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Rarely Faithful

The rarest songbird in Europe - and probably the most promiscuous…

The Aquatic Warbler is the rarest songbird in mainland Europe. Its numbers declined by 95 percent during the 20th century. But this is not for lack of effort on the warbler's part… [The Aquatic Warbler (Acrocephalus paludicola) was once widespread and numerous on fen mires and wet meadows across continental Europe. But in the 20th century most of these habitats were drained for agriculture, and the bird is now confined to strongholds in Eastern Poland, Belarus and Ukraine, a small but growing number in Hungary, and tiny, diminishing populations in ‘Pomerania’ (Eastern Germany and North West Poland) and West Siberia. A hundred years ago, the German Land of Brandenburg alone probably held around 100,000 singing males. Now the entire world population is believed to be between 12,000 – 20,000 pairs. Surveys in Poland and Ukraine have found new breeding sites and an increase in the core population. But the population fluctuates according to conditions in the habitat. Between 1996 and 1998, the low numbers of birds at the Zvanets reserve in Belarus (the largest breeding site in the world holding around 20% of the world population) were due to low water and fires. In 1999, the problem was flooding. BirdLife have now built a system of dams around the reserve to maintain the optimum water level and prevent the fen mire being taken over by reeds, bushes and trees.]Over the last fifteen years, researchers investigating the sex life of this small, retiring brown bird have uncovered a pattern of promiscuous behaviour, with male birds "continuously ready to mate and testing every female for her willingness to copulate". Almost two-thirds of all broods of young Aquatic Warblers have more than one father. [Different teams of researchers are trying to identify the bird’s African wintering grounds – there may also be conservation issues here, but this won’t be known until their exact location is discovered. The RSPB is examining stable isotopes in feathers taken from Aquatic Warblers. These isotopes vary in different regions, and when birds take them up by feeding, they build into their feathers a history of where they have been.

Aquatic Warblers pass through many other European countries on their way to and from their wintering grounds, including France, Belgium, Holland and Spain. In most years some turn up on the south coast of Britain. Indeed the first migrants of the Autumn have just passed through the UK, being seen at Radipole (Dorset) on 6 August and Farlington Marshes (Hants) on 7 August.]From 18-20 August, scientists from all over Europe will be meeting in Palencia, Spain, at the Aquatic Warbler International Conference, to share their knowledge of conservation best practice, and plan for the future. They will also hear latest research on its breeding biology. Male Aquatic Warblers enjoy a carefree lifestyle. They are "completely emancipated from brood care" – in other words, female Aquatic Warblers do all the work of building nests, incubating eggs and feeding young. The males frequently change their ‘activity ranges’ within their large territories, devoting their attentions to different clusters of females. If fertile females are few and far between, the males will cover long distances to find them.The male birds are particularly well endowed (in proportion to their size) with "cloacal protuberances, testes and seminal glomera being extraordinarily large compared to other Acrocephalus (warbler) species and birds in general".

One paper is titled "Prolonged Copulation, sperm reserves and sperm competition in the Aquatic Warbler". In contrast to most birds, which get the business over with a mere 1-2 seconds' sexual contact, Aquatic Warblers spend up to 35 minutes copulating, with an average of 23.7 minutes – thought to be a record among songbirds.

Mating for this length of time prevents other males slipping their sperm in (a practice known as ‘contact mate guarding’) and may also "allow the female to assess the quality of the male". The female is able to "escape" at any time she chooses.

Females seem to benefit from this promiscuous behaviour. Multiple matings could allow females to correct an initial choice of partner. Other incentives for mating with several different males include "fertilisation insurance" and the avoidance of in-breeding. The rate of unhatched eggs and starved or disappeared nestlings is also lower in broods with more than one father.Since 1998, the BirdLife International Aquatic Warbler Conservation Team has been searching for the remaining breeding populations of the bird, and devising ways to stabilise and improve them. The latest findings on the warbler’s breeding biology have been compiled by Dr Alexander Kozulin, the Scientific Director of APB, the BirdLife Partner in Belarus, who has taken the lead in protecting and restoring the bird’s habitat. "Given that the aquatic warbler is Europe's rarest songbird, we are lucky that they are also the most amorous - which makes our work to increase the size of the population so much easier! We are concentrating on recreating over 40,000 hectares of fens and bogs to make sure that they can increase their numbers and expand their range," he said. "The story of the Aquatic Warbler is a nice example of how much can be achieved when people from different countries and organisations join together," added Dr Kozulin.

4th July 2014