Do any birds hibernate?

Question

A friend said she heard there's a species of bird that hibernates during the winter. I was surprised because I've only heard of birds that either migrate to warm weather as it gets cold; or stay active and behave normally if they're designed to thrive in cold temperatures.

I know some hummingbirds go into a state called torpor. Their metabolism slows down and they lower their body temperature to conserve energy. A variety of animals do it, but it's different from hibernation.

Is there scientific proof that a species of bird goes into genuine hibernation?

If so, I'd like to know the general characteristics, including:

• What is the species name?
• Where does it live?
• When does it hibernate and for how long?
• What type of areas does it hibernate in, such as den, tunnel, under trees?

Any other information, especially a picture, would be appreciated.

If there's more than one known species, I can split this up into separate questions so it won't be too broad.

New species, and traits of existing species, are discovered over time, so I'm asking for data as of June 1, 2018.

Answer 1

According to the RSPB, which @muru found spelled out at the bottom of the linked page below as the Royal Society for the Protection of Birds, there is one species of bird that does truly hibernate. According to the RSPB website, the common poorwill, which lives in the western US, hibernates through the winter.

This small relative of the nightjar is found in western states of the USA such as California and New Mexico where it inhabits open areas of low vegetation and rocky outcrops.

As a nocturnal hunter of insects, the poorwill would struggle to find enough food to survive during the cold winters. When faced with harsh weather and little food, this unique bird hides away in rocky crevices and hibernates through the winter, emerging in spring when the temperature has risen and more insects are active.

The website goes on to explain that no UK bird hibernates. Instead they migrate, or engage in colonial roosting, which is huddling together for warmth.

The wren has been documented to share a nest box overnight with up to 60 other wrens!

One species in the UK, swifts, goes into torpor, to conserve energy for a short cold period, such as a cold night. Most of the research on torpor has been done in North America.