The Cook's's Swift is a small 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...

, superficially similar to a House Martin

House Martin

The Common House Martin , sometimes called the Northern House Martin or, particularly in Europe, just House Martin, is a migratory passerine bird of the swallow family which breeds in Europe, north Africa and temperate Asia; and winters in sub-Saharan Africa and tropical Asia...

. It is, however, completely unrelated to those 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...

 species, since swift

Swift

The swifts are a family, Apodidae, of highly aerial birds. They are superficially similar to swallows, but are actually not closely related to passerine species at all; swifts are in the separate order Apodiformes, which they share with hummingbirds...

s are in the order Apodiformes. The resemblances between the groups are due to convergent evolution

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

 reflecting similar life styles.

These birds have very short legs which they use only for clinging to vertical surfaces. The scientific name comes from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 απους, apous, meaning "without feet". They never settle voluntarily on the ground. Blyth's Swifts spend most of their lives in the air, living on the insects they catch in their beaks.

Cook's Swifts breed in limestone caves of Thailand

Thailand

Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...

, Myanmar

Myanmar

Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....

 and Indochina

Indochina

The Indochinese peninsula, is a region in Southeast Asia. It lies roughly southwest of China, and east of India. The name has its origins in the French, Indochine, as a combination of the names of "China" and "India", and was adopted when French colonizers in Vietnam began expanding their territory...

. The species has a green iridescence, a shallow tail fork and is a short distance migrant. A 2011 study has many taxonomists splitting this species from the Fork-tailed Swift

Fork-tailed Swift

Fork-tailed Swift is the historic name of a kind of bird which has since been divided taxonomically into four species. It could refer to any of four different species of swifts:*Pacific Swift, Apus pacificus*Salim Ali's Swift, Apus salimali...

 complex.

These swifts build their nests on cliffs, laying 2-3 eggs. A swift will return to the same site year after year, rebuilding its nest when necessary.

Cook's Swifts are similar in size to Common Swift

Common Swift

The Common Swift is a small bird, superficially similar to the Barn Swallow or House Martin. It is, however, completely unrelated to those passerine species, since swifts are in the separate order Apodiformes...

, and they are black except for a white rump. They can be distinguished from a partially leucistic

Leucistic

Leucism is a condition characterized by reduced pigmentation in animals and humans. Unlike albinism, it is caused by a reduction in all types of skin pigment, not just melanin.- Details :...

Common Swift by the deeper tail fork, longer wings, bigger head and larger white throat patch.