As a beautiful April evening draws to a close, a small group of people are wandering through Berlin's Tiergarten park in search of some music.

They don't have to venture far to find what they're looking for. Male nightingales, recently returned from Africa, are singing at full pitch.

"Listen, it's a battle rap," says ornithologist Kim Mortega, describing the sing-off between two males trying to attract female attention.

For the six weeks the males are in town, and filling the night air with their catchy compositions, Mortega leads natural history museum-organized night excursions to hear them. Some want to see the birds close up, others just want to hear their love songs.

As for the birds, their astonishingly rich repertoire has a single purpose: to find a partner and mate.

The city's conservation authorities put the current number of nesting pairs at between 1,300 and 1,700 and say the population is increasing by six percent every year.

The migratory species generally breeds in forest and scrub in Europe and south-west Asia, and winters in sub-Saharan Africa. So why is it drawn to Berlin?

"We think the nightingales come here because of the many wild and not yet over maintained green spaces," says biologist Silke Voigt-Heucke. She heads the team at "Forschungsfall Nachtigall", a citizen science nightingale project at the Museum of Natural History in Berlin.

The birds like to breed in nettles, hops and raspberry bushes, building their nests just above ground level, and the neglected city parks offer them the right materials to build a home.

The Luscinia megarhynchos, or common nightingale might only be 16 centimeters (6 ½ inches) long and quite unspectacular to look at, but when it comes to song, it is a virtuoso. Males have been known to sing for up to 20 hours non-stop, and the crescendo effect of their music has been an important symbol in literature since ancient times.

"Poets have run out of words to describe the emotion they feel when they hear a nightingale," says biologist and artist Sarah Darwin, who also happens to be a descendant of Charles Darwin.

"I think many people have heard more about the nightingale than actually heard the bird itself."

Darwin, who is also part of the project team at "Forschungsfall Nachtigall"  says the birds sing in verses. "Each verse is slightly different," she tells DW, adding that the birds have an average of 180 individual verses in their repertoire. "That makes it one of the most varied bird songs  that exists."

Involving citizens 

Darwin is hoping to see the tiny species sing its way into the heart of the German public.

"As far as reengaging people with nature the nightingale is a bit of a gateway species", Darwin says.

Researchers at the Natural History Museum are now encouraging citizens not only in Berlin, but all over Germany to record nightingales and upload the files to their research-database using "Naturblick",  a free app often referred to as "Shazam" for birdsong.

Last year the team received more than 1,700 nightingale-recordings, and they've already collected more than 500 so far this year. An algorithm detects whether the uploaded sounds are really nightingales, and if so, the location shows up on an interactive map. 

Once they see where the birds were located, the researchers can look for insight into preferred breeding locations and even try and identify varying regional dialects. They hope to find different sounds in Germany's capital from those in other parts of the country. Some Berliners even believe the nightingale's verses feature electronic beats.

"You definitely hear it’s got techno. Yeah, more techno than Paganini", says Sarah Darwin, adding that the nightingale's song isn't very melodic, "but powerful and determined, exciting if you like." 

City lights, the hum of traffic and construction work don't seem to bother the nightingales. On the contrary. At one point, the night excursion stops close to the main road to record a burst of song. 

"There are even speculations that nightingales actually like to sing against noise, because they can show that they can sing over the traffic," says biologist Silke Voigt-Heucke. 

In evolutionary terms it makes sense, as the loudest and fittest males will be the first to find a match. Once they've found a female, they continue to sing during the day to defend their territory.

With 95,000 nesting pairs throughout Germany,  a relatively stable natinal population since the early 1990s,  and Nature and Biodiversity Conservation Union (NABU)  estimates of between 4 to 11 million breeding pairs across Europe, the species is in no immediate risk of complete extinction. Nonetheless, habitat loss is taking its toll in some places. Nowhere more than in the UK, where the nightingale population has witnessed a dramatic 90 percent decline over the past fifty years. 

There are no scientifically proven explanations for the crashing numbers, and while researchers say it could be partly attributed to dense deer populations that feed on the bushes where nightingales like to nest, they believe intensive land use is likely to be a driving force. 

The biggest risk is habitat loss

And other European countries are not immune to that.  According to NABU, large-scale farming and forestry  in southern Germany have also contributed to a destruction of breeding grounds over recent years. 

"We have to be aware that nightingale habitats need to be protected so that we can still enjoy their lovely song in hundred years", says Silke Voigt-Heucke.

That is exactly the future that the team at "Forschungsfall Nachtigall" is hoping to secure, simply by passing on knowledge and encouraging people to go out and discover the rich biodiversity around them - even in the beating heart of scruffy Berlin.