BTO: German visitors hog the birdtable

There is a noticeable upturn in the use of the garden bird feeders as the first real winter weather begins to make itself felt. Alongside the familiar tits, finches and sparrows, you may also see the occasional wintering blackcap, a species more often regarded as a summer visitor.

The presence of these visiting blackcaps is the result of rapid evolution in their migratory behaviour. While the blackcaps breeding in Suffolk’s scrubbier habitats migrate south in the autumn, a journey that takes them to the countries bordering the western end of the Mediterranean, another group of blackcaps arrives to take their place. These late autumn arrivals originate in Germany and studies have demonstrated that the autumn move to Britain gives them a competitive advantage over others of their species, wintering further south. It turns out that the German blackcaps wintering in Britain arrive back on their breeding grounds earlier than their rivals, something that enables them to secure the best territories and, ultimately, produce more young. Genetic studies show that the tendency to migrate to Britain is genetically-coded; as these birds produce more young, so we see increasing numbers of German blackcaps wintering in Britain.

Two theories have been put forward to explain why these central European blackcaps have been able to overwinter in Britain so successfully. The suet and sunflower hearts that we provide in our gardens may be particularly important, providing the birds with food at a time of year when other foods are in short supply. Temperature may also have a role to play, with our changing climate reducing the severity of our winter weather and lowering the energetics stresses that these small birds face. That the BTO’s weekly Garden BirdWatch survey (www.bto.org/gbw) sees greater use of gardens by blackcaps in the southwest of England than elsewhere, may indicate the importance of overwinter temperature. Southwest England benefits from the warming effects of the Gulf Stream and is the mildest part of the country.

Blackcaps are not always the most sociable of bird table visitors, and individuals may drive off other garden birds in an attempt to keep a food source to themselves. It is only the adult males that have the black skull cap after which the species is named, so watch out for the brown-capped females too.

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