Exploiting the fly-by-nights

Nyctalus lasiopterus, the giant bird-eating bat

As winter approaches, billions of birds embark on long southward migrations across the Mediterranean to warmer climes. Falcons, whose colonies are found on coastlines and islands along the autumnal migration routes, prey on these migrating birds. However, the falcons’ keen eyesight, which they use to hunt, is useless at night, and the migrating birds minimize predation by travelling mostly under the cover of darkness. These streams of nocturnally migrating birds therefore represent a huge, and largely unexploited, source of high quality food – perhaps as much as 100,000 metric tonnes of it. Now, new research published in PLoS One provides evidence that migrating birds face another predator. But, unlike falcons, these hunters are blind, and use echolocation (or biological sonar) to locate their fly-by-night prey.

Six years ago, Carlos Ibañez and his colleagues at the Doñana Biological Station in Seville, Spain examined droppings from the great noctule bat Nyctalus lasiopterus. They found bird feathers in some 14,000 samples of droppings; this led them to suggest that the bats prey on migrating birds. Skeptics said that finding feathers in the droppings was not evidence that the bats preyed on birds; they could instead be catching and digesting the feathers after mistaking them for flying insects. Furthermore, they pointed out that all other European bat species prey only on invertebrates, and that there were no birds’ bones in the bats’ droppings. Ibañez further suggested that the bats consume the breast meat and discard the rest.

The present study, led by Ana Popa-Lisseanu, confirms that great noctule bats do indeed prey on migrating birds. Lisseanu’s team collected samples of blood from the great noctule bats, and measured the concentrations of carbon and nitrogen isotopes in the blood samples, which vary according to diet. They found a seasonal variation in the isotope composition of the bats’ blood – in fact, there was an almost perfect correlation between isotope levels and the times of year when birds migrate. In the summer, when there is no bird migration, the composition of the bats’ blood corrsponded with a diet consisting purely of invertebrates, while in autumn, when birds migrate south, and in spring, when they undertake their return migration back north, the isotope levels indicated that the bats fed on birds. Thus, it seems that the great noctule bat preys solely on invertebrates during the summer, but then opportunistically switches its diet in the autumn and spring to exploit the huge flocks of migrating birds.

The great noctule bat is found almost exclusively in restricted parts of the Mediterranean. Their geographical distribution coincides with the stopover regions at which the streams of migrating birds congregate. The vast majority of these migrating birds are small, with a body mass of less than 20 grams. This is less than twice the size of the great noctule bat, which, with a body mass of 70g and a wingspan of 45cm, is Europe’s largest bat species. When capturing insects, the bats wrap their wings around them and kill them with a bite. These bats are aerial foragers, but it is, as yet, unclear exactly how the bats subdue their avian prey. They are certainly large enough to capture birds in the same way they capture insects, and the altitude at which the birds migrate – over 700 metres – means that there are few obstacles for the bats to collide with while they wrestle with their prey.

This is the first known example of a vertebrate exploiting the nocturnally migrating birds as a source of food. Lisseanu’s team now hopes to observe the great noctule bats catching and eating birds on the wing, but this may prove difficult. They would also like to investigate whether or not other bat species also prey on migrating birds.

References:

Popa-Lisseanu, A., et al. (2007). Bats’ conquest of a formidable foraging niche: The myriads of nocturnally migrating songbirds. PLoS One DOI: 10.1371/journal.pone.0000205.

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