Martha Boalch and Seán Kerr, 29/11/2018
Bird behaviour is changing, from feeding to migration. There are a variety of causes linked to these changes, but climate change is often a factor as well as other human behaviour. The Nature’s Calendar species of blackcap, chiffchaff and song thrush are increasingly spotted earlier in the year than normal, here we explore why that might be.
Blackcap are considered summer migrants in the UK. However, since the 1960s more and more blackcap have been seen during the UK winter and we believe this is due to milder winters and more people putting out bird food during the winter. Some of these winter visitors are flying in from Germany.
Fifty years ago, most German blackcap flew south to Spain or Portugal for the winter. If any did end up in the UK for winter they would have died and their genes would not have been passed on to the next generation. However since the 1960s, a growing number of the birds from Germany now successfully migrate to the UK for winter. This change seems to be affecting the genetics of this species.
In 2009, Martin Schaefer and his colleagues studied two populations of blackcap in Germany, living in forests 500 miles apart. Despite Iberian and UK migrants returning to Germany and mingling over the summer the study showed that blackcap shared more genetic material with their migration companions than their close neighbours in Germany. This is because the two migrating populations are less likely to breed together, even if they spend summer in the same German forest, those that overwinter in the UK return to Germany earlier than those coming from Spain.
The two types of blackcap are even starting to look different. German blackcap wintering in the UK possess browner plumage on their backs, rounder wings and pointier beaks than blackcap that migrate south. UK birds no longer need to rely on a large beak to eat large fruits such as olives as they get powdered bird food instead. This evolutionary split has occurred in a very short time, it could eventually cause the blackcap to split into two distinct species. This is unlikely as it would require at least 100,000, maybe even a million, years of continued bird feeding during the UK winter.
You can record blackcap on Nature’s Calendar.
UK breeding chiffchaff historically overwintered in southern Europe or Africa. But since the 1970s, growing numbers of chiffchaff are wintering in the UK. There are benefits to this, since the migration is shorter or non-existent, they can secure the best breeding territories before long distance migrants arrive. But it’s all dependent on the weather. If the weather is mild, as current trends are now showing, they can find enough insect food to sustain them though the winter.
Chiffchaff now over winter in many places throughout the UK, especially in southern England and south Wales. They are fond of lowland areas near water where they can find small flying insects, even in midwinter. Their numbers are even increasing in summer, where the breeding population has more than doubled since 1970 (104% increase). They are able to breed further north and at higher altitudes as climate change makes these areas more favourable.
You can record chiffchaff on Nature’s Calendar.
Song thrush is not considered a migratory bird. Although in severe winters UK birds may move southwards as far as north-west France and northern Spain. Our winter population is also boosted by migrant birds from Holland.
Song thrush is not considered a migratory bird. Although in severe winters UK birds may move southwards as far as north-west France and northern Spain. Our winter population is also boosted by migrant birds from Holland. In winter, they establish their breeding territories for partnering and nesting, they don’t tend to start singing until January. This is the time we ask you to start listening out for the first singing song thrush. It is becoming more common to hear the song thrush sing before January, if that is the case our recorders can submit a record of ‘heard all winter’ rather than ‘first heard’. Find out more here.
The song thrush was once a very common bird in the UK. Between 1970 and 2010, the UK song thrush population declined by 54 per cent. It is thought that one of the causes could be lack of invertebrate food for fledging birds due to pesticide use. Their decline could also be linked to loss of habitat through hedgerow removal, loss of feeding sites as farmers remove wet ditches, reduction in earthworm numbers due to land drainage and poor quality habitat due to woodland degradation.
You can record song thrush on Nature’s Calendar.
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