The mystery of why songbirds arrived late in their European breeding grounds in 2011 has been solved. The answer is published in today’s edition of the leading journal Science. Dr Raymond Klaassen, who works as a researcher at the University of Groningen, is second author of this article. The publication by Klaassen and his colleagues from the University of Lund (Sweden) and University of Copenhagen (Denmark) puts an end to speculation about the cause of this delay.
Every year in May, various species of songbirds arrive in Europe to breed after having made the long journey from Africa. However, in 2011 many of these birds arrived much later than usual. Concerned bird-watchers wondered why this was. ‘There was all kind of speculation,’ explains Klaassen. ‘Internet was rife with rumour; the birds had been hampered by strong headwinds over the Sahara, or they’d encountered thunder storms in Turkey. But nobody really knew.’
Nobody knew – until the birds eventually arrived. Klaassen and his colleagues had fitted small sensors to the backs of dozens of red-backed shrikes and thrush nightingales. The sensors monitored the birds’ location by means of the amount of light. On arrival, the researchers studied the data from the geolocators and discovered the reason for the delay: the birds had been held up by extreme drought in the Horn of Africa.
As they do every year, in 2011 the birds stopped off in the Horn of Africa to fatten themselves up. In normal circumstances, they accumulate enough fat to enable them to fly almost non-stop to Northern-Europe. But last year, the serious drought meant that the birds took longer than usual to fatten themselves up. They spent an average of ten more days in Africa in 2011, almost double the time they spent there in 2010 and 2012, when it was not as arid.
To be more certain that this had in fact caused the delay, the researchers compared droughts in the Horn of Africa over previous decades with data about the arrival of the songbirds. Delays in previous years also turned out to correspond with periods of prolonged drought. Furthermore, species that did not stop in the Horn of Africa to feed arrived on time in 2011.
But this research does not only solve the mystery of the delay. ‘The geolocators have also shed more light on the habits of songbirds’, explains Klaassen. ‘This is helping us to protect these birds. We now know that drought in Africa can represent a real problem.’
‘The research findings are also important in terms of showing the huge impact of an event in a relatively small area’ continues Klaassen. ‘A problem in just one link of the annual cycle (in this case the food supply in a certain region) can interrupt the entire migration system.’
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