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http://www.sciencedaily.com/releases/20 ... 113243.htm
Dar viena ištrauka iš straipsnio: Niels C. Rattenborg "Do birds sleep in flight?", Naturwissenschaften (2006) 93: 413–425
"Candidates for sleep in flight
Common swift
Of all the birds thought to sleep in flight, perhaps the
common swift (Apus apus) has received the most attention.
The idea that swifts spend the night on the wing arose from
observations of swifts ascending into the sky above their
colony after sunset and descending the following morning
(Edwards 1887; Masson 1930; Weitnauer 1952, 1954,
1960; Lack 1956). Although collisions with and observations from airplanes showed that swifts did, in fact, fly at
night (Weitnauer 1952, 1954–1956; Lack 1956), it was not
until Weitnauer used radar to track swifts throughout the
night that aerial roosting was confirmed (Weitnauer 1960,
1980; Bruderer and Weitnauer 1972). Nevertheless, these
radar studies were unable to determine whether individual
birds flew for an entire night or the number of consecutive
nights spent in flight.
Recently, Tarburton and Kaiser (2001) used radio
tracking to follow individual swifts in flight for periods
up to two consecutive nights before loosing the signal. The
birds participating in aerial roosting were primarily prebreeders; breeding adults roosted, and presumably slept, in
the nest box and only engaged in aerial roosting toward the
end of the fledging period. Although the tracking of prebreeders was discontinuous, due to temporary signal loss,
all but one of the pre-breeders tracked were always
encountered while in flight, regardless of whether it was
day or night. The exceptional individual roosted at night on
a building, a behavior rarely observed at this study site.
Even fledglings typically departed from the nest for the
first time after sunset and spent their first night outside the
nest in flight. In other reports, recently fledged juveniles
have been found roosting on foliage or other substrates
more often than adults, however (Holmgren 2004). Nonaerial roosting occurs primarily during low ambient
temperatures, suggesting that juveniles are more energetically vulnerable than adults, and therefore, more likely to
resort to non-aerial roosting, presumably a less energetically expensive behavior than aerial roosting. Although
there is substantial evidence showing that pre-breeding
adult common swifts often spend the night in flight, the
average and maximum length of time spent in continuous
flight remains unknown. In conjunction with the results
from tracking studies, the fact that observations of
nonbreeding birds roosting terrestrially are noteworthy
suggests that aerial roosting is the typical nocturnal
behavior for common swifts, at least during the breeding
season. Aerial roosting may also occur in other species of
swifts, but the evidence is not as substantial as that for the
common swift (reviewed in Holmgren 2004). The evidence
for sleep in flying common swifts, however, is circumstantial, depending solely on the notion that if swifts must
sleep, such sleep must occur on the wing.
Aside from spending the night in flight, perhaps, the
only other piece of evidence for sleep in flight comes from
a radar study of the flight patterns of common swifts at
night. Bäckman and Alerstam (2002) tracked individual
swifts flying at night for periods up to 1 h. In general, the
swifts flew into the wind, a flight pattern that reduced
displacement from the colony. Interestingly, the swifts’
orientation oscillated with a period of 1–16 min, around the
direct heading into the wind, a flight pattern expected under
wind speeds lower than the swifts’ lowest flight speed,
assuming that the swifts’ goal was to remain over the
colony. However, this oscillation occurred at all wind
speeds, including those higher than the swifts’ flight speed.
Bäckman and Alerstam (2002) suggest that this flight
pattern may reflect oscillations in the level of alertness,
possibly related to sleep in flight. Weaving from left to
right of the direct heading may result from a sensory bias
related to flying alternately with only the left or right eye
open. Indeed, pigeons flying with one eye occluded tend to
veer toward the side with the open eye (Prior et al. 2004)."