In discussing this we are discussing animals, perhaps one lineage of
animals, where evolutionary transitions were occurring (where one
lineage gave rise to proper birds). I am not an expert on
evolutionary theory, but there must be a state that sometimes occurs
in evolutionary transitions where organisms have not yet adapted
anatomically to a new function or habitat, but are instead adapting
by sometimes straining to the maximum of their physical abilities.
This strain, in turn, provides the natural selection that leads to
the anatomical adaptation. In other words, some small paravian or
basal bird was in trees without a hallux, and this provided the
selection pressure for a hallux to evolve. It is the same with
powered flight. There must have been animals that were poor and
tentative fliers, that lacked large sternal keels, and in which the
selection pressure for a large carina arose. Now, of course, I know
that this is adaptationism, and I am skeptical of strict
adaptationism, but there must be a subset of cases in evolutionary
history where features arose not sheerly through exaptation or evo
devo consequences, but through simple adaptation: If you have a
slightly better grip on the branch you are a little more likely to
survive and leave offspring who also survive.
There are also known cases where a group of animals perform a
function which is an important part of their biology and they simply
never show an anatomical adaptation to this function, it simply never
arises. I now collect photos of turkeys and other basal neornithines
brooding their chicks in trees. Looking at these images the parents
certainly look awkward and we'd all feel a lot more comfortable about
the situation if they had feet like Cracids. But they don't, they are
somewhat ill suited to this sub-habitat, and future Paleontologists
will probably never ever be able to demonstrate that they behaved
this way.