It's almost ten and almost dark.

The Nidda lies still in the twilight.

"You might think it's a quiet evening," says Ulrike Balzer.

Then she turns on her detector.

Suddenly you hear an electronic click - the device makes the bats' ultrasonic sounds audible to humans.

Balzer smiles.

"They are among us."

Rainer Schulz

Editor in the Rhein-Main-Zeitung.

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Where the Rosa-Luxemburg-Strasse between Ginnheim and Heddernheim crosses the Nidda, bats hunt for prey in the evening.

The flying artists can just about be seen with the naked eye.

However, if you want to watch the spectacle, you have to be patient.

Because before that it means: wait.

The noctule bats start first

It is 8 p.m. when Ulrike Balzer – wearing jeans, a T-shirt, her gray hair tied back in a ponytail – gets out of the van that brought her from her home in Lich to Frankfurt.

Also present: bitch Paikea, 17 years old, very frugal and almost deaf.

"She doesn't hear anything anymore," says her mistress and pours water into a bowl.

Then the two set off on their walk.

When the sun goes down, bats take off for the night.

The noctule bats fly out first.

Then the other species will follow.

They are hungry and looking for food.

The table is richly laid at the Nidda: mosquitoes, flies and beetles buzz in the air.

Bats eat a quarter to a third of their body weight on insects every night.

The water bat creates 3000 to 5000 mosquitoes at night.

The pipistrelle comes to around 1000. The greater mouse-eared bat can also devour larger insects: it is only full after 50 to 60 ground beetles.

Other bats devour small lacewings or cockchafers.

"They share the living space," says Balzer.

The biologist is like a living book.

There's hardly a question about bats that she can't answer.

When the environmental agency wanted to carry out a survey a few years ago, she was there and went through Frankfurt's parks, forests and green spaces with a safety net and detector.

She and a colleague found 14 species at the time, including the rare Bechstein's bat, the brown and gray long-eared bat, the common pipistrelle, the water bat and the common noctule bat.

That's a good cut.

There are 25 species throughout Germany and 19 in Hesse.

There are many insects in the warm city

There are many more around the world.

Balzer says: "A quarter of all mammals are bats" - i.e. flying foxes or bats.

The animals are not related to mice because they are not rodents.

Fledern is an old German word for flutter.

Not all bats flap.

Some sail elegantly and cannot be distinguished immediately from swallows or swifts in the twilight.