The Hen Harrier Read online

  There is generally an interval of 48 hours between egg laying but longer intervals, of up to eight days, were recorded occasionally by Balfour, and Hobson told Nethersole-Thompson that one hen, in Ireland, took twelve days to lay two eggs. According to Brown and Amadon, and from my own observations in south-west Scotland, the commonest clutch is five eggs. Clutches laid by a single female range between three and eight eggs, but first clutches of only two eggs probably occur and I believe that a clutch of nine in south-west Scotland was laid by a single female. Bent gives the clutch size of the Marsh Hawk as from four to nine eggs. In Balfour’s experience, larger clutches of up to twelve eggs were generally laid by two hens, although they may have been the result of one female laying a repeat clutch in the original nest (Brown). Repeat layings, after loss of eggs, take place within a fortnight according to Nethersole-Thompson (1933); in my own experience one female, in south-west Scotland, which would almost certainly have laid a clutch of seven, lost a nest with five eggs, built a new makeshift nest immediately and laid another two eggs within three days.

  When freshly-laid, most eggs are very pale blue but this colour soon fades and they become off-white within a week or ten days. Their only claim to beauty lies in the ice-green colour inside the shell. Rarely, however, clutches occur with rust-coloured spots or streaks, while Balfour said that some eggs were blotched or spotted with ‘mid-green’. Nethersole-Thompson wrote that Staines Boorman had two heavily marked clutches and that John Douglas twice found sets of marked eggs during many years of nest-hunting in Orkney. Spotted eggs are apparently more common in the Marsh Hawk, amounting to some 10% of sets (Bent). During incubation, eggs frequently become stained and their rather ‘coarse-grained and chalky shells . . . tend to become polished’ (Balfour). Eggs of the Hen Harrier may be difficult to distinguish from those of Montagu’s Harrier, as I have good cause to remember (see here). Although Hen Harrier’s eggs on average are distinctly larger, their dimensions can overlap with Montagu’s, according to the Handbook of British Birds. The mean dimensions of 901 eggs of the Hen Harrier, measured by Balfour, were 46.3 × 35.6 mm; and the average of 100 Montagu’s Harrier’s eggs, given by the Handbook, was 41.5 mm × 32.7 mm. Eggs of both species are therefore small for the size of the birds. They vary in shape from rounded to quite elongated.

  In the case of the Hen Harrier, incubation is usually entirely by the female and is said to start with the second egg, but it does sometimes start with the first and, according to Geroudet, may even be delayed until the last egg is laid. The only British evidence of a male incubating, apart from a suggestion by Richmond, comes from a single Hebridean record, mentioned by Brown, involving a clutch of addled eggs which the female had continued to incubate long after they were due to hatch. R. D. Lawrence (1975) cites an instance of incubation by a male Marsh Hawk. Certainly, male harriers normally refrain even from alighting at nests during the incubation period.

  Incubation usually takes about 30 days for each egg, but according to Brown the period varies from 29 to 37 days. However, Wilhelm (1960) found that the last egg of a Marsh Hawk hatched in 24 days, an exceptionally short period. The normal incubation period for the Montagu’s Harrier, which on average lays three or four weeks later than the Hen Harrier, is 27–28 days for each egg. Neufeldt found that, in the Pied Harrier, irregular brooding caused the embryo of the first egg to take up to two days longer to develop than in the other eggs.

  When trying to find all the Marsh Hawk nests in her study area, Hamerstrom was sometimes aided by observing that the female frequently left the eggs to preen herself in the early morning. The incubation period is much the most difficult time to locate a Hen Harrier’s nest. This is particularly true where nesting is sparse and it is impossible to spend the necessary time overlooking all the possible areas where a pair might nest. The hen sits very close and, once incubation has properly begun, she rarely leaves the eggs except twice or thrice a day for the food pass or, as already said, for a brief excursion at the start of the day. The cock usually disappears from the precincts of the nest very quickly after delivering food.

  * See also details of nest sites in south-west Scotland, Chapter 14.

  * Frank King tells me that a female Hen Harrier incubating eggs in Munster was attended by a male Montagu’s Harrier.

  † See also chapter 12; the subsequent disappearance of some of the males, which I saw in a Galloway forest breeding area in March—April, might have been due to conflict with and exclusion by other males, but I saw no direct evidence of this.

  * Up to seven females (Brown, 1976).

  CHAPTER SIX

  The Breeding Cycle: Hatching to Fledging

  Hatching

  Since incubation has usually begun well before the completion of the clutch, hatching is usually spread over several days. At one nest I noted that three eggs hatched over a period of six days, and in large broods the time between the hatching of the first and last eggs may be as long as 10–11 days. The newly-hatched chicks are clothed in very short pinkish-white down, sparsest on the underparts, but their pink appearance is increased by the skin showing among the down. At first their eyes are closed but they are opened, blearily, within a day. The eyes are then seen to be very dark; though actually dark brown, they look black from any distance. These and their little black, hooked bills (tipped with the whitish egg tooth at first) are their most striking features. All young harriers have broad flat skulls which give their heads a rather reptilian look. The waxy cere at the base of the bill is elongated, dull pink in colour, and the limp legs and feet are flesh-pink. Delamain pointed out differences between the young chicks of the Hen and Montagu’s Harrier; the former, he said, have rounder heads with a more prominent superciliary arch while the Montagu’s can be distinguished at this early stage by their yellow legs and cere; but he added that melanistic chicks of Montagu’s have dark brown legs and cere. Scharf and Balfour give the average weight of a newly hatched Hen Harrier as 19.8 grams.

  While chicks are actually hatching, or immediately afterwards, it is usual for the hen to remain on the nest when the cock arrives with food. Indeed, if the cock is seen to fly low and almost stall above the nest as he drops the prey, but the hen does not rise, an observer may guess that hatching is imminent or has begun. The hen may be equally reluctant to rise at any time while the chicks are small and require continuous brooding in bad weather.

  The shells of hatched eggs are often removed by the hen quite soon after hatching and may be dropped some hundreds of metres from the nest. Sometimes, they are also left by the nest; or eaten by the hen, as David Whitaker tells me was observed by F. G. Hollands.

  Feeding of chicks: hunting ranges and rhythm

  Small chicks are fed by the hen from small items of prey brought by the cock. I describe the delicate manner in which the food is presented to the chicks, in Chapter 13. Bird prey is already well plucked when brought to the chicks, and usually headless, until they are at least three or even four weeks old, when they begin to tear up unprepared prey for themselves. Differing views have been expressed on how far hens ensure an even distribution of food among the chicks. Certainly, a hen may make deliberate attempts to feed each chick in turn, but Balfour and Macdonald (1970) noted that chicks in a brood of four lunged forward to grab food, and the quickest often got the prize. The largest chick in this brood received more, and the smallest less, than a fair share. I have made similar observations. However, Balfour and Macdonald also noted that, when the oldest chick was able to feed itself, the hen sometimes took food from it and distributed it among the brood. Hamerstrom made the interesting suggestion that the hunger calls and white down of the smallest chick in a brood inhibited the others from taking food and caused them to move away, thus allowing the smallest a larger share, but Balfour and Macdonald did not observe this behaviour.

  When watching a nest from a hide I saw the first evidence that the hen was deliberately feeding gristle, bone and feather to a chick when it was 22 days old. From this stage,
at least, chicks produce pellets of indigestible matter; these, like uneaten remains of food, are fairly soon removed by the hen who thus keeps the nest clean. Shortly before fledging time, however, when chicks are feeding themselves, pellets and remains begin to be left in or around the nest so that in the final stage nests with large broods tend to become messy and fly-ridden. As I stress, in Chapter 16, it is usually only the skeletal remains of larger prey which are uneaten and these can give a misleading impression of the true range of prey brought. I have seen quite clean nests with young almost fledged; in such instances it is likely that all the prey has been fairly small. Very soon after hatching, nest-sanitation is always ensured by the chicks defecating over the edge of the nest.

  Heysham (1783), who killed many Hen Harriers at the nest, said that he had seen a cock feeding chicks and further claimed that a cock could rear a brood after his mate had been killed. It would not be unusual for a cock to continue to bring food to the nest in such a situation, but I can find no recent evidence of a cock feeding chicks and they would only be likely to survive if they were well advanced and able to tear up prey for themselves. Breckenridge said that males’ visits to nests were like ‘forced landings’; they ‘dropped in, released their talons from the prey and literally bounced back out of the nests with the evident attitude of an intruder’. This, however, is not invariable behaviour and I have seen a male which alighted repeatedly at a nest and stayed there for a minute or more, but I do not know whether he ever fed the chicks. Anthony Buxton described how a cock Marsh Harrier, which had lost its mate, continued to bring food to a larder 20 metres from the nest—too far for the chicks to reach it. He was not the only harrier enthusiast to take a hand in helping chicks to survive in such circumstances. Neufeldt, however, saw the male Pied Harrier tearing up food and feeding it to the chicks when the female was absent.

  For the first two, to two and a half weeks the chicks require much brooding by the female, especially in wet or cold weather. In this period she does not normally stray far from the nest and, when not feeding or brooding the chicks, she can often be seen patrolling the neighbourhood, circling to a considerable height or drifting down to perch for a long time on a favourite look-out, ever watchful for the cock’s return with prey. As already stated females which are insufficiently supplied by the male, in a polygynous group, are compelled to start hunting for themselves and their chicks while the latter are still small. Some hens, even in monogamous situations, delay taking their share of hunting longer than others, but it is usual for both sexes to hunt for the second half of the period that chicks are in the nest. Hens bring in a greater share of larger items of prey and their role in food provision at this stage is clearly of great importance. Schipper has listed the numbers of prey items supplied by male and female, after hatching, at 13 different nests in the Netherlands: during the first ten days males brought 82%, females 18%; during days 11–20, males brought 54%, females 46%; and from day 21 onwards males provided 41% and females 59%. As these figures suggest, some cocks supply a lesser share of the food as fledging approaches and it is not uncommon for a cock to disappear altogether during the last week before the chicks are fledged. In my own experience, however, this has only occurred at a minority of nests. Schipper found that, with one exception, spells of prolonged rain reduced the supply of prey to nests, but there was some indication that more prey was brought when the wind was strong.

  Females tend to hunt nearer the nest than males, but many females regularly travel two to three kilometres to hunting grounds and two marked hens have been observed by Balfour hunting at five and eight kilometres from their nests with young. Schipper found that males were often hunting more than three kilometres from nests in the Netherlands. Some cocks in south-west Scotland hunt at least four kilometres from nests and probably considerably further. Geroudet gives two to four kilometres as the hunting range of ‘a pair’. Clearly the distances travelled from nests vary according to the availability of good hunting grounds, as Neufeldt found for Pied Harriers, some of which hunted no further than one and a half kilometres from their nests while others, separated from open ground by a belt of forest, ranged up to five kilometres. Breckenridge stated that the nesting Marsh Hawks which he studied in Minnesota, hunted over an area of ‘only one square mile’ (259 hectares).

  When watching Hen Harriers’ nests containing young, I have always noted the times at which food was brought and, when possible, whether the kill had been made by the male or the female. Using notes for 136 hours watching, at four nests of known brood sizes, I have calculated that the mean number of prey items brought per chick in a 14 hour hunting day was 3.2. This agrees closely with Schipper’s result from his much more systematic study, which gave a mean figure of 22.3 items per chick ‘per 100 hours of nest observation’, or an average of 3.12 prey items per chick in a 14 hour day. Clearly the number of deliveries of prey to a nest may depend greatly on the number of mouths to feed, but I found no lessening in the feeding rate after a brood of two was reduced to one. Variable hunting success, due to weather conditions or fluctuations in easily available prey, may also affect the feeding rate. Tinbergen (1940) found that prey supply by Kestrels gradually increased until the chicks were about three weeks old; and A. J. Watson, in a study of Merlins (1973), demonstrated an increasing feeding rate until chicks were 25–29 days old, followed by a decrease. Breckenridge (1935) recorded changes in the feeding rate for a family of five young Marsh Hawks (only three of which fledged). The rate rose from less than four feeds per day for the brood on the eighth day, to a maximum of 24 on the twenty-fifth day, with a decline to only six feeds on the thirty-second day, near fledging time. Schipper, however, said that in his study of Hen, Marsh and Montagu’s Harriers, it proved impossible to conclude that at any particular stage of development of the young, more prey was supplied than at any other. In any attempt to calculate weights of food supplied to chicks, it must be remembered that some proportion of the prey items brought is eaten by the female parent. A surprisingly high figure of 19.9 grams is given by Schipper as the average weight of prey, per chick, per hour.

  Hen Harriers, in the breeding season, do not usually hunt with equal consistency at all hours of the day. I have suggested 14 hours as a hunting day in south-west Scotland, during the breeding season, from my impression that very little hunting is done before about 06.30 or after about 20.30 hours. In my own experience, there are peak periods of hunting activity, most often between 08.00 and 10.00 and between 14.00 and 19.00 but the ‘hunting rhythms’ of individuals vary to some extent. Schipper found little sign of a pattern in hunting activity.

  Development of chicks; mortality and cannibalism

  Within a week, the chicks begin to lose their first rather naked pinkish-white appearance and grow a longer, dingy covering of down, well described by Richmond as ‘rabbit grey’ in colour. On the crown of the head, however, this down is distinctly paler, like a ‘skull cap’ sprinkled with hoar frost. The size of the body increases rapidly and the head soon ceases to look absurdly and disproportionately large. Compared with most other young birds of prey, harrier chicks of any age have a very distinctive head and bill shape, caused by the combined effect of the broad skull with eyes frontally set, like an owl’s, and the long tapering base to the bill (see illustrations).

  The legs and cere of a week-old chick are already becoming yellow, very pale at first and tinged with green on the cere, while the legs are a different, more creamy tint. I have found variation in the rate at which these parts become brighter yellow, but it is not usually until chicks are nearly four weeks old that the legs turn an astonishingly brilliant chrome yellow and the cere becomes bright lemon. I suspect that good feeding hastens these changes. In small chicks, the inside of the mouth is flesh-pink at about ten days and later crimson. The long tongue is notched, mid-way, with black and tipped pale pink. The gape is outlined with yellow, becoming orange at the base.

  Towards the end of the second week, dark feathers on scapulars
and wing-tips break their sheaths, forming double chevrons among the down. The feather sheaths are steely blue. An almost bare patch of grey-green skin remains on the lores, between eyes and bill, and a fringe of dark brown and chestnut feathers begins to show on the ear coverts. Between the ages of three and four weeks, feather growth rapidly changes the appearance of chicks and most of the down is lost, with the help of much preening. The forehead and crown of the head are the last parts of the body to lose their prominent tufts of down and as long as these remain a chick is far from ready to fly.

  In their first complete feathering the young are dark chocolate brown* on the upper parts, with numerous narrow salmon-buff fringes to wing and scapular feathers especially prominent on the wing coverts. Their freshly-grown tail feathers are strikingly beautiful, with their broad bands of the same colour and a lighter more creamy salmon-coloured terminal bar. Unless the wings are raised or spread, the white upper tail coverts are generally concealed but, when revealed, are very striking: some or all of these white feathers may have a narrow or fairly broad dark brown mesial streak. Further study of this feature is required to find out whether only females have the broad mesial streaks—it is possible that this might prove to be an additional way of sexing young birds. Below the dark crown the pale, almost white eyebrows and face set off the frowning deep-set eyes, and the deep russet fringe to the dark ear coverts overlaps the delicate lace-like border of the facial ruff.