Birds fly at tens of thousands of metres high. Ice forms on the wings of planes at this altitude, so why don鈥檛 the birds freeze?
Lucy Hawkes, University of Exeter, Devon, UK
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Birds can sometimes reach astonishingly high altitudes. The record is a massive 11,278 metres by a R眉ppell鈥檚 vulture that collided with an aircraft. The resulting 鈥渟narge鈥� 鈥� that is to say, the bird鈥檚 remains 鈥� left on the plane provided .
Birds almost certainly don鈥檛 experience icing while flying at altitude for three reasons.
First, birds normally maintain their body temperature at about 40 to 41掳C, higher than that of mammals. Flying is such hard work that it probably uses about twice as much oxygen per unit of time as running does, so birds are plenty warm as they fly about, even at very high altitudes.
Second, birds鈥� feathers are a fascinating evolutionary adaptation to not only flight, but also life outdoors. As well as being beautiful and providing lift and trim in flight, feathers keep birds warm and waterproof. They do this via complex microstructures that trap air and repel water. Birds also spend a lot of time preening their feathers to coat them in a layer of 鈥減reen oil鈥�.
鈥淭he bird that holds the record for altitude, a R眉ppell鈥檚 vulture, collided with a plane. The remains provided the proof鈥�
The content of preen oil varies between bird species, but mainly contains monoester waxes and triglycerides, which are oily and repel water. Next time you spot a bird in the rain, marvel at how raindrops sit on top of its feathers as beads, rather than making the bird wet. That is preen oil at work. Indeed, one of the solutions used to prevent accidents in aircraft flying through icing conditions is to use hydrophobic surfaces.
Lastly, most birds flap their wings at least some of the time when in flight, and this action would probably dislodge ice crystals if they were able to form.
Mike Follows, Sutton Coldfield, West Midlands, UK
Birds don鈥檛 freeze because they generate heat during flight. In fact, they have to flap their wings even harder than usual to stay aloft at high altitudes because the thinner air provides less lift. Also, they aren鈥檛 travelling as fast as a plane, so there is less wind chill. Besides, if they fly in formation, they shelter behind the leading bird.
Although the R眉ppell鈥檚 vulture holds the altitude record, I have a new respect for the unsung mallard ducks that frequent lakes and ponds in the UK. They can migrate at altitudes of around 6.5 kilometres.
Niall Leslie, Yarker, Ontario, Canada
As a pilot in the two disciplines of gliding and powered flight, I can say that for appreciable ice to form, usually the surrounding temperature must be below freezing and there must be a quantity of liquid water (such as raindrops) or vapour (such as clouds) around.
You can avoid icing by simply flying where these things are absent, such as below the clouds, where it may be above freezing, or at high altitudes where the atmosphere is generally pretty dry. Birds do this quite easily, and I imagine that there may also be a degree of 鈥渁nti-icing鈥� built into the physical properties of their wings.
With aircraft, ice normally accretes along the leading edges of projecting items, such as wings, propeller blades and tail assemblies. For extended flight in icing conditions, aircraft typically rely on these areas being heated or having flexible bladders that are periodically moved pneumatically to break up the ice formations.
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