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The swifts are
birds superficially similar to
swallows but are actually not closely related to those
passerine species at all; swifts are in the separate
order Apodiformes, which they formerly shared with the
hummingbirds.
The resemblances between the swifts and swallows are due
to convergent evolution reflecting similar life styles based
on catching insects in flight.
The family scientific name comes from the
Greek απους, apous, meaning "without feet", since swifts
have very short legs and never settle voluntarily on the
ground, perching instead on vertical surfaces. The tradition
of depicting swifts without feet continued into the Middle
Ages, as seen in the heraldic martlet.
Swifts are the most aerial of birds and some, like the
Common Swift, even sleep and mate on the wing. Larger
species, such as white-throated needletail, are amongst the
fastest flyers in the animal kingdom. One group, the
Swiftlets or Cave Swiftlets have developed a form of
echolocation for navigating through dark cave systems
where they roost. One species, Aerodramus papuensis
has recently been discovered to use this navigation at night
outside its cave roost also.
Like swallows and martins, the swifts of temperate
regions are strongly
migratory and winter in the tropics.
Many swifts have a characteristic shape, with a short
forked tail and very long swept-back wings that resemble a
crescent or a
boomerang. The flight of some species is characterised
by a distinctive "flicking" action quite different from
swallows.
The nest of many species is glued to a vertical surface
with saliva, and the genus Aerodramus use only that
substance, which is the basis for bird's nest soup.
Systematics and evolution
The
treeswifts are closely related to the true swifts, but
form a separate family, the Hemiprocnidae.
In the
Sibley-Ahlquist taxonomy, the old order Apodiformes is
split. Swifts remain in that order, but
hummingbirds are put into a new order, Trochiliformes.
This might be correct, but further research on the
interrelationships and evolutionary history of the
Apodiformes is necessary.
The taxonomy of this group is in general complicated,
with genus and species boundaries widely disputed,
especially amongst the swiftlets. Analysis of behavior and
vocalizations is marred by common parallel evolution, while
analyses of different morphological traits and of various
DNA sequences have yielded equivocal and partly
contradictory results (Thomassen et al., 2005).
The Apodiformes diversified during the Eocene, at the end
of which the extant families were present; fossil genera are
known from all over temperate Europe, between today's
Denmark and France, such as the primitive Scaniacypselus
(Early - Middle Eocene) and the more modern Procypseloides
(Late Eocene/Early Oligocene - Early Miocene). A prehistoric genus sometimes assigned to the
swifts, Primapus (Early Eocene of England), might
also be a more distant ancestor.
Chantler, Phil & Driessens, Gerald
(2000): Swifts : a guide to the swifts and treeswifts
of the world. Pica Press, Mountfield, East Sussex.
ISBN 1-873403-83-6
Thomassen, Henri A.; Tex, Robert-Jan; Bakker,
Merijn A.G. & Povel, G. David E. (2005): Phylogenetic
relationships amongst swifts and swiftlets: A multi
locus approach. Molecular Phylogenetics and Evolution37(1): 264-277.
DOI:10.1016/j.ympev.2005.05.010
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