Bats hunt migratory birds

Giant evening sailor with unusual prey scheme

Giant Noctule. © Ana Popa, Estación Biológica de Doñana, Seville
Read out

Bats in Spain have specialized in foraging for an unusual prey: they hunt for migratory birds at night. This surprising behavior has now been discovered by a Swiss-Spanish research group in a new study.

The many dangers that migratory birds experience twice a year on their journey are joined by another: the scientists found that the billions of Eurasian songbirds are a welcome source of food for a particular bat species in the Iberian Peninsula: the giant eel Nyctalus lasiopterus, with just over ten inches of body length, 46 centimeters wingspan and up to 70 grams of weight largest European bat species.

Already in 2001, Spanish researchers had discovered feather leftovers in bat dung and suspected that giant eel in Spain to a large extent feed on birds. These feather remnants were found mainly in spring and autumn, ie during migration of migratory birds. For European bat species, which are actually feeding exclusively on invertebrates, this would be a completely new predator-prey system. So the theory from Spain was first received with the utmost skepticism by bat researchers.

Opponents of the theory found another explanation for the bird feathers in the stomachs of the giant eel: the bats would mistake the feathers for insects and swallow them by mistake. This could not be ruled out, especially during the migratory period of birds, because many free-floating feathers would then be in the air. The sole presence of feathers in bats should therefore not be sufficient evidence that bats chased birds.

Unusual behavior

However, a Spanish-Swiss research group has now been able to demonstrate by means of the concentration of certain isotopes in the blood of giant eels that they actually eat birds. On the basis of the isotopes, which serve for the analysis of the food, showed a clear pattern: The Giant evening sailors fed in the summer only by insects, in the spring additionally by migratory birds and in the autumn almost only of it. display

The more frequent traces of bird meat in autumn compared to spring explain the researchers with the larger bird migration in the fall: At this time, both parents and young fly to their wintering places to Africa, while in the spring only those birds from the south return have survived the winter. In addition, the inexperienced young birds in the fall are probably an easy prey, according to the researchers around Ana Popa-Lisseanu and professors Carlos Iba ez and Rapha l Arlettaz from the Universities of Seville and Bern.

Unique ecological niche

According to Arlettaz, this nocturnal hunt of the giant eel on trawlers is a unique ecological niche: "Although there are other carnivorous bat species, they live in the tropics and feed on small prey animals, A few hawks in turn hunt migratory birds along the Mediterranean and African coast, but only during the daytime Owls as typical nocturnal hunters finally do not move in open space : They locate prey animals over the rustling sounds they make on the ground. "

The unique cological niche of giant egrets may explain some of their unique natural historical features: the species occurs only in special areas of the Mediterranean, where swarms of migratory birds often gather. The Giant Evening Sailors are also among the largest European and Asian bats and are even among the heaviest bats in the world hunting in the air. Her body mass of up to 70 grams at 45 centimeters wingspan is a prerequisite to hunt a prey like a migratory bird, which has about the same body weight as other European bat species.

(idw - University of Bern, 15.02.2007 - DLO)