Blackcaps with backpacks

Over last couple of months I have been helping out with the beginnings of an exciting new study into Britain’s overwintering population of Blackcaps with fellow undergraduate Robbie Phillips. This three-year project is being run by Benjamin Van Doren from Oxford University and Greg Conway from the BTO, in conjunction with research and academic staff here at the University of Exeter’s Penryn Campus and the Max Planck Institute for Evolutionary Biology.

The studies’ aims are to shed light on the year-round movements and wintering behaviour of Blackcaps, whilst looking into both the genetics and morphology of those birds choosing to spend the winter in our well-provisioned suburban gardens.

The species in question: a blackcap (Sylvia atricapilla)

Until around 50 years ago, Blackcaps were about as rare to see overwinter in Britain & Ireland as would be a Swallow at present. This plump Sylvia warbler typically spends the winter on the sun-baked shores of the Mediterranean and north Africa, before migrating northward to its European breeding grounds across much of northern and western Europe.

It came as something as a surprise, then, when the number of overwintering birds steadily started increasing in the UK; surveys such as the BTO Garden Birdwatch tracked this proliferation in numbers over ensuing decades, until many thousands are now counted every winter. These surveys have also revealed a particular preference of Blackcaps for garden feeding stations, especially those offering tasty fat balls, halved apples, seeds and cake. This suburban banquet is further supplemented by an abundance of berries and fruits often adorning hedges and garden perimeters.

Ringing recoveries indicated that the birds overwintering in the UK were largely of German and central European origin, and had thus completely switched their migration strategy: flying north-west to good ‘ol Blighty come autumn as opposed to heading south! A fascinating study by researchers at the University of Freiburg discovered that the British-wintering Blackcaps were also genetically dissimilar to migratory individuals breeding in the same area in Germany.

So where does this leave us? Well, it essentially appears that these Blackcaps from central Europe have developed a new migratory route in response to the artificial provisioning of garden food here in the UK. This provides a number of advantages: by overwintering closer to home, these birds are able to arrive on breeding grounds sooner and in better body condition after a shorter flight, allowing first choice of breeding territories and a host of other fitness benefits.

Many questions still remain, however, which is where this new study hopes to fill in the gaps. Ringing recoveries have provided the basis for inferring the source of our overwintering blackcaps thus far, but these are very much thin on the ground. A small number of recoveries even indicate that some birds might be year-round residents in the UK! To shed light on some of these remaining questions, this new study is utilising Geolocator backpacks that are fitted carefully to the lower backs of blackcaps…

a male blackcap fitted with a geolocator. The devices weigh less than a gram and fit snugly over the bird’s back, with loops securing it to the upper thigh a bit like a climbing harness

The Geolocators couple an accurate clock with a tiny photoreceptor, which provides two key pieces of information: the latitude can be calculated from daylength, and the longitude from the time of sunset and sunrise. These devices are accurate to around 70km, and should allow us to discover where these wintering birds are spending their summer months…in the Uk? In Germany? Or even further afield? The only disadvantage of these bits of technology is that they have to be retrieved to get the data back, and so we’re just hoping that a proportion of the 40-or-so birds fitted over last winter will return to the same gardens!

In addition to fitting these fancy tags to the Blackcaps, we have also been attaching colour-rings to their legs as part of the study. The unique combination of colours allows individual recognition in the field without the need for recapture, and will hopefully provide a wealth of information on the wintering behaviour and movements of these birds.

Any blackcap we catch are fitted with a unique combination of colour rings and have a variety of biometric measurements taken, such as tarsus length, bill length and the length of various wing feathers

Cornwall, with its balmy climate and plentiful number of suburban gardens, is something of a mecca for overwintering blackcaps. This makes it an ideal place to carry out tagging and ringing, which is what Robbie Phillips (a Zoology undergraduate at the University of Exeter) and myself have been attempting to do on calm days this winter. After receiving the necessary training to attach the tags and carry out the procedure, we have been visiting a number of gardens around Penryn and Falmouth to catch overwintering blackcaps and contribute to the exciting study.

The great thing about spending hours trying to catch the desired birds (and yes, it can take a lot of time and effort to catch these birds sometimes!) is that you are always bound to catch a number of other species too. We’ve had some great birds pay a visit to the mist nets, from bullfinches, goldfinches and greenfinches, through to starlings and even a rook! All of these ‘non-target’ species are fitted with the ‘usual’ aluminium BTO rings, and will add valuable data to the on-going studies that bird ringing provides into the movements, survival and behaviour of common breeding birds in the UK.

A handsome male Bullfinch

The stunningly iridescent head patterning of a male Starling (left) and a bright male Blackbird (right)

So with migration well underway now, this winter’s efforts on the new project are at an end: our overwintering population is soon to be mingled with migrant birds fresh from warmer climes, and perhaps ‘our’ birds have headed off to their German breeding grounds.

However, come wintertime, please keep your eyes peeled for blackcaps sporting flashy bling and a tiny tag on their backs. If you do see a colour-ringed bird, please drop an email to or via the Euring reporting page. And even if you simply notice blackcaps overwintering in your garden, be sure to report your sightings to record databases such as Birdtrack and the BTO’s Garden Birdwatch scheme.

You can find out more about the project by reading an excellent blog post by Ben on the BTO ‘Demog’ Blog


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