When billions of songbirds migrate across Europe between their breeding and wintering grounds in spring and autumn, they often seek shelter in the dark of night. But this certainty is deceptive: the nocturnal hikers have discovered at least three species of bats as nutritious prey. A new study shows for the first time how Europe’s largest bat hunts birds in flight at night and even eats them in the air.
Twice a year, billions of songbirds in this country migrate from their breeding grounds to their wintering grounds and vice versa. To avoid birds of prey on this long, dangerous journey, some birds prefer to fly at night. But even in the supposed protection of darkness, the travelers are not safe. As excrement remains show, at least three large European bat species have discovered the nocturnal flocks of birds as a source of food. However, how exactly they go about hunting has been a big mystery until now.
Chases in the night sky
Since the bats hunt at night, it is impossible to film the hunt. And even more adventurous methods such as surveillance cameras at sleeping places, military radar, ultrasound recorders on hot air balloons and GPS trackers have not provided any significant insights in the past decades. Only now have researchers led by Laura Stidsholt from Aarhus University managed to witness a nighttime songbird hunt by bats “live” – with the help of ultra-light biologgers that they attached like tiny backpacks to the backs of 14 giant noctules (Nyctalus lasiopterus). With a wingspan of almost half a meter, these flying artists are the largest bats in Europe.
By recording the altitude, movement and echolocation of their carriers, the biologgers allowed Stidsholt and her team to track the hunting behavior of the giant noctules, even in complete darkness. Overall, the measurements revealed two nocturnal attacks on songbirds: one aborted and one successful. In both cases, the bats initially rose over 400 meters before finally detecting their prey using echolocation and then descending vertically like a fighter plane for up to 176 seconds. The predators beat their wings much faster and more powerfully than during the ascent and emitted a total of over 40 echolocation signals at short intervals – an indication of intensive, targeted pursuit.
Snap snap, wings off
A bat broke off its dive after just 30 seconds – probably because its agile prey escaped it. “We know songbirds perform wild evasive maneuvers like loops and spirals during the day to escape predators like hawks – and they seem to use the same tactics against bats at night,” explains Stidsholt. The other bat, on the other hand, was successful and captured a robin, as was later determined based on the emergency calls made by the bird. Once the bat killed the robin with its bite, 23 minutes of chewing noises followed. The giant noctule not only caught its prey in flight, but also ate it in the air – and without losing height.
“It’s fascinating that bats can not only catch songbirds, but also kill and eat them in flight. Such a bird weighs about half as much as the bat itself – that would be like me catching and eating a 35 kilo animal while jogging,” explains Stidsholt. In order not to lose their balance during this heavy meal, the giant noctules use a special trick. As DNA analysis of songbird wings found under hunting grounds shows, the bats apparently begin their meal by biting off the wings of their victims. This reduces both weight and air resistance. “It does awaken compassion for the prey, but it is part of nature,” says co-author Elena Tena from the Spanish Research Council CSIC in Seville.
The new images not only solve a long-standing scientific mystery, but could also be relevant for species conservation. The giant noctule is one of the rarest bat species in Europe. It lives in old forests, where it finds shelter in tree hollows – but such habitats are becoming increasingly rare. The new knowledge about its hunting style and behavior could now help to develop targeted protective measures.
Source: Laura Stidsholt (Aarhus University, Aarhus, Denmark) et al., Science, doi: 10.1126/science.adr2475