Batman Eats Robin!

What do bats eat? We all know about vampire bats--they suck blood. Although I personally think the most fascinating thing about vampires is that they share their bloodmeals with cave-mates who were less fortunate in finding a vein. Then there are the fruit bats, they eat ... well, fruit. Then there's the insect eating bats. In fact, much of the bat's famous sonar evolved to help them catch flying insects in the dark. It would also stand to reason that there would be omnivorous bats that ate some combination of the above. But do bats eat birds?

Well it seems that the giant noctule bat (Nyctalus lasiopterus) does just that.

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.

Bird eating bats?? Oh my!!

Scientists first suspected this might be the case a few years back when they found feathers in bat feces (I thought it was called guano?), but were met with much justified skepicism. To settle the issue, they looked at isotopes of oxygen and nitrogen in the bats and the suspected prey. All elements exist in isotopes, where the number of protons is the same, but the number of neutrons differs. The ratios of different isotopes varies in nature and can be detected using mass spec analysis. Since "you are what you eat," you can determine an animal's diet by comparing its isotope ratios to that of its suspected diet sources.

But how to study the foraging habits of an elusive predator that chases its favourite prey several hundreds if not thousands of metres above the ground in total darkness?

To elucidate the mysterious habits of giant noctule bats, an ambitious investigation led by young scientist Ana Popa-Lisseanu, under Prof. Ibañez' supervision, was launched by Spanish research teams based in Sevilla (Doñana Biological Station) and Granada (Zaidín Experimental Station) thanks to funding from the Spanish Environmental Ministry. The programme was soon joined by one of their once most virulent Swiss detractors (Prof. Raphaël Arlettaz, University of Bern and Swiss Ornithological Institute). The technique of stable isotopes was applied because this recently developed method allows tracking species' main dietary specializations and trophic level position along food chains. The researchers concentrated on carbon and nitrogen isotopes, which are among the best dietary tracers. They analysed their concentrations in bats' blood throughout the year, predicting major seasonal shifts in isotopic signatures towards bird prey in spring and, especially, during autumn. In parallel, isotopic signatures of whole insect bodies and passerine muscular tissues were tracked throughout the year, serving as a control. The results elegantly showed that the bats ate only insects in summer, included some songbirds' flesh in their diet during spring, and depended a great deal on passerines during autumn. Moreover, a higher fraction of songbirds' flesh in autumn than in spring was attributed to the more massive passerine migration in autumn, because both parents and offspring migrate then towards their wintering grounds in Africa, whereas in spring only birds having survived winter mortality return to their breeding area. In addition, young birds in autumn may represent naïve, inexperienced, i.e. more vulnerable prey.

(emphasis mine) Why, you ask, did I highlight the fact that these bird eating bats also eat insects? After all, everyone knows that bats eat insects. So what's the deal? The deal is that sometimes insects eat bats. You don't believe me? Then watch this.