Listening to variation and world history in the song of the nightingale

How does the nightingale’s singing change? Jorma Sorjonen’s old recordings have been brought back to life in Abel Souriau’s doctoral thesis.

On the computer screen, birdsong resembles a stock price chart or an ECG. A spectrogram converts sound into images, so it's possible to follow the song's pitch variation. It makes spotting the differences easier than by just trying to hear them.

''We humans are more visual creatures than birds,'' says Abel Souriau, a biologist who researches nightingales.

Souriau has been going through the thrush nightingale’s song in the Finnish Museum of Natural History’s archive samples. He came across them by chance; he met a Finnish researcher during a party to celebrate an acquaintance’s defence of a doctoral dissertation. The Finn tipped him off about a good collection of thrush nightingale’s song recordings in Helsinki.

Souriau, who is from France, is writing his doctoral dissertation at Charles University in Prague on the interaction of two species of nightingales and the geographical variation in their songs. He has recorded and observed thrush nightingales and the common nightingale (also known as the rufous nightingale), especially in Poland. He has also studied sound samples in Russian archives.

The tip stayed in his mind and he applied for an Erasmus exchange at the Finnish Museum of Natural History, part of the University of Helsinki. He is now analysing recordings made between the 1960s and 1990s by the biologist Jorma Sorjonen.

From warbling to organ music

Souriau has been utilising only a small portion of the 300 recordings. He is particularly interested in the recordings made in Siikalahti in Eastern Finland.

“They have elements that are completely different from the samples I recorded in Poland,” Souriau says, delighted.

He made a trip to Siikalahti last summer, to investigate whether the birds there still sing the same songs as the birds in Sorjonen’s recordings.

Jorma Sorjonen is pleased that his recordings are making a contribution to science. They have already been used in art: Harri Viitanen, the organist at the Helsinki Cathedral, has composed a piece based on them called Images d’oiseau pour orgue, ‘bird images for organ’.

When Sorjonen retired from the University of Joensuu in 2006, where he had worked as departmental secretary in charge of the collections, he donated his recordings to the Helsinki museum, which in turn digitised them.

 “I had been chasing birds day and night. For fifty years I’d tormented my family by constantly being away. Nowadays I listen to birdsong purely for aesthetic pleasure.”

Jorma Sorjonen

From alder woods to willow thickets

It was during a field trip to Siikalahti to observe the night-singing birds that, as a biology student, Sorjonen’s interest in nightingales was sparked. He chose the loud songster as one of the species for his doctoral thesis on the acoustic communication of birds.

Sorjonen began recording the birdsong of nightingales at the end of the 1960s. He travelled around Finland on recording trips, visiting any and every alder grove, wood or willow thicket that might be a suitable habitat for nightingales.

“Nowadays I wouldn’t be able to tell you where the birdsong was recorded, but back then on those recording trips, the thrush nightingales in Tammisaari [on the west coast] and Siikalahti sounded very different to me,” Sorjonen recalls.

Can an egg learn?

Male nightingales learn the singing style of their own area as nestlings, possibly even before they hatch.  They listen closely to their own father and other males singing in the neighbouring thickets.

Winter, on the other hand, is the time for young birds to practise. Abel Souriau has had the chance to listen to the recordings made by his colleague in the overwintering areas of nightingales in Tanzania.

"During the first winter the singing can hardly be recognized as nightingale song. But young birds are fast learners: the following summer they’re competing with the older birds almost as equals, even though their repertoire is still narrower, likely due to shorter life experience," Souriau says.

How does a dialect evolve?

In spring, most males aim to return to the area where they hatched. Females are not as loyal to their place of birth. The regional singing styles survive if enough males return. Then birds from other regions will adopt that style as well. A common song seems to correlate with successful nesting.

However, songs do evolve and change over the years. A new dialect can emerge quickly, depending on the preferences of the females and the patterns adopted by the younger birds.

On the other hand, sometimes the songs can remain exactly the same over a long period of time. Souriau presents two nearly identical recordings, the first recorded by himself in Poland and the second nine years later in Tanzania.

The singers of Kursk

Finland lies on the northern edge of the thrush nightingale’s habitat.  In any given year, prevailing weather conditions have an impact on the number of these birds migrating to Finland. While working in the Siikalahti conservation area, Jorma Sorjonen ringed 254 fledgling nightingale thrushes; only one returned to the area. He also ringed a number of male thrush nightingales that had moved into the area from elsewhere, and their rate of return was greater. It was through these newcomers that the birdsong so typical of Siikalahti was preserved, as they learned it from older males and would return and sing it themselves in subsequent years.

Birdsong is also affected by changes in the environment. In quiet, open environments, birds can make use of higher frequencies that do not carry far. In urban environments and dense forests, however, birdsong must be both plainer and louder.  

In the 1990s, Sorjonen travelled to the Kursk oblast in Russia, in order to study the region’s famed thrush nightingale. He compared his recordings of their singing to those discovered in Moscow.

World history can be heard in the recordings. During the Second World War, German troops set the oak forests of Kursk ablaze. Thrush nightingales came to nest in thickets growing in the charred clearings, and secured their reputation for exquisite singing. By the 1990s the trees had grown back and, as a consequence, the song of the nightingale thrush had become plainer.

It’s only Twitchers’ Rock & Roll

Abel Souriau’s interest extends beyond the thrush nightingale to its relative, the common nightingale, which is not found in Finland. Both species nest and compete for territory within a belt stretching from Poland to the Black Sea. The common nightingale has a repertoire of up to 200 distinct songs, whereas its northern cousin makes do with between 15 and 50 songs. However, whilst the thrush nightingale may learn to imitate the common nightingale, the compliment is seldom returned.

Jorma Sorjonen once tried to study the learning abilities of nightingales in Siikalahti with the help of a population of nine birds he had ringed himself. He rigged up a tape recorder in a bush, to play the singing of a common nightingale. But the study went pear-shaped because some birdwatchers thought that the recorder was a prank and recorded rock music over the birdsong. Not even nightingales could manage to pick that up.

Sweet little troublemaker

Souriau is interested in how and why thrush nightingales copy the common nightingales’ song. Apparently, it helps them in the contests over territory.

With the singing contest, the birds try to avoid unnecessary violence, but if a song does not expel the intruder, there will be a fight.

“You wouldn’t think such a cute bird singing so beautifully could attack so aggressively, in a mood to kill,” Souriau says.

Although the thrush nightingale is bigger and, for that reason alone often wins the territorial dispute, the common nightingales seem to be gaining ground in the Polish region studied by Souriau. Climate change and changes in habitat have led to the decline of both species in the region, according to Souriau.

A familiar sound in the toilet

Souriau aims to have his doctoral thesis ready soon. And he would gladly continue to study the nightingales even after that. Nightingales are close to his heart: even the ringtone in his alarm clock has the song of a common nightingale. On a recent cruise on a Viking Line ship, he was somewhat bewildered to hear a familiar sound. But it wasn’t that a common nightingale had flown on board — they were just playing the bird's singing in the boat’s public toilets.

“Working with the nightingales is pleasant because their singing is so musical,” Souriau says. 

According to him, there is a case for making a connection between birdsong and the development and learning of human language. “Language is not just a human property, as was previously thought. It’s found in other species, too.”


The article has been published in Finnish in the 5/2019 issue of the Yliopisto Magazine.

Translated by Muriel Cambruzzi, Timothy Chance, Jaana Helminen, Aleksi Iso-Pärnä, Niklas Hiltunen, Anton Jeskanen, Mia Jyrkinen, Anni Litja, Elmeri Naukkarinen, Katja Nummela, Terhi Pienimaa, Saana Pyyhtiä, Siina Sammalisto, Tuija Schmid and Suvi Valtanen-Tunkara , and post-edited under the supervision of John Calton, lecturer in English philology, Department of Languages, University of Helsinki

Abel Souriau

Compare the singing of a Finnish and a Polish trush nightingale. Recordings by Jorma Sorjonen and Abel Souriau. First recording is from Parikkala, Finland, and the second from Warta-river, Poland.