Editing Swiftlet (section)

== Echolocation ==
The genus ''Aerodramus'' was thought to be the only echolocating swiftlets.  These birds use echolocation to locate their roost in dark caves.  Unlike a bat's echolocation, ''Aerodramus'' swiftlets make clicking noises that are well within the human range of hearing.  The clicks consist of two broad band pulses (3–10 [[kHz]]) separated by a slight pause (1–3 [[millisecond]]s).  The interpulse periods (IPPs) are varied depending on the level of light; in darker situations the bird emits shorter IPPs, as obstacles become harder to see, and longer IPPs are observed when the bird nears the exit of the cave.  This behavior is similar to that of bats as they approach targets.  The birds also emit a series of low clicks followed by a call when approaching the nests; presumably to warn nearby birds out of their way. <!-- "The frequency of clicks does not aid in echolocation but rather the bird gathers temporal information about its surroundings." This makes no sense, or does it? -->It is thought that the double clicks are used to discriminate between individual birds.  ''Aerodramus sawtelli'', the [[Atiu swiftlet]], and ''Aerodramus maximus'', the [[black-nest swiftlet]] are the only known species which emit single clicks.  The single click is thought be used to avoid voice overlap during echolocation.  The use of a single click might be associated with an evolutionary shift in eastern Pacific swiftlets; determining how many clicks the [[Marquesan swiftlet]] emits could shed light on this.  It was also discovered that both the Atiu swiftlet<ref name=Fullard/> and the [[Papuan swiftlet]]<ref name=Price2005/> emit clicks while foraging outside at dusk; the latter possibly only in these circumstances, considering that it might not nest in caves at all. Such behavior is not known to occur in other species,<ref name=Fullard/> but quite possibly does, given that the Papuan and Atiu swiftlets are not closely related. However, it has recently been determined that the echolocation vocalizations do not agree with evolutionary relationship between swiftlet species as suggested by [[DNA]] sequence comparison.<ref name=Thomassen2006/> This suggests that as in [[bat]]s, echolocation sounds, once present, adapt rapidly and independently to the particular species' acoustic environment.

Three hypotheses are considered to describe how echolocation evolved in the genus ''Aerodramus'' and, as determined more recently, other taxa in the Apodidae.  One hypothesis states that echolocation evolved from an ancestral species of swiftlets and was lost in the genera which lack echolocation.  A second hypothesis is that echolocation evolved independently several times.  The third scenario involves a combination of the first two, i.e. a gain-loss-regain scenario.

Several functional subunits (like vocal muscles and brain areas) are needed to produce the echolocating system.  Past studies have thought that the loss of one of these subunits was more likely to occur than acquiring all the traits needed to echolocate. Yet a recent study suggests that the echolocation subunits were mainly located in the [[central nervous system]], while the subunits in the vocal apparatus were already present and capable of use before echolocation even evolved.  This study supports the second hypothesis of independent evolution of echolocation in ''Aerodramus'' and ''Collocalia'', with the subsequent evolution of complex behavior needed to complement the physical echolocation system, or even the third approach, as the vocal apparatus-parts of the echolocation system might even be inherited from some [[prehistoric]] [[nocturnal]] ancestor.<ref name=Thomassen2005/>