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the evolution of flight

I am an amateur when it comes to dinosaurs, and offer the following
criticisms in that spirit.  

The fish predator scenario has a number of problems, starting from the
beginning.  There are lots of fish-eating crocodilians, snakes, birds, and
mammals on our planet.  Many of these do not have hollow bones, which are
not particularly helpful for fishing per se.  The species that are superb
fishermen either fish submerged, while airborne, or from land.  Floating is
not really a good fishing strategy.

Neither are excessively long forelimbs particularly good for aquatic
propulsion.  Penguins and sea turtles illustrate the fact that if you are
going to use the forelimbs for underwater thrust, a relatively short, stiff
paddle works best.

Better insulation is only advantageous for an endotherm.  So starting with
the assumption that we are dealing with an endotherm, more severe climates
might indeed select for better insulation.  The problem is that a small
endotherm must already have some sort of insulation.  There is increasing
evidence that small non-avian dinosaurs did have feathers.  Making the
feathers better adapted for cold does nothing to enhance their design for
flight purposes.

"Flying" through the water and flying through the air are two very
different things.  Structures that help you aerodynamically often hinder
you underwater.  Flight feathers will just not develop via selection for
underwater thrust capability.

However, proposing bold hypotheses is always good, and in that spirit I
would like to propose the following.

Flight evolved in dinosaurs in the context of territoriality.  Defending a
territory successfully requires efficient mobility.  The ability to move
quickly and efficiently from one place to another within one's territory is
a tremendous advantage.  It is often important to get to a high,
conspicuous spot for territorial advertisement.  It is not difficult for me
to envision a feathered dinosaur developing long forelimbs and flight
feathers for this purpose.  Gliding would be an intermediate stage, with
the animal climbing a tree or bush, advertising its territory, then
launching off and gliding toward another such perch, climbing high again,
and repeating the process.

Eventually powered flight would evolve, enabling the animal to very quickly
move from perch to perch.  Such an advantage comes at a high energy cost,
but this is offset by rapid mobility and consequent ability to defend a
large territory.  There is also, obviously, the ability to escape
terrestrial predators.

That is my happy little story.  In more general terms, I think there is
often a tendency to overlook intraspecific interactions as explanations for
adaptations.  If we look at the adaptations of living birds, we can see
that they are very often the product of intraspecific forces at work.

Best regards,