A small bird with a big impact

Weighing in at less than 12 g and 10 cm long, the wren (Troglodytes troglodytes) is one of Britain`s smallest birds; only the goldcrest and firecrest are smaller. Nigel Buxton, our Natura Project Manager, tells more about this surprisingly successful bird.

It would not at first appearances seem to have much going for it; in shape it is undoubtedly dumpy. It is brown, with short rounded wings, a body which appears plump, a short narrow tail and long spindly legs! Even its Latin name does not seem to do it any favours – Troglodytes means a cave dweller or, alternatively, old fashioned and ignorant. It certainly is neither of the latter, being a very successful species over a very wide area covering not only the UK but also Europe and a wide band of Asia, from Iran and Afghanistan to Japan. However, because of its small size and agile behaviour, it is certainly adeptly able to live in and search, not exactly caves but small spaces, including holes in rocks for its food and can secrete its nest in small cracks and crevices.

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(C) Nigel Wedge, Creative Commons

The wren is able to thrive in a wide variety of habitats; in Scotland it is well known from gardens in villages and towns, in hedges, woodlands, reed beds, sea cliffs, heathery uplands and moorlands, including scree slopes and boulder fields. Although in Britain it can be susceptible to periods of cold weather, in Asia it is a resident species in higher parts of the Himalayas!

Another indication of its success is the number of wren species. There are 88 species in the Troglodytidae Family, which is divided into 19 genera; Troglodytes, with 10-15 species worldwide (depending on the authority) is just one of these. The Eurasian wren`s ability to prosper in a wide variety of habitats means in Britain alone four sub-species are currently recognised; on the mainland, St Kilda, Fair Isle and Shetland.

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Shetland wren, (C) Mike Pennington, Creative Commons

One aspect of the wren which is not small is its voice; what it lacks in size it makes up for in sound. Its song is particularly loud and vibrant, including a strident and characteristic trill. Even its angry “churr” cannot be ignored!

The wren is not just one of our smallest birds but also one of the most abundant; the latest population estimate is approximately 8,600,000 pairs in the UK. However, there is a downside to being small; it brings vulnerability to cold and, although wrens can live at high altitudes, the continued availability of food is paramount. Consequently cold weather in Britain can have a drastic effect on population numbers. But again the wren has inbuilt resilience. Feathers obviously give efficient insulation against heat loss but wrens have relatively long thin legs and, like most birds, these extremities are not feathered.

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A Fair Isle wren, (C) Dave Curtis, Creative Commons

How do birds cope with this and minimise heat loss? They have a heat exchange system, by which vessels carrying warm arterial blood out of the body to the legs and feet are closely integrated with those vessels carrying the cooler blood back into the body to the heart; heat is exchanged between these two sets of vessels, thereby minimising loss to the bird. This system is perhaps most obviously essential in waterbirds which swim in cold water or sit on ice!

Night-time, when birds can’t feed or move about to keep warm, is obviously another critical period. Wrens are renowned for their communal winter roosting, many birds cluster tightly together to minimise heat loss from a single larger object. As many as 60 wrens have been recorded in one nest box!

Perhaps most importantly, with its wide distribution and use of many habitats, it can rapidly replace numbers lost during winter. Wrens can be polygamous, with males having up to four actively breeding females in their territory, each capable of producing two broods of between five and eight eggs in the season. That’s potentially a lot of new birds if all the chicks fledge successfully!

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Wren with leaf for nest building, (C) Kentish Plumber, Creative Commons

Nests are built by the male; he builds several, often between four and six, with the female ultimately choosing which will be used. Nesting in crevices gives the opportunity for both a variety and abundance of nesting sites and perhaps may be the nearest it becomes to being a cave dweller!

In some places killing or harming a wren is thought to bring bad luck, resulting at the very least in broken bones or problems in the home or farm. However, in some places, such as Ireland on St Stephen’s day, in a surprising contrast, wrens used to be hunted so they could be attached to a decorated pole; nowadays I’m pleased to say a fake wren is used! Despite its size it is also known as the king of the birds and in folklore there is a story of how the wren tricked the eagle to enable it to fly the highest (clever not ignorant!). Look back to pre-decimal coins in the first half of the 20th century and  you’ll see that the wren figures prominently on the farthing – then the smallest extant coin with a value of ¼ of one (old) penny.

Perhaps the most obvious fact about the wren is that, with its abundance, it occurs on virtually all of our statutory sites, whether they are Sites of Special Scientific Interest (SSSI),  Special Areas of Conservation (SAC), Special Protection Areas (SPA) or Ramsar sites. These protected sites, with their variety of habitats, are vital in underpinning a significant proportion of one of Britain’s most common and typical bird species.

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