwick Moor. Ruttledge (1966) and Scott, commenting on the recent spread of nesting in Ireland, have also remarked on the attraction of sites with a strong growth of gorse among young conifers; and King has found some moorland nests which were built between the bare stems of flattened gorse bushes, the prickly extremities of which provided shelter and some concealment. He has also found that small islets of heather and bracken situated in river beds, and measuring only about 100 × 150 centimetres, are often favoured in Kerry. Sometimes these are completely surrounded by a moat which may deter predators. Over a period of five years, a pair nested successfully in such sites in two different rivers. Other island nests are in almost dry river beds. Scott stresses that, in Ireland, nests are generally in well drained situations. Blake (1961) said that nest sites in the Norwegian mountains were typically in thickets of hoary arctic willows, at elevations between 870 and 1,050 metres. On a much-favoured hill in Kintyre where until recently a colony of pairs nested, the eye was surprised by numerous purple rhododendrons flowering among the conifers in a luxuriant under-bush of heather. Many sites in conifer plantations are surrounded by heather, bog myrtle and grasses 60–120 centimetres tall, and are thus well sheltered and concealed throughout the summer, but many moorland nests and some in the youngest forests are very exposed early in the season. As shown in my regional study (Chapter 11), the most exposed sites of all have a high incidence of failure. The frequency with which the Hen Harrier chooses a site with a mixture of vegetation, such as heather and bracken or heather and rushes, is striking. The birds seem able to recognise that bracken, which is hardly out of the ground when the nest is built, will later provide valuable shade and shelter for the chicks.

  I have tried to discover a pattern in the selection of situations for nests. Very few are on exposed hill-tops and steep, rugged slopes always seem to be avoided, even if there is plenty of ground cover, though King has found one remarkable nest on a steep slope, about six metres above a river bed, in Kerry. It was built on one of a series of the horizontal track-like lines caused by the stretching and splitting of the land surface. In this instance the ground rose steeply above the nest and fell equally steeply below it.

  The most common situations are on gentle slopes, often just above or in a gully, generally well sheltered from the prevailing wind. In such places, of course, there is often a small stream near by. Delamain said that Montagu’s Harriers never visited a water hole which was within a few paces of their nest, but Hen Harriers certainly visit water for bathing, sometimes. This has been noted both by Balfour and Brian Turner.

  There seems to be no evidence that sites which provide a wide outlook over their surroundings are preferred—the bird on the nest is often much too surrounded by vegetation to see any distance and must then depend on her ears for warning of approaching danger. In conifer forests, nests are quite often in virtually flat ground at fairly low elevations but here, I think, it is significant that females with young in the nest make much use of perching places on trees as look-outs. Nesting in marshy ground could obviously have an advantage in reducing accessibility to mammalian predators—the old sites in the almost impenetrable bogland of Billie Mire, in Berwickshire, must have been excellent in this respect. Island sites could also provide a high degree of safety. Yet, the more typical nesting sites in Britain and Ireland at the present time are easily accessible to man or fox and generally depend for their survival (where they are not actively protected) on being located in large tracts of fairly uniform country without obvious features to advertise where they may be sited. In Orkney, John Douglas told Nethersole-Thompson of a nest within 500 metres of a crofter’s cottage. Hen Harriers are not commonly deterred by regular human activities such as peat-cutting, farm work or the passage of a shepherd and his dogs near the nest, but Jones suggests that the great amount of peat digging on the moors in Co. Limerick may sometimes compel birds to change their choice of site. Even moor fires, which often destroy good nesting cover early in the season, do not necessarily drive the birds very far away. Balfour said that the Orkney colony thrived among war-time manoeuvres! />
  Bannerman quotes evidence from Blair and Hagen that many nests in Norway are built close to some handiwork of man, such as a ditch, fence, snowbrake, railway line or station. He said that three nests were within ten metres of the main Oslo–Trondheim line. In my own experience a number of nests in south-west Scotland have been less than 100 metres from forest roads, and several have been less than 50 metres. More have been close to a forest ride than deeper into blocks of forest; there was sometimes a dead conifer nearby, conspicuously red or orange among the general greenery, which might have served the birds as a recognition mark. However, I am not sure that these features had any special significance as nest-markers for the birds. Balfour found that birds did not rebuild on the actual nest of the previous year, except sometimes when this had failed at an early stage, but there is widespread evidence of nests built within 20–200 metres of sites used the year before.

  Social nesting

  Delamain said of the Montagu’s Harrier: ‘It is sociable and when it has at its disposal vast uncultivated expanses it tends to form colonies.’ Hen Harriers also tend to nest in groups, but their nests are very rarely so close together as the Montagu’s’. Weis observed a colony of six or seven nests of Montagu’s barely 20 metres apart, and Chris Knights tells me that very similar spacing occurs in southern Spain at the present time. In the largest and best documented colony of Hen Harriers, in Orkney, Balfour had never seen two nests closer than 200 metres apart, but had heard of an exceptional instance in which only 15–20 metres separated two nests. Few nests in Orkney, however, are closer together than 500 metres. Remarkably, when I asked Peter Strang about the peak period (1958–60) for the largest colony in a Kintyre forest, he also gave 200 metres as the minimum distance between nests, and said that nine pairs had nested in an area of 250 hectares, the same number as that given by Balfour for an area of the same size in a particularly favoured moorland valley in Orkney in 1949. So the highest known density in Britain has occurred in two widely separated areas, at different times, and in very different habitats. Peter Strang believed that all the matings in the peak years in Kintyre were monogamous, and as far as can be judged from Balfour’s account there was little if any polygyny in Orkney in 1949. (Polygyny is discussed in this chapter.)

  In North America, Breckenridge found two pairs of Marsh Hawks nesting within 200 metres of each other. C. E. Douglas told me that only 150 metres separated nests of Hen and Montagu’s Harriers in a Border forest in 1965.* In south-west Scotland, the shortest distance between two Hen Harriers’ nests, in a polygynous group, has been 500 metres, but here, in a comparatively sparse population, pairs are generally much more scattered, nests being commonly separated by at least two kilometres. In Ireland, the nearest nests in David Scott’s experience were one kilometre apart within sight of each other, but closer when separated by a ridge. Densities of nests, comparable to those cited for Orkney and Kintyre, probably occur here and there in Scottish forests at the present time but, in general, it is likely that they are prevented by limitations of food or hunting habitat; and on grouse moors persecution nearly always acts as a check on the more social nesting.

  The tendency to social nesting is matched by mutual toleration on the hunting grounds and only the near vicinity of the nest is normally defended against other Hen Harriers - this may be an area of about 600 metres in diameter, according to Brown and Amadon. Even at the nest a bird may show no aggression to visiting individuals or pairs. Balfour observed two pairs nest-prospecting together and up to six individuals flying amicably together over a nesting site. Once he watched a strange, immature male alight at a nest and mate with the female in full view of her acquiescent mate. Dickson, on the other hand, has seen two first vear males driven away by the older resident male† and Geroudet (1965) saw two males confront one another and grasp talons, after apparently trying to intimidate each other by flying with wings partly folded and tails fully spread. Errington (1930) recorded frequent conflicts between three pairs of Marsh Hawks whose nests were close together, at 130 and 400 yards. The characteristic form of mild defensive behaviour shown by males, chiefly to other males, has been described by Balfour as ‘escorting flight’, in which a male shadows an intruder on the borders of his nesting area by flying just below it, as if preventing it from alighting. In polygynous groups, males defend the home areas of two or more nests in this way, against other Hen Harriers. Further away, on the hunting grounds, Schipper found that there was not even any conflict between harriers of three different species—Hen, Montagu’s and Marsh Harriers.

  Polygyny

  Polygyny occurs most often where there is a high density of nesting. Its most remarkable development has been in Orkney, where it was first noticed by John Douglas, at least as early as 1931, since when it has greatly increased. Balfour and Cadbury say that ‘in recent years the sex ratio among breeding birds has been approximately two females to one male but each year there are a few males with three and even up to six females,* usually simultaneously’. Elsewhere in Scotland, polygyny has been observed in Kincardineshire by Nick Picozzi, in Kintyre by Peter Strang (only once), at least once in south-west Scotland by myself, and by Donald Macaskill in an unnamed forest further north. It has also been noted in Ireland (where Scott considers it uncommon), in the Netherlands and in North America. Schipper tells me that there are ‘many more females than males’ in the breeding population on the island of Ameland in the Netherlands; but Hamerstrom knew of only seven bigamous matings, and one case of trigamy, in 99 nest histories in her study area in Wisconsin, USA.

  One obvious effect of polygyny is that some females are not supplied with sufficient food, or with no food at all, by the male and are forced to spend more time hunting. The Orkney study has shown that this may have an adverse effect on hatching success; this was lowest when two females shared a male, but higher again with four females. Although in polygynous situations losses of young occur on account of inadequate brooding by females which must hunt from an early stage, Balfour and Cadbury state that overall breeding success is not significantly affected. According to Brown, fledging success (66%) was actually higher with four females than with two or three, in a polygynous group. This is attributed by Balfour and Cadbury to the fact that older, probably more experienced, males tend to associate with more females. First year males are nearly always monogamous and older males sometimes acquire additional females which have been mated to young males. Although the available evidence suggests that polygyny was much rarer in a comparably dense nesting group in Kintyre, observation there was not detailed enough for a valid comparison on this point.

  One explanation for polygyny might be that it reduces the time spent in the conflicts which can arise when pairs are extremely close, and so releases more time for hunting. Balfour and Cadbury have shown that in Orkney more females than males (1.2:1) have been fledged since 1953, and this might favour the growth of polygyny. Evidence for differential mortality of the sexes after fledging is inconclusive, but in this respect it is interesting to note that both Balfour and Nethersole-Thompson firmly believed that males had the higher mortality rate in the Orkney population. It is likely that on some mainland nesting grounds the reverse is more probably the case because, where there is no protection, females are more likely to be killed at the nest. If this is true it might explain why polygyny appears to be exceptional on the Scottish mainland.

  The possibility that polygyny is largely the result of social behaviour, by which a proportion of males is excluded from the breeding population, was put forward by Balfour and Cadbury. Further research may be enlightening.

  Neighbours

  Possible prey species often nest successfully very close to a predator’s nest. I recall, for instance, Wrens feeding nestlings within a metre or two of a Merlin’s nest and Ring Ouzels habitually raising broods in the shadow of a Peregrine’s eyrie. I have seen the fledglings of several passerine birds, such as Robins, Wrens and Willow warblers, unconcernedly feeding among the conifers beside a
harrier’s nest with young. Eventually, of course, some of these may have been taken as prey (see Chapters 9 and 11), since female harriers do some hunting close to their nests. As a rule, however, there is probably a degree of safety for small birds in the immediate vicinity. There are interesting examples of several larger species of birds breeding close to harrier nests and no suggestion that such nests were ever raided by the harriers. I have already related Booth’s story of the Greyhen nesting at ‘six or seven paces’; and Picozzi found a Mallard on nine eggs at only 1.2 metres, in Orkney. Also in Orkney, Balfour recorded a Kestrel’s nest at 14.5 metres, and Merlins often nesting at distances of less than 100 metres. The close association of Hen Harrier, Merlin, Short-eared Owl and Kestrel nests has often been noted in Orkney where Kestrels, too, like the others, breed among the heather. In Galloway, I have found Short-eared Owls nesting within 200 metres but Merlins never so close. Dickson, in a different part of Galloway, found a Greylag Goose on its nest at less than 140 metres from a Hen Harrier’s and, another time, a Teal nesting closer than this. I have already mentioned that Hen, Montagu’s and Marsh Harriers sometimes nest in neighbourly association.

  Bent has told how a Marsh Hawk once incubated twelve eggs of a Prairie Chicken. Only one chick hatched and ‘promptly ran away’.

  Laying and incubation

  The start of laying may be delayed by a sudden cold spell, or even a snowfall, but in Scotland the first egg is usually laid between late April and about 20 May. Blake, however, referring to the 1950s, said that laying began as early as mid-April in Central Scotland; more recently, in 1971 and 1972, nests found by Bremner and Macdonald (1970; 1971) in east Sutherland must have contained their first eggs by 10 April, calculating from fledging dates in mid-June. This is about the time that laying begins much further south, in the Netherlands, for instance. It is possible that the early Sutherland nesting originated with birds from a southerly locality. In the south-east of Ireland (Kerry), King has found a full clutch on 4 April but Scott has found that birds did not usually lay until mid-May in Co. Wicklow. Balfour’s earliest date for eggs was 19 April in Orkney and generally he found that few had full clutches before the end of the month, while the peak laying period was 11–20 May. This is very similar to my own experience in south-west Scotland and there is little if any difference in the laying season in Kintyre (Strang). Early failures are followed by repeat layings in which the clutch may not be complete until well into June.