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Autumn Migration

Thursday, 25th August 2016

Birds like Swallows and Sand Martins will soon set off for their winter home in Africa and the Mediterranean.
So, how does a bird know when to migrate? Well, at roughly the same time every year a birds’ body will start to release hormones, which makes the bird eat more, ensuring that its body weight increases for the long journey ahead.

Migrating birds have their own traditional route and therefore know which way to go - they don’t just ‘wing’ it. They mostly follow coastlines and valleys which may take them across stretches of sea or desert, returning usually, along the same route. So it would seem that navigation plays a big part in migration. Young birds have no experience of the route and may have no help from their parents so it is now thought that the sun and stars play a big part. Even in poor weather birds detect the sun's position using polarised light; – using ultra-violet rays that we humans cannot see. This does not help night flying migrants. They depart at sunset and use the setting sun as their first compass setting, they then use the stars and moon to guide them on their way. Once they have used their extra body fat and their weight has returned to normal, they instinctively know that they have reached their destination and have no urge to migrate further.

At least 4,000 species of birds are regular migrants including the Cuckoo, Swift, Reed Warbler and House Martin. However a few, like the partridge, do not move very far from where they are born.

We are lucky in this country to have native migrants. These are birds like wrens and starlings, which move into urban areas from the countryside when the weather turns cooler. Thrushes will even move from northern to southern Britain to spend the winter.

Let’s not forget the birds migrating to Britain, geese and swans arrive here as our winters are not as severe as other countries, Blackbirds in your garden during autumn could be winter visitors from Eastern Europe.

Migration is a really exciting time but recording the departure dates of migrants can be difficult.

Take the county of Leicestershire for example, they have recorded the following data for the latest departure dates for these summer visitors:-
 
Latest departure dates for summer visitors

Cuckoo – 10th September
Swift – 25th September
Sand Martin – 6th October
Swallow – 2nd November
House Martin – 20th October
Reed Warbler – 27th September
(Source: British Trust for Ornithology (BTO) )

It has taken thousands of years for people to understand the mysteries of migration but there is still plenty we don’t understand but these are unanswered questions that are secret to the animal world only.

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