Hidden Britain: when wrens have to tolerate each other for warmth
REVEALING A FASCINATING WORLD OF WILDLIFE THAT WE OFTEN OVERLOOK.
WRENS IN WINTER
Every winter day, in late afternoon, the bird-feeders empty and the light fades quickly. Apart from a few species that make showy displays around roosting time, such as starlings, most birds just drift away into the darkness. The odd robin sings after dark, but on the whole few of us know – or have ever seen – what happens to small-bodied birds on long winter nights.
Blue and great tits creep into cavities in trees or walls, or into nestboxes, alone, where they sit out the darkness. Robins head for dense vegetation just above the ground, often among ivy (a godsend for little birds). Flocks of finches and sparrows use thick bushes and low conifers. They all need shelter and security from predators.ors.
Birds’ experiences of winter nights couldn’t be more different to ours, spent inside secure, heated homes. On cold nights, as the temperature falls towards freezing, small species are at risk – and many die.
Bringing loners together
Wrens are well aware that their lives hang in the balance on cold nights and change their behaviour accordingly. On mild nights wrens roost alone, often in one of their famous domed nests, made of moss, leaves, grass and other vegetation. They’re also known to use nests belonging to other birds as well as holes in trees, nestboxes and buildings. There’s even evidence that they might build nests specifically for roosting.
But when it’s cold, rainy or windy, wrens resort to sharing roost sites. More awkward still, they’re compelled to huddle together to keep warm. It is a simple law of physics that very small organisms, with high surface area to volume ratios, lose body heat more rapidly than larger species. And one way to combat this is effectively to make a larger organism – by cuddling up.
Wrens are highly territorial so this must be stressful for them, but the alternative is worse. The birds don’t sit comfortably; in largela roosts they squat in row rows, with heads facing in and tails out, and the precise arr arrangement is worked out b by much squabbling. Most wren roosts are small, so it must take extreme co conditions to produce some of thet remarkable counts that have been made: 61 wrens in a sing single nestbox; 30 in a hole in a thatchedth roof.
The case of long-tailed tits is slightly different. These tiny birds don’t roost in holes, but instead huddle along a perch low down in thick vegetation. They huddle much more often than wrens do, on all but the warmest nights, and their assemblage consists of related birds, usually a breeding pair with their latest offspring and some extra adults. It doesn’t stop the pre-sleep jockeying and jostling for position, though.
Centred on survival
Recently scientists measured how important this jostling could be. Studying captive long-tailed tits, they found that across a range of temperatures a long-tailed tit loses 0.75g per night on average, equivalent to 8.9 per cent of its body weight. But a long-tailed tit stuck outside the huddle loses more – an average of 0.79g per night, or 9.4 per cent of its weight. These differences might seem small, but could mean the difference between life and death.
The scientists also found that, despite the jostling, the same birds tended to occupy the chilly outer spots. Even in family groups, there was a clear hierarchy, with the older, dominant birds staying in the middle. On a freezing night, with life in the balance, there’s no room for sentiment.