Aquatic Warbler (Acrocephalus paludicola)

The Aquatic Warbler has an unusual mating system in which the pair-bond between male and female is reduced to the bare minimum. It begins and ends with the act of copulation, which, unusually for a small bird, is considerably prolonged, lasting about 30 minutes instead of a few seconds. Once this has been achieved, male Aquatic Warblers contribute absolutely nothing to the rearing of the young, not even providing a single insect throughout the nestling and fledgling period, instead leaving this task entirely in the hands of the female – which is very unusual. Furthermore the female, it seems, habitually mates with more than one male; in a study in Poland, for example, it was found that the six eggs from a normal clutch were sired by between two and five different fathers. Males usually attempt to copulate in the evening, when their chances of fertilising the egg to be laid the next morning are highest, but they do not always succeed.

By Ron Knight from Seaford, East Sussex, United Kingdom – Aquatic Warbler (Acrocephalus paludicola), CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=74705965

Males advertise their presence by song, which is mainly delivered from a perch and, less often, during a song flight in which the bird lifts up to a height of 3-30 m and then raises its fanned tail, causing a steep plummet. The male has at least two types of song: a short, blunt one used for territorial encounters with other males, and a longer, more complex one designed to impress the females.

This unique warbler has a very restricted distribution and habitat. Confined to Eastern Europe and Russia, it breeds mainly in marshes dominated by clumps of sedge up to 80 cm tall, with water in between a few centimetres deep. The presence of a few willow bushes is also popular. Such habitats, it seems, are able to provide the female with so many large insects and spiders with which to feed the young that the one-parent family option can succeed. Despite large populations having been found recently in Belarus to swell the known numbers, the Aquatic Warbler is still Europe’s rarest small migratory bird.

From ‘Birds: A Complete Guide to All British and European Species’, by Dominic Couzens. Published by Collins and reproduced with permission.