Birds perch standing up, bats upside down. Are there any bird or bat exceptions to this? And why do the two perch differently?
鈥� There is a misconception that bats can鈥檛 take off from an upright position owing to their small leg bones and muscles, which have been reduced to make flight more efficient.
However, flapping their tail membrane allows bats to launch upwards. Granted, such an ungainly take-off would make them vulnerable to predation if they nested on the ground, which makes dropping into flight from a perch a better strategy.
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It鈥檚 not just risk of predation that explains the difference, though. Bats also have superior aerobatic ability. These animals can invert and come to a virtual standstill in flight, which allows them to grasp a suitable roosting position from underneath. This means bats can monopolise the ceilings of caves and other inaccessible roosting sites.
Bats are more manoeuvrable because their wings are larger relative to their body mass than is the case for birds. In addition, though wings have evolved from arms in both cases, the bone is limited to the front edge of a bird鈥檚 wing whereas in bats the fingers extend across and to the rear edge of the wing, giving bats finer control of the wing surface.
Conversely, birds鈥� lack of manoeuvrability might limit them to more accessible roosts. And this may help explain why birds don鈥檛 sleep as we understand the term 鈥� they rest each side of the brain in turn so that they stay alert to predators and possibly to avoid falling off their perches. However, there are a few birds, including the vernal hanging parrot (Loriculus vernalis), that roost upside down in trees.
The tendons in bats鈥� and birds鈥� feet are arranged to close the claws and lock the feet to the perch when the creature is relaxed, minimising energy expenditure. If a bat dies in its sleep, it doesn鈥檛 automatically fall to the ground, and needs to be knocked off its perch.
鈥淭endons in bats and birds close the claws and lock the feet to the perch when they are relaxed鈥�
Mike Follows, Sutton Coldfield, West Midlands, UK
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This article appeared in print under the headline 鈥淧erfect perch鈥�