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More and more red kites are spending the winter in Switzerland

Red kites are increasingly not flying south in winter. Whether a red kite migrates south or winters in Switzerland depends on its age, size and sex, as a new study by the Ornithological Institute Sempach LU shows.

"Forty years ago, it was still very rare for a red kite to spend the winter in Switzerland," biologist Stephanie Witczak from the Swiss Ornithological Institute told the Keystone-SDA news agency on Tuesday. According to the expert, red kites were observed wintering in Switzerland for the first time ever in the 1960s. Like many other bird species, red kites are now considered to be partial migrants.

For the study, which was published in the January issue of the "Journal of Animal Ecology", Witczak investigated the factors that determine whether a red kite migrates to the south of France or Spain or stays in Switzerland. Together with other researchers from the Sempach Ornithological Institute, she tracked 381 red kites over several years using GPS.

Sedentary birds stay sedentary

In their first year of life, almost all of the red kites with GPS transmitters migrated south. The likelihood of them switching to sedentary behavior increased with age. According to the Sempach Ornithological Institute, it is estimated that around two thirds of adult red kites spend the winter in Switzerland. According to Witczak, one possible explanation for this is that the birds may have a higher breeding success if they are in their breeding grounds earlier.

The age at which a bird stayed in Switzerland for the winter also depended on the size of the female: larger female red kites settled earlier in life than smaller ones. According to Witczak, this could be related to the fat reserves required for breeding, for example.

Once settled, red kites rarely became migratory birds again. According to the study, the adult males also appear to derive a considerable survival advantage from remaining sedentary.

"It is a first step towards understanding how and why such migration patterns change," said Witczak about the study results. Further studies could shed light on the exact role played by environmental factors such as the increasingly mild winters.

©Keystone/SDA

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