Re: which bird flies upside down ?

First we need to define what is meant by flying up-side down.  For 
argument's sake, lets say: A sustained powered flight in an inverted 
position.  

As far as I know there is no record of any bird capable of this.

What prevents birds from being able to achieve sustained flight up-side 
down is physics. The bird is aerodynamically designed to fly right side up. 
There are several complex processes involved in attaining flight.    

A quick physics lesson: 

In order for a bird to fly it must obtain an upward force call "lift". This 
applies to airplanes as well. After all, a plane's wings are designed on the 
basic physics of the bird wing. 

The shape of a bird’s wing enables the bird to achieve lift because the 
wing is basically an airfoil. The front edge and the upper surface is more 
convex than the lower surface. So, when air strikes the leading edge of 
the wing, it divides- some air passes underneath the wing and the rest 
passes across the upper surface. Since the upper surface of the wing is 
curved, it has a longer surface than the underside of the wing. In order 
for air to travel to the rear of the wing at more or less the same time as 
the air on the underside, the air on the upper surface must travel faster 
than the air underneath the wing. The faster air travels across a surface, 
the lower the pressure it exerts on that surface. As a result, the wing’s 
upper surface experiences a lower pressure than under the surface and 
produces lift.

There are other processes involved as well including thrust and drag. But 
to keep things simple we'll avoid those right now.  If we understand the 
physics of an airfoil or wing, then we can see that it works in only one 
direction.. up.  An inverted wing will no longer provide lift.  This is why 
birds are not capable of sustained flight in an inverted position. To learn 
more about the physics of flight, follow this link. 
http://www.ucmp.berkeley.edu/vertebrates/flight/physics.html

Okay I know what your thinking: if a plane is based on the physics of a 
bird, and planes can fly up-side down why can't a bird do the same?

This is a zoology question right? My strength is nature not physics, so I 
will let those with greater interest post that question in the physics 
category for the full answer. But...I believe the problem with birds versus 
planes is the direction of thrust.  In a plane, thrust is provided by the 
engines or propellors and this force is always parallel to the wing. In birds, 
the wings are additionally providing the thrust through flapping. This 
makes it harder for birds to compensate and remain inverted like planes 
can.  Please don't quote me on this. 

Back to the original question. What birds can fly upside down? 

Although they can't "sustain" an inverted flight, many birds can turn over 
for brief periods while in the air.

Birds are quite agile and maneuverable and the ability to turn up-side down 
could be beneficial in avoiding capture, hunting prey, or attracting the 
attention of a mate.  For example, the courtship display flight of the 
Peregrine Falcon involves dives, barrel rolls and summersaults.  Falcons 
have also been known to turn upside down briefly to grasp prey on the wing. 
Bald Eagles have a dramatic courtship display in which the male and female 
clasp talons together in flight- sometimes one bird appears to turn upside 
down to do so.  

In the "Audubon Nature Encyclopedia" there is a section on the aerobatics 
of birds. The book describes Osprey doing loops and summersaults in playful 
flight.  The text goes on to relate a story of a Bald Eagle that was 
observed warding off intruders by turning up-side down and remaining in the 
inverted position for several wing beats without great loss of altitude. If 
you had the opportunity to see David Attenbouroughs latest nature series, 
"The Life of Birds", there is great footage of Sea Eagles battling for 
territory.  In the scene, one bird turns completely up side down to grasp 
the talons of the intruding bird. This position is sustained for many 
seconds.

The anecdotal info I found on the aerobatics of birds mostly covered fast 
agile species like falcons, eagles and hawks.  I would not rule out the 
possibility of many other birds being capable of this feat, but it is not as 
easily documented in smaller species.  

I hope this helps to answer your question


Steve A Williams
biodswill@prodigy.net