Monday, November 01, 2010
ATROGLODYTE is a cave dweller.
The term is not normally used in a flattering way because it has associations with the primitive and the unintelligent but I had one in my conservatory the other morning and found it to be full of charm and interest.
It was a wren, Troglodytes troglodytes, that had crawled into a crack between the lean-to conservatory and the house to shelter from the night frost and then become disorientated and exited into the conservatory rather than the garden. I caught it in my hand and looked at it for a second or two before releasing it outdoors.
It was very alert and slightly aggressive, not at all intimidated by being captured by a giant, but its most outstanding characteristic was the fact that it was almost unbelievably tiny. Baby wrens, when they first fledge, are the size of large bumble bees, with a similar whirring flight. They grow up to be the second smallest Irish bird, after the gold-crest.
They have no close relations among European birds, though modern genetics suggests they are distantly related to both dippers and robins. But a number of wren species, including ours, are found in North America. The Americans call our species the ‘winter wren’, to differentiate it from other wrens. It’s a good name because one of the things that characterises the little wren is how it has adapted to survive through the winter.
Two main problems start to arise at this time of year – cold nights and a lack of food. Wrens mostly eat very small invertebrates, though they may occasionally swallow a few seeds when food is very scarce. Today they are found in every sort of habitat in Ireland, from cities to remote and treeless offshore islands, but they appear to have evolved as forest birds.
They use their relatively long, slim beaks to probe under bark scales to find over-wintering insects, larvae and small spiders in much the same way that tree-creepers do. Most Irish small birds become vegetarians for the winter but the wren survives by persisting with a high-energy diet.
The smaller a bird is the greater the danger that it will suffer from hypothermia. This is because it has a larger ratio of surface area to body mass and this surface area radiates out the core heat during a long winter night. To counter this wrens become troglodytes, burrowing into places with good insulation. Sometimes they will use their domed nests – very few bird species sleep in their nests, they only use them for breeding, but wrens are an exception. They will also crawl into cracks in old trees, spaces behind fascia boards, holes in stone walls or my conservatory.
Most of the year they are fiercely territorial but they abandon this in cold weather and will fly distances of up to several kilometres to roost together for extra warmth. These communal roosts normally contain ten or a dozen birds huddled into a tight ball. But communal roosts have occasionally been discovered with up to a hundred individuals in them.
The extent to which they also migrate to escape cold weather is a mystery. Concentrations of wrens have been noted in spring and autumn in our eastern coastal areas, suggesting there is some movement. The tiny birds are certainly capable of remarkably long flights. One autumn an exhausted wren dropped on to a ship in the middle of the North Sea and one remarkable bird was ringed in Sweden and recovered in southern Spain. But the majority of Irish wrens stay at home and survive the winter nights by becoming troglodytes.
* dick.warner@examiner.ie
© Examiner Publications (Cork) Limited, City Quarter, Lapps Quay, Cork. Registered in Ireland: 73385.
