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Re: Hone and Benton 2007 (their second paper)

Arboreal birds use gravity assisted launches, as do cliff-dwellers like tropicbirds. Arboreal birds do not require gravity assistance, while tropicbirds (and frigatebirds) often do.

Can you expand on this a bit?

Sure thing. If you watch an arboreal bird launch from a branch (be it a passerine, raptor, etc) you'll probably notice that it usually drops below the level of the branch as it starts its flight path. This does not always happen, of course. Those same animals are perfectly capable of taking off nearly straight up, perhaps using a leaping launch to reach another branch above themselves. When launching into level flight, however, most arboreal birds will get a little gravity boost and drop a bit as they leave the branch. Exactly when the wings are opened depends on the flight dynamic and size of the bird in question. Bounding passerines will often stay highly tucked for a moment, large raptors usually start a flapping cycle right from the branch (they generally elevate the wings as they launch or just after).

As for tropicbirds and frigatebirds, they both have extremely reduced hind limbs. By "reduced", I mean not only overall size and length, but also strength. Mass-specific femoral strengths in tropicbirds, for example, fall below just about any other birds I've looked at thus far (I suspect that apodiforms and/or some caprimulgiforms may be close, but I have yet to confirm that). Tropicbirds and frigatebirds nest on cliffs or trees on coastlines or islands. They cannot run much at all, nor do they leap powerfully like most birds. Instead, they rely to a great extent on gusts and/or gravitational acceleration in order to launch. A frigatebird placed on level ground, in still air, will have a great deal of trouble taking off. Given its lifestyle and pelagic habitat, however, it obviously will not tend to find itself in such a situation without very strange circumstances (such as human intervention).


--Mike H.