A little bird with a very big heart

CONTRARY to popular belief, the wren is not our smallest bird.

The ‘king of the birds’ weighs 10 grams, but there’s another little songster only half as heavy; the goldcrest weighs the same as a 20-cent coin.

However, this does not make it a contender for the world lightweight title; the bee hummingbird of Cuba doesn’t even weigh two grams. Being small can be a problem in winter; goldcrest numbers fall drastically during cold spells, like the one we’re experiencing at present. Most warblers avoid hard weather by spending the winter abroad, but Irish goldcrests stay put and brazen it out.

These little birds are not shy of people and don’t mind being stared at. Light brown above, paler underneath, the bright crest on the crown is unmistakable. Females have yellow crests, males have mixed orange and yellow ones. Insects and spiders living on the branches of conifers are their main source of food. Spruce and fir trees are especially favoured, but goldcrests also like big redwoods – the soft bark offers rich insect pickings and snug places to roost. It’s ironic that Europe’s tiniest bird has taken a shine to the world’s largest tree. British and Irish goldcrests don’t confine their hunting to evergreens; they visit willow and alder scrub along riverbanks, and turn up in gardens. The female produces a clutch of 11 eggs, one-and-a-half times her body weight. She will lay a second batch in another nest, and may do so while her mate is raising the first brood.

There are more goldcrests to the hectare in Britain than in mainland Europe, thanks to the milder Atlantic climate. Conditions in Ireland are even less severe, so numbers here are higher still; British bird ringers are impressed with the numbers we catch. Lack of competition with other species benefits Irish goldcrests. We have no firecrests, willow, or marsh tits here, and there are fewer visiting warblers. Irish birds can afford to exploit a wider ecological niche than is available to their British counterparts.

In 1847, the German biologist, Christian Bergmann, formulated a rule that bears his name; it asserts that animals living in cold environments tend to be bigger than those in warm ones. The association is a loose one; its validity has been questioned, but the rule works most of the time. It’s a matter of physics rather than biology; small things gain and lose heat more quickly than large ones. This has to do with the ratio of surface area to volume; bigger objects have proportionately smaller surface areas and heat must enter or leave a body through its surface. Tea pots cool more quickly than kettles and that huge volume of water, the Gulf Stream, stays warm for months. Animals in cold environments, therefore, tend to be as big as food resources allow. Dinosaurs depended on the sun to heat their bodies; being large was an advantage when temperatures dropped. Whales are big, so that they can keep warm in cold seas; weight is not a liability for a swimming animal and there’s enough food to sustain the over-sized lifestyle.

The little goldcrest, however, is at the other end of the Bergmann scale – when temperatures fall, it can be in trouble. But dealing with the cold is a goldcrest skill. A study carried out in the 1980s found that Scandinavian ones, with only six hours of daylight in which to feed, could survive huddled together in temperatures of -25º C, losing a fifth of their body weight overnight. So how are our goldcrests faring in the current cold spell? Numbers always crash during severe winters. In 1920, one observer thought that the goldcrest “could have little more than an obituary notice”. The winters of 1947 and 1963 were especially severe and the bird almost disappeared. Then it staged a come-back. Laying 20 eggs each season is part of the recovery strategy. When the death-rate is high, large broods are insurance against disaster; there are plenty of youngsters to make good the losses. American goldcrests, closely related to our European species, are known as ‘kinglets’. Shouldn’t we depose the wren and put the resourceful little goldcrest on the avian throne?