This passage is adapted from Ingo Schiffner, et. al., “Minding the Gap: In-Flight Body Awareness in Birds.” © 2014 by Ingo Schiffner et. al.
When traversing cluttered environments at nearly cruising
speeds, birds need to be constantly aware of the distances to
oncoming obstacles and the spaces between them, in order to
make split second decisions about whether a gap can be
traversed, and to determine whether a change in the wing
posture is necessary to facilitate an injury-free passage.
Do birds fly through passages that are narrower than their
wingspan? If they indeed do so, what postural changes do they
make to accommodate the passage? Are the wings held up,
held down, held forward, or held behind, tucked close to the
body? Furthermore, very little is known about a bird’s ability to
assess the width of a gap in relation to its own body size, and
about how this assessment is made. In principle, there are a
number of ways in which this could be accomplished.
In the experiments presented here, seven budgerigars
(Melopsittacus undulatus) were confronted with an aperture of
variable width. We aimed to investigate their flight manoeuvres
through the aperture and to enquire whether they display
awareness of their body size while doing so. The aperture was
a vertical slit, presented as a gap between two cloth panels. A
total of 560 flights were recorded, with 10 flights per bird in
seven experimental and one control (unobstructed tunnel)
condition.
During the normal flapping flight mode when the birds were
not negotiating an aperture, the birds’ wingbeat cycles proved
to be very stable, with the duration of the downstrokes being in
the range of 38 ± 3ms and the upstrokes in the range of 22 ±
2ms, resulting in a mean wingbeat period of approximately
60ms. Even though there was a slight variation in the duration
of the downstroke and the upstroke, the ratio between the two
remained almost constant at 0.59 ± 0.07. These figures were
constant across individuals and across different experimental
conditions.
As the width of the gap was reduced to approach the
wingspan of each individual bird, the normal wingbeat cycle
was interrupted during the actual passage through the gap. The
duration of the upstroke was then longer than the duration of
the downstroke, indicating that the birds actively held their
wings in such position to avoid touching the panels during the
passage. During traversal of the gap, the birds either held their
wings in a position corresponding to the end of an upstroke, or
tucked them in against the body (a behaviour very reminiscent
of flap bounding, i.e. intermittent phases during which normal
flapping flight is interrupted). In either case, the birds closed
their wings, projectiling themselves through the gap, rather
than actively flapping through it. The choice of the mode of
traversal depended upon the duration of the passage: during
longer traversals the wings were always tucked in completely.
Only in one instance (out of a total of 490 narrow-gap
traversals) did we observe a bird holding its wings pointing
downwards, and once a bird holding one wing up and the other
down. For simplicity, we shall refer to all of the projectiling
behaviours as “wing closure.”
We found that, as the gap was made narrower, the birds
were more likely to interrupt their wingbeat cycle and close
their wings. Furthermore, the birds maintained wing closure for
a longer duration as the gap was made narrower. More
importantly, the birds are not simply adjusting the phase of
their normal wingbeat cycle so as to ensure that wings are in
the closed position when passing through the gap. They are
definitely prolonging the period of wing closure during passage
through the gap.
When traversing cluttered environments at nearly cruising
speeds, birds need to be constantly aware of the distances to
oncoming obstacles and the spaces between them, in order to
make split second decisions about whether a gap can be
traversed, and to determine whether a change in the wing
posture is necessary to facilitate an injury-free passage.
Do birds fly through passages that are narrower than their
wingspan? If they indeed do so, what postural changes do they
make to accommodate the passage? Are the wings held up,
held down, held forward, or held behind, tucked close to the
body? Furthermore, very little is known about a bird’s ability to
assess the width of a gap in relation to its own body size, and
about how this assessment is made. In principle, there are a
number of ways in which this could be accomplished.
In the experiments presented here, seven budgerigars
(Melopsittacus undulatus) were confronted with an aperture of
variable width. We aimed to investigate their flight manoeuvres
through the aperture and to enquire whether they display
awareness of their body size while doing so. The aperture was
a vertical slit, presented as a gap between two cloth panels. A
total of 560 flights were recorded, with 10 flights per bird in
seven experimental and one control (unobstructed tunnel)
condition.
During the normal flapping flight mode when the birds were
not negotiating an aperture, the birds’ wingbeat cycles proved
to be very stable, with the duration of the downstrokes being in
the range of 38 ± 3ms and the upstrokes in the range of 22 ±
2ms, resulting in a mean wingbeat period of approximately
60ms. Even though there was a slight variation in the duration
of the downstroke and the upstroke, the ratio between the two
remained almost constant at 0.59 ± 0.07. These figures were
constant across individuals and across different experimental
conditions.
As the width of the gap was reduced to approach the
wingspan of each individual bird, the normal wingbeat cycle
was interrupted during the actual passage through the gap. The
duration of the upstroke was then longer than the duration of
the downstroke, indicating that the birds actively held their
wings in such position to avoid touching the panels during the
passage. During traversal of the gap, the birds either held their
wings in a position corresponding to the end of an upstroke, or
tucked them in against the body (a behaviour very reminiscent
of flap bounding, i.e. intermittent phases during which normal
flapping flight is interrupted). In either case, the birds closed
their wings, projectiling themselves through the gap, rather
than actively flapping through it. The choice of the mode of
traversal depended upon the duration of the passage: during
longer traversals the wings were always tucked in completely.
Only in one instance (out of a total of 490 narrow-gap
traversals) did we observe a bird holding its wings pointing
downwards, and once a bird holding one wing up and the other
down. For simplicity, we shall refer to all of the projectiling
behaviours as “wing closure.”
We found that, as the gap was made narrower, the birds
were more likely to interrupt their wingbeat cycle and close
their wings. Furthermore, the birds maintained wing closure for
a longer duration as the gap was made narrower. More
importantly, the birds are not simply adjusting the phase of
their normal wingbeat cycle so as to ensure that wings are in
the closed position when passing through the gap. They are
definitely prolonging the period of wing closure during passage
through the gap.