The blackcap is a new bird in our midst. Take the chance to get to know it

This winter visitor has a fluting song, giving it the nickname of "lesser nightingale"

(Richard Vaughan)

New creatures always excite us. I defy anyone not to be stirred by the discovery of Beelzebub’s tube-nosed bat, a devilish-looking creature found in Vietnam in 2011, along with 125 other species new to science in the Greater Mekong region alone, including a walking catfish and a frog whose croak sounds more like birdsong.

Vietnam is a special case: an area which was closed to naturalists by war for 40 years, and is now yielding up a quite extraordinarily rich biodiversity. In Britain, we long ago sussed out all our wildlife, and such discoveries as we make are on a much more modest scale, but nevertheless there is a fascination when we do make one, as in our realisation in recent years that we have a new winter bird.

Birds are the best part of our winter wildlife: after all, there are no orchids, and (generally) no butterflies to entertain us in the depths of January, are there? But every autumn we have a fresh infusion of avian species, as birds from colder climates farther north fly south to winter with us, ranging from the great flocks of wild geese and wild swans to the Scandinavian songbirds, the redwings and the fieldfares and the waxwings eating our berries.

The relatively new winter arrival among us is a warbler: the blackcap. It has always been a summer visitor to Britain, and one of the most appreciated, as besides its restrained but handsome grey plumage – males with a black cap, females with a chestnut-brown one – it has a wonderful, liquid, fluting song, giving it the old nickname of “the lesser nightingale”. (If you go to Kew Gardens to look at the bluebells next May, there is almost always a blackcap singing in the south-west corner of the bluebell wood.)

Like most of our warblers, British-breeding blackcaps migrate south at the end of summer, mainly to North Africa. But over the past 25 years or so of warmer winters, something singular has happened: German-breeding blackcaps have begun to migrate for the winter, not south to Africa, but west to Britain.

It makes ecological sense for them: they can survive in our mild westerly climate, and have a much shorter journey back to their German and Central European breeding grounds, meaning that they can have first choice of the best nesting sites. The process is also thought to have been boosted by the massive expansion here of garden bird feeders, which ensures a constant supply of nourishment.

Birds are the best part of our winter wildlife: after all, there are no orchids, and (generally) no butterflies to entertain us in the depths of January

Our winter blackcaps from Germany are quite separate from our summer birds. Indeed, they are starting to evolve differences, such as rounder wings (for shorter migrations); eventually they might mutate into a separate species, and this month the first survey of them is taking place.

The British Trust for Ornithology is asking anyone interested to spend one day in January (how long is up to you) observing their garden for blackcaps. The BTO wants to get an answer to three questions. What are they eating? What is the balance in numbers between males and females? And how aggressive are they towards other bird species? Taking part will give you a feel for a charming new addition to winter bird tables: go to www.bto.org.

Now even more places to kiss

Our new winter blackcaps are already having an effect on Britain’s own ecology: they appear to be helping the spread of mistletoe, our mysterious Christmas kissing plant which grows parasitically on trees. Although most birds shun the milky-white mistletoe berries, which appear in the cold months, they are among blackcaps’ favourite foods, and it is thought that the birds’ habit of separating out the berry seeds and wiping them directly onto a tree branch is now causing mistletoe to spread far beyond its stronghold of the western counties.

Most mistletoe in Britain is traditionally found in Worcestershire, Herefordshire, Gloucestershire and Somerset, especially in old cider orchards, but recently there has been evidence of it spreading in Surrey, Essex, Cambridgeshire and other areas, and it is strongly suspected that the arrival of wintering blackcaps is playing a part.