BEHAVIOURAL ADAPTATIONS OF THE WREN (TROGLOD YTES TROGLOD YTES)
Authors: ARMSTRONG EDWARD A. , WHITEHOUSE HAROLD L. K. 
Published: January 2008

SUMMARY

This paper includes observations made and references to relevant literature published since the appearance of The Wren (Armstrong, 1955). It includes comparisons between the Wren's behaviour and that of other birds.
The Wren is exceptional and perhaps unique in having extended its range in comparatively recent times from North America to Asia, Europe and North Africa. Fossil remains accepted as evidence that it reached Britain some 100000 years ago were wrongly identified.
The Wren is also exceptional in its adaptability. Behaviour, especially the character of the pair‐bond, is related to habitat and foraging opportunities, with polygamy occurring in favourable habitats and monogamy in more marginal areas. This facultative pair‐bond is believed to be related primarily to the food supply. The life‐style is a complex in which the size of the bird and its territory, nature of the display, song and calls, character of the food supply, structure of the nest, roosting behaviour and other adaptations constitute a delicately integrated complex, in which foraging is probably of primary importance. This principle is applicable to many species.
Size of population is very variable as numbers are greatly reduced during severe winters but the remnants surviving in prime or ‘refuge’ areas form nuclei enabling the population to expand widely during a series of mild winters, invading marginal areas where the size of territories tends to be smaller when the population reaches a peak.
Song serves various functions and is partly innate and partly learned. It is readily recognizable throughout the bird's range but local groups utter distinctive songs and there is a tendency for northerly birds to utter longer songs. Individual birds can recognize the songs of neighbours.
The number of nests built is determined by factors as different as the maturity of the male and the amount of rainfall. The enclosed nest facilitates variability in the pair‐bond as a single bird can rear at least some of the young. The complex of behaviour involved, characteristic of a number of tropical species, is further evidence of the Wren's tropical or subtropical antecedents.
There is variability as to when the eggs are laid, before or after the female's first sortie of the day, and also in the inception and duration of incubation. Published estimates of the incubation period not based on data obtained with automatic recording apparatus can be misleading.
When his mate disappears, a male who has done little to tend the nestlings may change abruptly to feeding them with extreme diligence, even spending the night in the nest with them. The male visits his nests whether or not they are occupied and is usually aware of the state of affairs at any nest.
Occasionally a Wren may visit a neighbour's nest and possibly feed the nestlings or a bird of another species may feed young Wrens but there is no evidence that, as happens with some other species, young birds feed nestlings of a later brood. Such ‘altruistic’ behaviour in some species is believed to be advantageous.
As in many other passerine species there are morning and evening peaks in foraging activity for the nestlings. The number of feeding visits tends to be related to the number of the young as well as to the vigour of their begging.
Social roosting is a form of behaviour characteristic of some tropical species of wren but which in them has no close relationship to the air temperature. Its persistence in the otherwise asocial Wren enables it to survive the low temperatures of northern winters.