It was until now believed that nocturnally migrating songbirds, while venturing into the unfamiliar night sky for accomplishing their long, challenging trans-continental migrations, could at least release anti-predator vigilance thanks to the concealment of darkness. A new study by Spanish and Swiss scientists published this week in PLoS ONE shows that migration at night is not without predation risk for passerines. A unique creature is indeed capable of exploiting the formidable food source represented by the billions of high-flying, Eurasian songbirds which engage twice a year into long-distance, north-south or south-north nocturnal movements. The danger seems especially acute where birds' flight routes converge around the Mediterranean basin, such as the Iberian Peninsula. This newly recognized hazard adds to the numerous obstacles that sea and desert crossings already represent for fragile migratory passerines. Actually, the newly uncovered danger comes from the deep black sky, in the form of a 45 cm wing-spanned aerial-hawking mammal, equipped with sharp canines and an efficient radar system which remains probably largely inaudible to songbirds.
In 2001, Carlos Ibez and his colleagues at the Doana Biological Station in Seville, Spain, suggested that the giant noctule bat (Nyctalus lasiopterus), a rare European species occurring principally in the Mediterranean, may feed to a large extent on birds (Proceedings of the National Academy of Sciences 98, 9700-9702). They had found numerous feathers in the faeces of Spanish giant noctules, with occurrence peaks in the diet in spring and autumn, i.e. during main songbirds' seasonal migration. This contrasted strikingly with food composition of other European bat species which all feed exclusively on invertebrates!
This finding brought about a heated controversy among bat scientists. Some claimed that eating feathers was no proof of preying upon birds: they suggested that giant noctules could
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