The Eurasian wren (Troglodytes troglodytes) is one of our commonest birds; it is very widely distributed, with an estimated population of eight million breeding pairs.
They are tiny birds, only the goldcrest is smaller, and they live most of their lives in deep cover, difficult to see and even harder to photograph.
The male is highly territorial and he announces his claim with surprisingly loud song. He builds several unlined nests for visiting females to choose from and when one of the females has made her choice, the pair line the nest together, with moss, hair and feathers.
Wren song recorded by W. Agster
Wren singing by Keith Gallie [CC BY 2.0]
The male defends his territory vigorously while he and his mate are rearing young but come winter, his behaviour changes. Wrens store very little body fat and, in the winter, keeping warm can be a problem; in fact, some authorities have estimated that a severe winter can kill half our wren population. The male wren with the most suitably accommodating territory will invite other wrens in the area to share his winter roost. He calls and makes short flights to announce that he has the room to do this.
The birds huddle together for warmth, with their heads pointing inwards, sometimes in astonishing numbers. In 1969, in Norfolk, 61 wrens were counted coming out of the same nesting box and in February of 1979, somewhere in Gloucestershire, over 90 wrens were found to be roosting in the eaves of a single cottage.
The male wren, whose territory is used as a winter roost, usually gets to mate with one of the females that has shared his roost. His behaviour, therefore, in inviting other wrens to overwinter with him may not be as altruistic as it first seems: he not only stays warm all winter but, in the spring, he gets first choice of all the females in the immediate area.
A wren’s nest with nestlings
The header picture is by Ian on Flickr (CC BY-NC 2.0)