A field guide to mesozoic birds and other winged dinosaurs

using their wings to passively extend leaps between branches.
On the other hand, a strict ground-up model would predict flapping
flight to emerge first, as running animals used their wings to extend
leaps, propel themselves up steep inclines, and ultimately achieve groundbased
launches into the air. Furthermore, it might predict that the reversed
hallux would only begin to evolve after a complete flight stroke had been
achieved, allowing previously terrestrial animals to begin perching in trees
(Ohmes 2012).
Unfortunately, the fossil record doesn’t seem the be a good fit for
either of these scenarios. The most primitive winged dinosaurs, such as
Microraptor and Confuciusornis, had hind limbs apparently devoid of any
clear climbing adaptations. The perching foot wasn’t perfected until the ornithothoraces,
when powered flight had probably already been achieved,
but the first stages of modification to the hallux are seen as early as the
confuciusornithids, if not the deinonychosaurians. However, these primitive
forms also did not have the shoulder anatomy that would have allowed
a flight stroke, and may have been incapable of launching from the ground.
Even early ornithothoracians may have needed to climb to become airborne.
At the moment, a middle ground hypothesis regarding the origin of
flight is the only one favored by this evidence. It is likely that in the ancestors
of birds, flight and arboreality actually co-evolved. The arboreal adaptations
of bird ancestors seem to have advanced simultaneously with those related
to flight. It seems that very rudimentary gliding flight or incline running
was utilized in the earliest winged birds, slightly extending the range of
these animals into the trees to escape predators or access prey out of their
competitors’ reach. This can be seen as a hybrid hypothesis, “ground up into
the trees.” Once birds had become fully arboreal, flight could be perfected,
but the initial stages of flight evolved hand-in-hand with the initial stages of
arboreality.
In modern birds, flight takes a variety of forms, which can be broadly
defined in terms of five distinct styles: flapping, flap-gliding, bounding,
soaring, and bursting (Close & Rayfield 2012). “Flapping” refers to continuous
flapping, a style employed today by birds like ducks and flamingos,
which flap almost continuously during flight. Flapping birds come in a variety
of sizes, but all tend to have a high wing loading (i.e. small wings relative
to the weight of the bird). Flap-gliding is a common form of flight, seen in
crows and gulls among others, where flapping is intermittent, interrupted
by periods where the wings are extended in a glide. Bounding is another
form of intermittent flight, seen mainly in small birds, where instead of
gliding, birds enter a “ballistic” phase in between flapping phases. Here, the
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