Crossing paths with the crossbill

VISITING the woods above Glendalough last week, I came upon some of Ireland’s most elusive birds; crossbills were feeding in a spruce tree.

The crossbill, it has to be said, won’t win any beauty competitions. Slightly bigger than a greenfinch, rather plump and thick-necked, it vaguely resembles a parrot. Adult males are dull red all over except for their wings which are grey-brown. Females are greyish green with bright yellow rumps.

The bird’s most distinguishing feature, however, is its extraordinary bill. The tip of the upper mandible curves downwards like a hawk’s. The lower one curves upwards. The tips don’t meet but slide past each other giving the impression that the beak is deformed. The flight pattern is distinctive; crossbills have a high-pitched ‘glip-glip’ flight call. They don’t bounce up and down in the air as much as other finches do.

The bill is a highly specialised tool. Inserted in a spruce or pine cone and closed, it prises open the case enabling the tongue to reach the seeds inside.

This accomplished acrobat will hang upside-down, cracking open a cone kept pressed against the branch. The bird can carry cones as heavy as itself. It can strip bark from trees to expose creepy-crawlies underneath and chop up apples.

Each of Darwin’s Galapagos finches has its own unique bill. The seeds available to these famous birds differ from island to island and each finch species developed a bill specialised for its local ones.

Something similar may have happened to crossbills, but for them conifers, rather than broad-leaved plants, call the shots. Cones vary in strength size and shape between the species, dictating the design of the bills needed to harvest them. This has led to great variation among crossbills. The two-barred crossbill, recorded very rarely in Ireland, lives in the forests of sub-Arctic Russia and Canada. A few pairs of parrot crossbills, Scandinavian visitors with particularly large bills, have bred in Britain. There are about 20 distinct species of the ‘common’ crossbill, the species which breeds here.

The celebrity among crossbills, however, is found only in the north of Scotland; the Scottish crossbill is the only bird species found in Britain and nowhere else.

Scots pine cones are its main food and to tackle them it has a bill larger than that of the common species but smaller than a parrot crossbill’s. Scots pines produce seed every two years, shedding their crop between March and June, so Scottish crossbills breed in early spring.

Crossbill babies are fed on regurgitated seeds mashed in their parents’ bills. The Norway spruce, the great conifer of Scandinavia, produces a crop annually but the seeds must be harvested before they are released, usually in April. Birds nest when food supplies are most abundant and some crossbills start doing so as early as November. English ones breed from December to June. The Irish season is a month or so later.

Breeding at this time of year isn’t easy; there is not much light, it’s cold and eggs chill quickly. The female crossbill does most of the work, building the well-insulated nest and incubating the eggs. The male only lends a hand, or rather a bill, when young have to be fed. There are usually two broods.

Conifer crops are erratic with lots of seeds in some years and hardly any in others, so crossbills experience both feasts and famines.

Facing starvation, they move to areas where they hope the harvest will be better. Food shortages drive them south and west through Europe and on to Britain and Ireland. Large-scale ‘irruptions’ take place from time to time and there are ‘crossbill years’. Influxes have been recorded since the 18th Century but the earliest well-documented one was in 1821. A spectacular irruption occurred in 1888 and an even bigger one in 1962. Unlike waxwings, our most famous irruptive visitors, crossbills don’t always return to their countries of origin at the end of the season.

If they find sufficient food in an area, they stay on and breed. Some colonies become permanent, at least until the seed crop fails and the birds are forced to leave.

The widespread planting of conifers in recent decades must be a bonus for these birds. One swallow does not a summer make but could the Glendalough crossbills be a sign that an invasion is pending?