The Arctic Warbler, Phylloscopus borealis, is a widespread leaf warbler

Leaf warbler

Leaf-warblers are small insectivorous birds belonging mainly to the genus Phylloscopus. They were formerly included in the Old World warbler family but are now considered to belong to the Phylloscopidae, a family created in 2006. The genus is closely related to Seicercus and some species have been...

 in birch

Birch

Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...

 or mixed birch forest near water throughout its breeding range in Fennoscandia

Fennoscandia

Fennoscandia and Fenno-Scandinavia are geographic and geological terms used to describe the Scandinavian Peninsula, the Kola Peninsula, Karelia and Finland...

 and northern 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 has established a foothold in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, breeding in Alaska

Alaska

Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

. This warbler is strongly 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...

; the entire population winters in southeast Asia. It therefore has one of the longest migrations of any Old World

Old World

The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....

 insectivorous bird.

It has traditionally included populations that breed in Kamchatka, the Kuril Islands

Kuril Islands

The Kuril Islands , in Russia's Sakhalin Oblast region, form a volcanic archipelago that stretches approximately northeast from Hokkaidō, Japan, to Kamchatka, Russia, separating the Sea of Okhotsk from the North Pacific Ocean. There are 56 islands and many more minor rocks. It consists of Greater...

 and Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, but genetic

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 and vocal evidence strongly suggests these should be treated as separate species, with P. examinandus in Kamchatka, Hokkaido

Hokkaido

, formerly known as Ezo, Yezo, Yeso, or Yesso, is Japan's second largest island; it is also the largest and northernmost of Japan's 47 prefectural-level subdivisions. The Tsugaru Strait separates Hokkaido from Honshu, although the two islands are connected by the underwater railway Seikan Tunnel...

 and the Kuril Islands, and P. xanthodryas in Japan (except Hokkaido).

The nest is on the ground in a low shrub. Like most Old World warblers, this small 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...

 is insectivorous.

This is a typical leaf warbler in appearance, greyish-green above and off-white below. Its single wing bar distinguishes it from most similar species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 except the Greenish Warbler

Greenish Warbler

The Greenish Warbler and Green Warbler are widespread leaf-warblers throughout their breeding range in northeastern Europe and temperate to subtropical continental Asia. This warbler is strongly migratory and winters in India. It is not uncommon as a spring or early autumn vagrant in Western...

, Phylloscopus trochiloides. It is larger than that species and has a heavier, dagger-like bill, with a dark tip to the lower mandible. Its song is a fast trill.

This species occurs as an autumn vagrant in 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...

 and is annual in Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

.