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I'm seeing and hearing a lot of activity of common swifts (Apus apus) here in Europe, and the flight and call behaviour reminds me a lot of echolocating bats hunting insects. (The call has strong high-frequency and rapid AM, which might give them useful broadband components.) Has echolocation in these birds been studied? Ruled out?

I read that there are some Southern Asian/Oceanian/Australian birds in the swift family, in Aerodramus, that do: see their wikipedia entry or this paper "The evolution of echolocation in swiftlets". But I don't spot any literature telling me, positively or negatively, about echolocation in common swifts or their closer relatives.

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You are correct that there (to my knowledge) isn't any literature on echolocation in common swifts. We are, however, finding more and more evidence that animals other than bats and dolphins use echolocation, or rather what is considered "echoranging" because it is thought some of these animals lack the fine-resolution discrimination abilities that bats and dolphins have demonstrated. Some of these animals include oilbirds and swiftlets, shrews, hippopotamus and most recently a very comprehensive study on soft-furred tree mice. What most of these species have in common are click or pulse-like signals, which give broadband, short duration echoes that aid in object detection and are similar to dolphin and tongue clicking bats signals.

It's important to note that producing bioacoustic signals with similar properties to those of bats and dolphins can suggest, but not "prove" echolocation. In order to determine if an animal is echolocating one needs to eliminate any other sensory possibilities and look for behavioral patterns consistent with using sound to locate objects. For example, the Italian priest Spallanzani surgically removed the eyeballs of a bat to determine it was not using vision for navigation. For obvious reasons you wouldn't do that today, but an example of what can be done, and something I use in my lab to investigate shrew echolocation behavior, is have a completely dark room and monitor the acoustic and movement behavior of animals using infrared illumination and IR sensitive cameras, or thermal imagery. With a setup like this one can investigate how the animal links sensorimotor behavior to get more evidence that a novel species may be using echoes to navigate their environment. This evidence can include: do sound emission rates increase when exposed to a novel environment? Is the animal exhibiting head scanning behavior during sound emission? Do sound emission rates increase as an animal approaches an object?

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In addition to Laura's answer, it would be useful to determine the spatial characteristics (directivity) of the emitted sound and if reflections from prey items, obstacles, etc. can be observed.

It should further not be forgotten, that regular sound emission during flights are very useful for intra species communication (flight co-ordination).

There is evidence that echolocation trains of deep diving cetaceans are supporting co-operative foraging behaviour.

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