Wrens – The Fascinating Life of Troglodytes troglodytes
Wrens are confident birds, but difficult to photograph because of their cryptic colouring and speed, and their penchant for the dapple of undergrowth and nooks and crannies, hence their wonderful Latin name,Troglodytes, the cave dweller.
They are a relatively common bird found throughout Europe, North America, and much of Asia. They occupy a wide range of habitats, and are often associated with human habitation in urban areas, villages and farm buildings.
Wrens are more often heard than seen during the summer. Views are often fleeting, a quick view of that erect tail as they dive into the secret areas of gardens, sheds and woodpiles.
Most Males Fail Dismally in the Sharing and Caring Role
A couple of years ago, during mid-March in Woodcock Wood, we were able to watch a male Wren building one of several of his nests. He spent a lot of time in the open plucking moss to build his domed nest in a wooden “insect house”.
He was building what is known as a “cock nest”. This will be one of several nests he will construct, and we did find another under loose tarpaulin in the gutter of our storage container.
Typically, Wrens will build about five nests but some prolific individuals will construct ten or twelve in one season.
Having completed the structures, his next task is to attract a female through song, and show her the nests. If she’s impressed by his display and his nest building prowess, she will choose a nest and complete it by lining it with soft material, mating will take place, and then the rest is left to her … he is off to attract another female to another of his nests.
Most males are polygamous, and take no part in the rearing of the brood. However, some have been recorded as staying with a single female and even helping rear the young.
Territories
Males are strongly territorial during the breeding season, defending large territories against other males. Singing from song posts is an important part of this defense.
Females however have no territories. They will wander through different males’ territories until they find a mate, and will play no part in defending the patch, even when they are nesting.
Favoured Nest Sites
In the countryside, they will choose sites low to the ground and in thick cover, often in damp areas near streams. Around human habitation they often select nooks, crannies, and ledges in outbuildings.
A study1 in Spain specifically on Wrens nesting in farm buildings, found that a favoured spot for their nests was abandoned Swallows’ nest cups in barns. These are usually built high up on rafters and therefore safe from predation. Indeed the study observed a fledging success of 100% in these sites.
Other sites in farm buildings included ledges and holes, bundles of rope, jacket pockets – such is the ingenuity of the Wren. Overall in the study, 93% of chicks fledged, which is probably a better success rate than those choosing to nest in the countryside.
Wrens in Winter
Like all small insectivorous birds, Wrens struggle during severe winter cold spells. Daylight and foraging time is short, and there is a need to increase body fat in order to survive each night, and this inevitably has an impact on the population.
For instance the infamous “Beast from the East”, that brought extreme cold and heavy snow to the UK for eight days at the end of February 2018, was estimated to have reduced the Wren population by 21%2 in 2019.
Small birds have of course developed mechanisms to combat this. For instance members of the tit family (parids), can survive extremes of cold in Siberia by lowering their night-time body temperature from 42 ° down to 35 °C. They remain in this torpid state throughout the night, and thus lessen their energy needs to maintain life3.
Another strategy is for small birds to shelter in nesting holes and boxes during cold spells. Wrens can to take this to the extreme. The BTO suggests the record is 63 Wrens in one box4 – cozy yes, suffocating, presumably not!
An interesting set of observations were made by B.D. Harding5 on this behaviour, over a period of four months – January to April. The number of Wrens recorded entering a box in his garden at night ranged from 2 to 28. The calls they made during their approach were assumed to be a stimulus for the gathering. The number per night did not seem to correlate with either particularly cold or wet conditions. For Wrens, therefore, communual roosting in an enclosed space seems to be a winter strategy rather than a behaviour stimulated by particularly severe weather.
Wrens are, indeed, fascinating little birds, and fortunately not on the endangered list!
Footnotes:
- “Nest-site and breeding success of the Wren Troglodytes troglodytes inside human habitations”, Pedro J. Cordero, Ringing and Migration (1992) 13,122-124.
- https://community.rspb.org.uk/ourwork/b/science/posts/the-breeding-bird-survey-2018-report
- “Adaptive temperature regulation in the little bird in winter: predictions from a stochastic dynamic programming model” Anders Brodin, Jan-Åke Nilsson, and Andreas Nord, Oecologia. 2017; 185(1): 43–54.
- British Trust for Ornithology BTO: https://www.bto.org/about-bto/press-releases/birds_feather_sleep_together
- “Observations at a Wren Roost” B. D. Harding, Brit. Birds, 82: 48-52, February 1989