Enterprising blackbirds hatch early

Richard Collins sheds some light on why birds seem to have tricked nature.

TWO Cork birds deserve the Entrepreneur of the Year award. Blackbirds start laying in the middle of March with the more precocious pairs getting going a week or two earlier. Then, a most unusual nest was discovered on a Christmas tree in the grounds of University College Cork. Professor John O’Halloran of the Zoology Department had a web-cam installed and began monitoring the nest, reporting on its progress for RTÉ’s Mooney Show. The first egg, he thinks, was laid on January 5, an extraordinarily early date. There were three eggs in the clutch, but it seemed unlikely that any would hatch. Would they even be fertile? Against all the odds, however, they were; three chicks were produced. One died but, on February 4, the remaining two fledged.

Human entrepreneurs want to generate profits. For birds, offspring are the equivalent of profits. Producing plenty of descendants helps ensure your immortality; the genes are passed on to future generations. Most blackbirds have either two or three clutches in a season. Occasionally, a pair has a fourth, and there are a few records of a fifth. All going well the UCC pair should manage at least three broods this year.

That blackbirds nested successfully in the depths of winter is extraordinary. It should be impossible to produce eggs that early in the year. Songbirds lapse into a sexless state in the autumn; their reproductive organs becoming so small they almost disappear. In some cases, the gonads may shrink to a thousandth of their summer weight.

Breeding in birds at our latitudes is controlled by hormones and by light. In Ireland there is enormous variation in the length of daylight between winter and summer, crucially important when it comes to laying eggs. Light, penetrating the skull, reaches photoreceptors in the brain. These trigger the release of hormones. During short winter days very little hormone is produced but, with spring’s arrival amounts increase steadily, stimulating the gonads to grow. Other hormones induce courtship behaviour, nest-building and singing.

But the UCC birds produced eggs at the darkest time of the year, so what brought them into breeding condition? Did the lights on the Christmas tree fool their systems into believing that spring had arrived? It seems unlikely; Christmas-tree lights are not bright. Streetlights should have a much more powerful effect, but blackbirds living in brightly-lit cities don’t breed during the winter.

A female must reach a particular weight if she is to ovulate, and getting enough food to form eggs is a tall order during short winter days. The fruit and berries are gone from the trees. Worms stay well below the surface when the soil is cold and few creep-crawlies are out and about. Calcium, needed to form eggshells, can be scarce even in spring; thrushes get their supplies by eating snails, but these are difficult to find in winter.

Blackbirds are resourceful and quick to spot new opportunities. The males, for example, have broken with thrush tradition and don’t wear the same brown outfits as their mates. A glamorous singing cock, with his shiny black plumage and bright orange bill, is a sitting duck for predators such as hawks. Such showy behaviour seems foolhardy, but it draws attention away from his female and the nest, giving his youngsters a better chance of survival.

Another blackbird enterprise began in the 19th century; to a greater extent than the other thrushes, blackbirds began colonising city parks and gardens. It was a clever move; there are now about 10 times more blackbirds per hectare in suburbia than on farmland. Estimating the size of bird populations is difficult, but the Atlas of Breeding Birds in Britain and Ireland came up with a figure of 1.8 million territories in Ireland each year from 1988 to 1991, far more than the estimates for the blackbird’s more conservative relatives. The figure for song thrushes was 390,000 territories, while there were only 90,000 mistle thrush ones. Entrepreneurship, therefore, seems to pay off.

So, is the UCC nest a once-off freak or is it the start of a trend? Will early breeding in a globally warming environment be the blackbird’s next business venture?