And a Wagtail in a Plane Tree

In these meditative dusks, with their indigo skies and early stars above the mountain, I am likely to be visited by incongruous memories of O'Connell Street, mercifully muted of its worst Christmas rush-hour sounds and smells. Standing still as a rock in the human flood, in a canyon of light and noise, I attend one of the strangest phenomena in Irish wildlife.

As I watched it for the first time, a couple of decades ago, the arrival of the pied wagtails began about 20 minutes after sunset - about half-past-four. The birds dropped down from every point of the deepening blue sky like honey bees returning to a hive. They alighted in the twigs of the plane trees, above the Christmas fairy lights, and moved restlessly among the dark, round bobbles of the tree's fruits. Their twittering evensong was vigorous enough to carry above the traffic, but within an hour they were still and silent in the branches.

The roosting of the O'Connell Street wagtails was once known to almost every Dubliner: it was something you took your kids to see. But now, apparently, the old plane trees are to go, to be replaced at some future date with 28 lime trees, trimmed to formal box-shapes in the Parisian style to flatter the gleaming spire of The Needle. It was in the autumn of 1929 that one of the plane trees, in what some still called Sackville Street, became the nightly roost of about 100 pied wagtails. They slept there each night through the winter, indifferent to the clanging of the electric trams that ground up and down at each side of them, almost touching the trees, at the rate of more than two to the minute, until midnight.

When the birds returned the following autumn, their numbers built up to close on 600, and nightly crowds began to gather around the tree covered with silver leaves. There was much popular argument as to whether the birds were swallows or long-tailed tits. As they assembled each night, they broke into a twittering so loud that, as one observer noted, a big crowd of men tramping home from a Saturday football match was held up by the unexpected clamour.

In the winter of 1931-32, the roost numbered well over 1,000 birds, some of which stayed on until July, even through the crowds, bands and flag-waving of the Eucharistic Congress. By 1934, when the roost amounted to 2,000 birds and had spread into a second tree, the wagtails were, as the naturalist C.B. Moffat wrote, "recognised as a distinct asset to the city [and] they never receive the slightest molestation".

Since then, successive generations of the birds have roosted in the street, changing their trees from time to time and varying in their mid-winter numbers from about 500 to more than 3,500 (in 1950). The roost has dwindled, to fewer than 1,000 birds, but has continued to fascinate ornithologists, one of whom made his study from the top of the old Nelson's column, 53 metres up. The emblem of The Dublin Naturalists Field Club shows the Corporation's coat of arms, with two wagtails perched in a plane tree above it.

The usual roosts of wagtails are reedbeds and rural thickets, but the extra warmth of city centres attracts the urban birds that exist unobtrusively in suburban parks and on canal banks and wasteland sites, feeding on insects and spiders. A Trinity geographer found that the temperature in O'Connell Street on a winter night could be as much as two degrees higher than in the surrounding countryside, and that the centre of the street was warmer than the ends. The three trees just north of the GPO are, therefore, where the wagtails concentrate in the coldest weather, spreading out more widely when it gets milder. The ornithologist Tom Cooney has seen wagtails fly into the Corporation Christmas tree on especially bitter nights, seeking denser cover and the extra warmth of the bulbs.

The urban roosts of wagtails are known from all over these islands - in London plane trees just down the road from Buckingham Palace, for example - but O'Connell Street seems to have held all the records for numbers and site-loyalty. What will happen when the birds find themselves dispossessed?

Some wagtails have used the trees on Burgh Quay (a shockingly draughty location, I would have thought) and others may follow. But the species has also long discovered the virtues of glass in keeping warm, roosting on or under glass roofs in sheltered places. Railway stations, factories, powerstations have all attracted wagtails in their hundreds, and roosts in commercial greenhouses have sometimes made a terrible mess of carnations and tomatoes.

Thus, the glittering new constructions in Dublin, with their glass atria, roof lanterns and penthouses, should offer any number of alternative temporary dormitories, but none will have the optimum ambient temperatures just north of Anna Liffey.

Meanwhile, I continue to enjoy the pied wagtail as the bird it was meant to be, the restless, excitable familiar of the water's edge, unstable as a ball of thistledown, in W.H. Hudson's phrase. I love the regularity of our encounters round the year - with a courting pair among the pebbles at the ford in spring; with travelling parties on the grassy banks of the shore at the end of summer; with lone birds on my roof in autumn, rummaging for spiders in the slates and insects in the gutters; with corner-boys in the village street in winter, attacking their reflections in car wing-mirrors.

Why do wagtails wag their tails so constantly? In one explanation, which convinces me, it is to echo the movement of water and thus to confuse their enemies. Even the winter wagtails of O'Connell Street regard the city rain with pleasure. In the 1930s, one observer was walking home past the plane trees at 1 a.m. on a February morning and was enchanted to see that, while the roost as a whole was silent and sleeping, half-a-dozen birds were splashing and sporting in some big rain puddles.

That, of course, was in the days when people strolling home down O'Connell Street at 1 a.m. felt free to watch birds and not their backs.