British Wildlife of the Week:
In global terms, the rarest bird you stand a chance of realistically seeing in the wild in the British Isles is probably the aquatic warbler. Even then, you need to be in the right place at the right time, and ideally also have a liberal sprinkling of good fortune on your side. This is because the aquatic warbler doesn’t breed here, or even spend the winter – it’s only a rare passage migrant to our shores.
Despite its name, the aquatic warbler can’t swim, but it does like wet areas. However, vast swathes of the insect-rich wetlands and soggy, sedge-filled marshes where it breeds in eastern Europe have been drained, resulting in widespread habitat loss and fragmentation. As its range shrunk across Europe, its population dropped by 95% during the 20th century. With estimated numbers now as low as 11,000 – 16,000 pairs and declining, the aquatic warbler is the rarest and only internationally threatened songbird found in mainland Europe.
After breeding, the aquatic warbler migrates to its wintering grounds in West Africa. Unlike most birds heading from Europe to Africa, though, it does not take a direct southerly route. Instead, it first heads west, into Germany and France, before finally turning south. En route, if the winds are heading our way, small numbers get blown off course and end up spending a few nights in coastal reedbeds along the south coast of England in late August and early September. Usually, only a couple of dozen of these skulking, secretive birds are recorded here every year so your chances of seeing one are slim, but the RSPB reserve at Marazion Marsh in Cornwall tends to yield annual sightings.
For most birds, mating is a rather brief affair that merely involves the touching of genital openings. In the case of the aquatic warbler, however, sex can last for as long as half an hour. The reason for this is because the male aquatic warbler has massive testes, which store large amounts of sperm. During one coupling, a male may inseminate a single female seven or eight times to flood her reproductive system with sperm and ensure that the egg she is producing will almost certainly be fathered by him.
Afterwards, the female goes back to a nest she has constructed and lays a single egg. The next day, she will go in search of another male and mate with him. Thus, he will be the father of her second egg. She will continue this until her clutch of six eggs is complete, with every one of them having a different father.
Next time on British Wildlife of the Week, we’ll be looking at a summer visitor to the British Isles and one of the heaviest fish in the world – the sunfish.