Wonderful Wrens
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ABOUT BIRDS Wonderful Wrens iny, restless and pugnacious, the Wren of the upland fringe. The species also breeds is one of our most familiar songbirds, on the sea cliffs of offshore islands, where it T widespread across Britain & Ireland occurs in a number of distinct island forms (see and a regular user of the garden environment. page 11), exploiting the tide line for feeding Many books describe the Wren as being opportunities. secretive but, while it is often overlooked, Male Wrens establish their breeding it certainly does not hide itself away from territories in early spring, each proclaiming Man. It would, perhaps, be better described as ownership through a surprisingly powerful being indifferent to us and to our activities. song. Such territory holders are highly The Wren population is at something of aggressive towards other males and use a By Mike Toms a peak at the moment, thanks to a run of combination of song and posture to see off GBW Organiser mild winters which have boosted overwinter their rivals. Each male will court any female survival. Monitoring work carried out by the that enters his territory, tempting her to use British Trust for Ornithology has highlighted one of the nests that he has built there. Males how particularly cold winters can reduce the nesting in the south of Britain will build five breeding population by a quarter or more. or six different nests, the female inspecting each in turn to determine which, if any, she The nesting habit wishes to use. Females appear to select nests The preferred nesting habitat of the Wren that are the most cryptic, presumably because is the understorey of deciduous and mixed such nests are less likely to be predated. Once woodland, especially the damper areas where selected, the nest will be lined by the female this vegetation is particularly lush. However, and breeding will commence. when numbers are buoyant (such as following Males nesting further north, and in a run of mild winters), other habitats are particular on islands off the coast, build fewer increasingly well-used. Among these other nests (in some cases just a single nest) and are habitats are gardens, orchards, farmland more attentive partners than those further hedgerows and the sheltered scrubby valleys south. Such differences in investment relate Wrens (this page and opposite) by Steve Round | Spring 2008 Bird Table 53 WREN to the environmental conditions under which maintaining a compost heap, turned regularly the bird is nesting. With a shorter season and to reveal a wealth of tiny insects upon which FACT BOX less food available in the north of the breeding the Wren can feed. Even so, the species tends ranges, males here have to work harder to rear to favour cover and is unlikely to be seen Common name: a successful brood. Males in the south have visiting bird tables. Wren access to more resources and so can indulge Scientific name: in establishing relationships with multiple Winter roosts Troglodytes troglodytes females, at the same time investing less in The establishment of communal roosts is a Family: means by which individual Wrens can reduce each individual nesting attempt. Similarly, Wrens the species tends to be single-brooded in the heat loss during the long winter nights. extreme north of Britain, managing two broods Such roosts usually involve small numbers of UK population: elsewhere within Britain & Ireland. individuals but there are records of several 8 million pairs (summer) dozen roosting together. There is even a record winter numbers unknown Foods and feeding of 61 roosting together in a Norfolk nestbox Wrens feed on insects and spiders, occasionally during the winter of 1969. In the confined Conservation status: adding small seeds to their diet. Their small space afforded by many of these roosting sites Green listed size and agility mean that Wrens are able to the birds may squat on top of one another, Migratory status: access holes and crevices that would be out of heads in and tails towards the entrance. Resident / winter visitor the reach of many other birds. Many of these The establishment of communal roosts Breeding: sites remain available during periods when appears to be initiated by the male within snow has fallen and so provide a lifeline for whose territory the roost is located. He will Clutch size: 5–6 eggs these diminutive insectivores. Wrens have attract other birds to the roost through calls Incubates: 16–18 days even be known to forage under the snow cover and short flights out. Individuals appear to Young in nest: 15–18 days itself, seeking out the small invertebrates that enter the roost shortly after sunset, and depart Number of broods: 1 (2) remain active within the somewhat protected before dawn, so they may be using sites within Breeding season: Mar–Aug environment afforded by the snow. your garden without your knowledge. Late in Being so small has its disadvantages, the the winter, with the approach of spring, the Age at first breeding: 1 most prominent of which are that the bird resident male attempts to prevent other males Typical lifespan: chills rapidly and is unable to store much in the from entering the roost, favouring females, one Two years way of body fat. Wrens can be tempted to take or more of whom may go on to mate with him supplementary food by providing breadcrumbs, come the breeding season. Certainly, paired Max. recorded lifespan: a little oatmeal or finely grated cheese. A females will often roost within the preferred Seven years more valuable service can be provided by nest prior to the initiation of egg-laying. www.bto.org/birdfacts Bird Table 53 Spring 2008 | ABOUT BIRDS Winter territoriality and we receive a small but regular passage Many Wrens establish winter territories, (in autumn and spring) of birds from the near which they use to defend their preferred continent. Some of these birds, seemingly of feeding areas. Such territories clearly make Scandinavian origin reach Scotland and there an important contribution to overwinter is a regular passage of Wrens reported from oil survival, since it has been shown that territory platforms located in the North Sea. Further holders have much better survival rates than confirmation of the arrival of such birds comes birds without a territory. Presumably having from the spring recovery in Sussex of a Wren a territory provides access to known food ringed the previous autumn at a Russian sources and accrues knowledge of where ringing station on the Baltic coast. these food sources are, plus the locations of suitable roosting sites. Competition for Shout about it winter territories may be fierce and can begin A question that is frequently asked by Garden as early as the preceding July. Even migrant BirdWatchers is how such a small bird can Wrens (of which more later) may establish have such a loud song. In birds, the larynx lacks winter territories, often returning to the same any vocal cords and so has little or no role in territory in subsequent winters. Many of the song production. Instead, birds have an organ winter territories of these migrant individuals known as a syrinx. This is located just above are associated with reedbeds and other damp where the windpipe forks into the two lungs. It habitats, presumably because they hold good has several components, including a resonating populations of invertebrate prey species. chamber (the tympanum), a series of vibrating membranes and controlling musculature. On the move Interestingly, the syrinx is far more efficient at Within Britain & Ireland, Wrens are largely producing sound than our own larynx and its sedentary in habit. However, those birds associated vocal cords. In comparison, while Wetland habitats, such breeding in upland areas may move to lower the syrinx is able to vibrate just about all of as reedbeds and wet altitude sites for the winter. Populations on the air coming out of the lungs, our vocal woodland, are well used our northernmost islands are able to persist chords manage to utilise a mere 2% of the air by Wrens, especially without the need to move elsewhere because of passing over them. In addition, the syrinx can access to the tide line, a habitat that supports produce two different sounds simultaneously during the winter good numbers of invertebrate prey even in (one from each half) and this goes some way months. They are rich in the most testing of weather conditions. Other to explaining the complexity of sounds that a invertebrates. breeding populations may be more mobile bird can incorporate into its song. Wren by Steve Round 10 | Spring 2008 Bird Table 53 ISLAND FORMS Some 43 subspecies of Wren are recognized globally, six of which occur in Britain & Ireland. Two occur on the mainland; troglodytes in southern England (the race also found on the near Continent) and indigenus, found across northern England, Scotland, Wales and Northern Ireland. Then there are a number of island forms; Shetland Wren (zetlandicus) on Shetland, Fair Isle Wren (fridariensis) on Fair Isle, Hebridean Wren (hebridensis) on the Outer Hebrides and St. Kilda Wren (hirtensis) on St. Kilda. These island forms typically differ from the mainland forms by being darker in the ground colour of their plumage and more heavily barred - for example, the Shetland Wren is a sooty brown colour and not the rufous brown of mainland birds. Their songs also differ and some of the subspecies are notably different in size. The development of island races or subspecies derives from their isolation. Separated from the mainland populations, there is scope for divergence in characteristics, like song or body colour. The island forms also show differences in their behaviour, with the males typically more supportive of their mates during the breeding season.