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Re: More on Argentavis

In any case, not quite. Flight capabilities of
galliforms for example are not too well-developed, but
also very rarely lost (not quite as rarely as in
passeriforms, but far more rarely than in the usually
well-flying Anseriformes).

Actually, galliforms have extremely well developed flight abilities, but they are specialized for burst launching. The tradeoff is that galliforms have reduced sustained flight ability (low endurance muscle, relatively high drag at moderate speeds). Galliforms also develop the flight apparatus early in ontogeny. They hatch with a well developed pectoral girdle (compared to other birds), and it grows and fuses early in life compared to other clades. This would appear to be a major mechanic behind the low incidence of flight loss in galliforms. In addition, galliforms generally have relatively low wing loadings, at least at small to medium sizes (galliforms have a fairly steep loading scaling relationship, because the m. pectoralis major and m. supracoracoideus mass increases very rapidly with increased body size in order to maintain the burst launch performance).

By contrast, Anseriforms tend to mature the flight apparatus a bit later. Perhaps even more importantly, anseriforms have gracile humeri, relatively small sternal keels (which reduces total bone investment a bit), and high wing loadings. These are all associated with rapid, high endurance flight powered by mostly aerobic muscle. Such long-distance flight creates an impression of "better" flight abilities, but the morphology and structural requirements associated with high endurance, highly loaded flight in open environments are somewhat less steep than for burst flight. Endurance flight morphology actually predisposes endurance flyers to flight loss under the proper conditions.

One related sidenote is that while galliforms (and columbiforms, which also have burst takeoff adaptations) are popular avian flight models, they are poor models for the "minimum" flight requirements. This has been a problem for the conclusions of a number of studies trying to reconstruct avian flight evolution.


--Mike H.