How birds hear songs
Song of a whip-poor-will, recorded in Ontario, Canada, by Tom Cosburn.
Do birds hear birdsong as we do?
The first step towards answering this question was provided by sound recordings of a whip-poor-will, a North American species of nightjar. The bird's name is a verbalisation of its song. It says “whip-poor-will” - a three note phrase. When, however, you slow down the recording, it is clear that the bird sings five notes, but their temporal arrangement is such that, when heard in the wild (or at natural speed) the five notes sound like three. There is no means of knowing whether other whip-poor-wills hear three or five, though it is not unreasonable to suggest that if they were deaf to the subtlety, the subtlety would probably not exist.
Whip-poor-will song (slowed down)
This recording of a whip-poor-will's song has been slowed down by 90%. It was recorded in Ontario, Canada, by Tom Cosburn.
The second, highly ingenious stage in the answer, was to slow down a recording of a mockingbird copying a whip-poor-will. Lo and behold, the mocker produces a five note imitation! It can thus be assumed that other whip-poor-wills also hear five-note songs.
The wren's song phrase
It is to be expected that the smaller the bird, the better able it will be at separating out a succession of fast-arriving sounds. For a bird like the wren a factor of ten is suggested. Thus by slowing down a recording of a wren's song phrase three times, one to two, two to four, and four to eight times as long (and three octaves down as well) we approach the degree of separation that one wren experiences in another wren's song - furthermore it is a musical revelation. One song phrase 8.25 seconds long, when stretched out to 66 seconds, reveals 103 notes, which means that the bird is singing at a rate of 740 notes to the minute. Slowed down this way, a wren's song phrase is at once more intelligible. If you follow it by the same recording at normal speed, the impression is of a hurried song.
Song of a wren, recorded in Hampshire, England, by Phil Riddett.
Wren song (slowed down)
This recording of a wren's song has been slowed down by 70%. Recorded in Hampshire, England, by Phil Riddett.
When a typical songbird, say a chaffinch, is singing, he is saying “I am a chaffinch”, “I am a cock chaffinch”, “I am a particular cock chaffinch”, “I am here”, “I am in my territory”, “I am ready to posture at and drive off other cock chaffinches”, and, if unmated, “I am ready to take a mate”. Each time a cock chaffinch sings "chip, chip, chip; cherry-erry-erry; fissychooee-o!" it takes him about 2.5 seconds, in 12 hours he says that phrase 3,300 times.
The song of a chaffinch, recorded in Cumbria, England, by Richard Ranft.
The world record for songs per day is held by the North American red-eyed vireo, a male sang “see me - hear me” 22,197 times. Such a bird must reiterate its song a million times a season.
Red-eyed vireo song
The song of a red-eyed vireo, recorded in Mansfield, Ontario, Canada.
Seeing with sound
There are a few species of bird, breeding in the total darkness of large caves, that use sounds not for communication but to “see in the dark”. Like many bats, these creatures echolocate. As is well-known in bats, a short sharp sound is created by the bird, the echo is then listened to, the time gap assessed and an appropriate response made, e.g. turning left if you are about to collide with a stalagtite.
One such feathered echolocator is the oilbird of north-eastern South America, a large nightjar-like bird that feeds at night on fruit. The others are certain species of swiftlet from the Oriental and Australasian zoogeographical regions. They are the birds whose nests are used for soup, the best-known species and colony being the black-nest swiftlet of the Great Niah caves in Sarawak (Borneo).