That's exactly right. You see this effect with both aeroplanes and
helicopters. For an aeroplane to fly really slowly, the angle of attack is
increased (the plane points upwards) - this results in increased drag (hence
the slower speed), lift is decreased because the airflow over the wings is
reduced and must be compensated for by increased power. The reason a plane
can't hover is because the only way that airflow occurs over the wings is by
forward movement. A bird generates lift by moving its wings (like a
helicopter generates lift by moving/rotating its "wings"). By adopting the
appropriate angle of attack (generally reflected in the body position of a
bird with respect to the ground), a bird can effectively hover by moving its
wings in such a way that lift exactly equals weight. The greater the
headwind, the lower the angle of attack - in other words, the more
horizontal the body position (or the flatter the flight mode). Sometimes you
just need a diagram!
I've seen this with Nankeen Kestrels - in conditions from high wind to
still.
Paul Dodd
Docklands, Melbourne
-----Original Message-----
From:
On Behalf Of John Tongue
Sent: Wednesday, 16 July 2008 2:32 PM
To:
Cc:
Subject: Hovering Kestrel?
Hi all,
Kestrels are fantastic at hovering. I has seen them doing it in quite
strong wind, and almost still conditions. Te difference seems to be
that they are more "horizontal" in stronger breeze, and less so in
lighter breezes. Also, they probably do not hang stationary for as
long when the wind is lighter, but I have seen them hover for quite
some time even in still conditions - whether that constitutes a "true"
hover, I'll leave to those more expert in the field. Sadly, there are
VERY few Nankeen Kestrels in Tasmania, so it's not something I observe
as often as when I was growing up.
John Tongue,
Ulverstone, Tas.
On 16/07/2008, at 1:25 PM, Andrew Taylor wrote:
> On Wed, Jul 16, 2008 at 12:35:58PM +1000, Chris Sanderson wrote:
>> I have observed Black-shouldered Kites, Nankeen Kestrels and Brown
>> Falcons
>> hovering on regular occasions. I understand Peregrine Falcons are
>> also
>> capable of the feat, though I have never seen them do it.
>
> Evan is right. In most cases they would have maintaining a stationary
> position with respect to the ground by flying into a headwind -
> impressive
> in itself but not strictly hovering. For most birds energy
> requirements
> are thought to follow have a U-shape with flight and very low speed
> flight is either expensive or impossible. Despite kestrels'
> adaption to
> this hunting mode there are suggestions they are incapable of
> hovering.
> I don't know if this is correct - as a variety of birds seem capable
> of
> transient hovering - e.g I think I've seen spinebills, kingfishers,
> tyrants and owls hover in still air - and of course hummingbirds.
>
> Andrew
>
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