Aquatic warblers are rare and temporary migrants to parts of southern England each autumn, en-route from their breeding grounds in Poland, Ukraine and Belarus to their winter territories in West Africa. Only around 40 sightings are reported each year, with the species classed as vulnerable and in decline.
Aquatic warblers are rather secretive songbirds, well-camouflaged against their preferred sedge and reedbed habitats.
They have a striped crown with a pale line through the centre and prominent dark markings on either side. Facial markings include a distinctive pale eyebrow, a darker brown stripe next to the eye, and yellowish cheeks.
Their upperparts are olive-brown, heavily streaked with much darker black-brown, and their underparts are pale, with fine streaky yellow-brown markings, particularly on the flanks and breast. Aquatic warblers have yellowish-pink legs and a pointed black-brown bill.
Males and females are identical in appearance, although males have slightly longer wings than females, and are marginally heavier.
Juvenile aquatic warblers are similar to adults in colouring, although the breast of young birds lacks the streaky markings seen in adult birds.
Aquatic Warbler in natural habitat
Aquatic warblers are medium-sized members of the warbler family, around the same size as a reed warbler and a sedge warbler. Males and females are the same size, although in females, wings may be slightly shorter.
The song of an aquatic warbler is rapid and chattering, followed by a series of whistles and frequently heard from perches and in flight. Calls are churring ‘chuk’ notes.
Aquatic warblers are insectivores, with caterpillars, dragonflies, spiders, larvae, grasshoppers, earwigs, and water snails being the main elements of their diet.
They are mainly ground foragers, finding insects in sedges and marsh close to the nest when breeding. In late summer, their diet may be supplemented with small berries.
Young aquatic warblers are fed on large insects and their larvae by their parents.
Aquatic Warbler feeding on insects
During the breeding season, aquatic warblers prefer open marshy, lowland landscapes, for example, fen mires with some sedge cover and scattered willow bushes. A water depth of between 1 cm and 10 cm (0.4 in and 4 in) is required by the species, and flooded grasslands and short reedbeds alongside slow-flowing rivers may also be used.
Constant management of habitats by humans is important to provide the suitable conditions this species needs for breeding, for example, well-timed mowing and the removal of old biomass.
In winter, aquatic warblers head to wetland environments in West Africa, particularly marshlands and shallow sedges, and reedbeds.
Breeding grounds of aquatic warblers are limited to areas of north-east Germany, Poland, Lithuania, Belarus, Ukraine, and a tiny isolated population in western Siberia. Winters are spent in West Africa, south of the Sahara from Senegal to Ghana.
There are less than 60 known breeding sites for aquatic warblers, which are spread across Poland, Belarus, Ukraine, Lithuania, and a handful in Western Siberia. Poland and Belarus are home to the largest number of pairs.
The European population of aquatic warblers is estimated at between 12,000 and 20,000 pairs and has been recognised as Europe’s rarest songbird species. Each year, only around 42 sightings are reported in the UK, with passage migrants spotted in coastal regions en route to their West African wintering grounds.
Passage sightings are limited to the south coast of England, with around 40 birds arriving in August at scattered coastal sites before continuing on to their winter grounds in West Africa. Cornwall and Devon are particularly popular spots, with Marazion Marsh, Cornwall and Radipole Lake, and Lodmoor in Dorset reporting visiting birds each year.
Back view of an Aquatic Warbler perching on reeds
Little is known about the typical and maximum life expectancy of aquatic warblers, but it is thought that they breed for the first time at one year old, and live for around 2 and a half years on average.
Shrews are among the main predators of aquatic warblers, regularly destroying their nests and attacking their eggs and young. Marsh harriers prey on adult aquatic warblers as well as their chicks.
In the UK, aquatic warblers are protected under the Wildlife and Countryside Act, of 1981, which legislates against the species being killed, injured, or taken into captivity.
Additionally, aquatic warblers have officially protected status in Poland, Belarus, Germany, and Hungary, with the most important breeding sites in these countries found within legally protected areas. Djoudj National Park in Senegal, a leading wintering ground for aquatic warblers, is also a protected site.
Globally, aquatic warblers are classified as a vulnerable species, due to their low and declining numbers and the concentration of breeding grounds limited to such a small geographic area.
In the 20th century, breeding was more widespread across Western Europe, but the species no longer breeds in France, Belgium, Netherlands, Austria, or Italy.
Habitat loss is a critical factor in the decline of aquatic warblers and their vulnerable conservation status. Drainage of wetlands for peat extraction and agricultural clearance has led to a contraction of breeding grounds, while early mowing of agricultural fields destroys a number of nests each year.
Aquatic Warbler singing in natural habitat
Nest sites are built between 3 cm and 30 cm (2 in and 12 in) above ground level in swampy marshes, with nests often constructed beneath the shelter of dead sedges and bent reeds. A cup of grass, stems, leaves, spiders’ webs, and soft plant matter is woven, then lined with softer plant down.
Aquatic warblers arrive back on their European breeding grounds in early May, with the first clutches usually being laid in mid-May. Second broods, usually with different mates, are common and are usually laid between late June and early July.
Incubation, which takes 12 to 15 days, is solely undertaken by the female, and parental care of the young before fledging up to 16 days after hatching, is also down to the female alone.
Aquatic warblers lay between 3 and 6 brown eggs, which are spotted with greyish-yellow markings. The eggs are tiny, measuring 17 mm by 13 mm (0.7 in by 0.3 in).
Aquatic warblers do not form long-term pairs and both males and females mate with numerous different partners during the course of a breeding season. Chicks from a single brood are usually fathered by several different birds.
Aquatic Warbler perching in reed beds
Males defend small territories in the breeding season but are not known to be especially aggressive or confrontational, and in winter, no territorial behaviour is displayed.
Aquatic warblers are migratory, with breeding largely concentrated in a small area of east-central Europe and lasting for only around four months.
At the end of the breeding season, aquatic warblers disperse, heading east and southeast across Europe towards warmer wintering grounds in western sub-Saharan Africa, where the largest portion of the year is spent.
No aquatic warbler pairs breed in the UK and the species is only seen as a rare passage migrant in August each summer, with birds making brief and temporary stopovers along the south coast of England on their way to winter territories in Senegal, Mali and Ghana.
16.5cm to 19.5cm
10g to 14g
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