Hen Harrier (Circus cyaneus)

Of all our four harriers, this is the hardiest. It is the only one that reaches into the Arctic to breed in large numbers, and the only one that does not retreat at all to tropical Africa for the winter. It is generally found in chilly, windswept places, including moorland, wetlands and the fringes of the tundra, but outside the breeding season it will also hunt over agricultural land.

Male Hen Harrier. By Isle of Man Government – Hen Harrier, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=30598443

In common with all harriers the Hen Harrier hunts by quartering low (less than 5m) above the ground. At such heights it does not just look, but can also often hear prey, too, and will interrupt its slow flight for a quick pounce into the vegetation. It takes a wide range of prey, including mammals, birds and insects, and the local abundance of each will be reflected in the diet. In some parts of the range up to 80% of food taken to nests may consist of small birds (almost half of these chicks), but in the far north the Hen Harrier depends more on voles and lemmings, and indeed, the populations of harriers often mirror the rodents’ 3-4 year cycle of boom and bust.

The breeding behaviour of this bird of prey is highly unusual. In some populations, there is a marked tendency towards males having more than one mate, whereas in others, monogamy is the rule. Where males are polygynous, most take just one extra mate, but there are records of exceptional individuals with as many as seven – these are almost always older, experienced birds. The nests of females making up a “harem” can be as little as 50m apart. In monogamous pairs the male does all the hunting and provisions both female and young until the latter are at least two weeks old; but in polygyny the female must do some hunting for itself.

Female Hen Harrier. By Rob Zweers – Blauwe Kiekendief – Hen Harrier, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=44025926

The Hen Harrier nest is, in common with those of other harriers, placed on the ground. It is usually little more than a low pile of nearby vegetation, hidden in a thicket, but if the ground is wet it may be much larger, as much as 45cm tall. The young usually leave it when they begin to grow feathers at about 15 days old, preferring to remain nearby in the vegetation rather than on the structure itself. They keep contact with the provisioning adult by calling.

The Hen Harrier’s displays are just as spectacular as those of the Marsh Harrier and others of the genus. The male’s sky-dance consists of a series of steep undulations, said to be steeper than those of any harrier, and as the bird performs it often rolls over or rocks from side to side. Most birds rise and fall only three or four times, but there is one energetic male on record that flew up and down no less than 149 times consecutively before landing near the nest site. Hopefully such a persistent individual was displaying to a harem of females.

From ‘Birds: A Complete Guide to All British and European Species’, by Dominic Couzens. Published by Collins and reproduced with permission.