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FLIGHT VERSUS BROODING
Hopp and Orsen's idea that brooding was the selective force behind the
evolution of the long feathers and folding arms of bird-like dinosaurs and
their descendents is one of the more interesting nonflight explanations - and
may prove applicable if the dinosaurs prove less avian than Archaeopteryx -
but it has some problems.
The arms and feathers of large ratites are reduced to varying degrees, even
though they brood large numbers of eggs laid in ground nests. A photograph of
a brooding rhea (Hoyo et al. 1994) shows the over all body feathers covering
the nest like a blanket. This suggests large arms bearing arrays of well
developed arm feathers are not critical to avian style brooding.
The brooding hypothesis is pretty good at explaining the folding arms - better
than predation since most theropods did fine without them. The core problems
center around the large sternal elements, ossified sternal ribs and furcula.
Even large feathers are not heavy, and it does not make sense to develop all
this shoulder girdle strength and muscle attachment area simply to drape
feathery arms over a nest. Nor to fold up the arms.
A better scenario has the strengthened shoulder girdle and enlarge muscles,
pennaceous feathers and folding arm that so dramatically characterize birds
and their immediate relatives first evolving for flight, not brooding.
Instead, the wing feathers were secondarily used to help cover exposed eggs,
helping initiate a new mode of egg incubation. When flight was quickly lost in
some protobirds just above the level of Archaeopteryx, this system was
Also note that the well developed distal tail feathers on Protarchaeopteryx
and Caudipteryx are too far aft to help cover the eggs, judging from the
position of the tail in the nesting Oviraptor. This too argues for a flight
heritage rather than brooding as the better explanation for the development of
I do not argue for multiple evolution of flight, although it is a possibility
that must be considered. Nor am I particuarly arguing that loss of flight in
early dino-birds occurred multiple times. One event may have led to the entire
radiation of small Cretaceous theropods. Is it parsimonious to invoke flight
loss in early fliers? Why not? Loss of flight has occurred multiple times in
advanced fliers, even on continents. Parsimony is useful only when one mode is
clearly simpler. This is not one of them.
So much for idle chatter. Have to get back to the book detailing these