The Thick-billed Warbler (Iduna aedon) is an Old World warbler

Old World warbler

The "Old World Warblers" is the name used to describe a large group of birds formerly grouped together in the bird family Sylviidae. The family held over 400 species in over 70 genera, and were the source of much taxonomic confusion. Two families were split out initially, the cisticolas into...

 that breeds in temperate east Asia

Asia

Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

. It is migratory

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

, wintering in tropical south east Asia. It is a very rare vagrant to western Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

.

This passerine

Passerine

A passerine is a bird of the order Passeriformes, which includes more than half of all bird species. Sometimes known as perching birds or, less accurately, as songbirds, the passerines form one of the most diverse terrestrial vertebrate orders: with over 5,000 identified species, it has roughly...

 bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 is a species found in dense vegetation such as reeds, bushes and thick undergrowth. 5-6 egg

Egg (biology)

An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

s are laid in a nest in a low tree.

This is a large warbler, at 16-17.5 cm long nearly as big as Great Reed Warbler

Great Reed Warbler

The Great Reed Warbler, Acrocephalus arundinaceus, is an Eurasiaan songbird in the genus Acrocephalus. It used to be placed in the "Old World warbler" assemblage, but nowadays is recognized to be part of the marsh- and tree-warbler family .-Description:This is a large thrush-sized warbler,...

. The adult has an unstreaked brown back and buff underparts, with few obvious distinctive plumage

Feather

Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...

 features. The forehead is rounded, and the bill is short and pointed. The sexes are identical, as with most warblers, but young birds are richer buff below. Like most warblers, it is insectivorous, but will take other small prey items.

The song is fast and loud, and similar to Marsh Warbler

Marsh Warbler

The Marsh Warbler, Acrocephalus palustris, is an Old World warbler currently classified in the family Acrocephalidae. It breeds in temperate Europe and western Asia and winters mainly in south east Africa...

, with much mimicry and typically acrocephaline whistles added.

It was sometimes placed in the monotypic genus Phragmaticola (or Phragamaticola) and for a long time as Acrocephalus and in 2009 found to belong to the Iduna clade.