Page 254 - The Origin of Birds and Flight

Many species of birds, such as ostriches, domestic chicken and pen-
guins, are classified as flightless. And there are a number of differences
between the anatomies of flightless and flying birds. The former lack one or
more components of the complex anatomy that makes flight possible—
most importantly, feather asymmetry, the deeply keeled sternum (breast
bone), and their flight muscles.
Flying birds’ asymmetric feather structure gives them an aerodynamic
advantage. The number of strands on the right side of the feathers haft is
different from the left, creating the same aerodynamic effect as the differ-
ence between the upper and lower surfaces of a plane’s wing. Lift force is
produced thanks to the upper surface being more curved, and the bird is
better able to take to the air. In flightless birds, however, the feathers are
symmetrical—one of the reasons they are unable to fly.
In addition to feather asymmetry, the deeply keeled sternum also plays
a major role in bird flight. If we think of the skeleton of a domestic chicken,
this is an extension that extends in a line in the middle of the breastbone
(sternum). Although this exists in the majority of flightless birds, the breast-
bones of such species as the kiwi, emu and ostrich, part of the ratitae clas-
sification, lack such a heavy sternum.
Although chickens, with a deeply keeled sternum are able to fly a few
meters, species with none, such as the ostrich, are unable to fly at all.
These flightless birds’ tails are very short. Thanks to their powerful legs,
however, ostriches (Struthio camelus, a species of bird peculiar to Africa)
can run very fast. The wing bones in these and some other flightless birds
are similarly very small, and they have few flight muscles.
For these birds, however, the lack of anatomical structures that permit
flight is not a deficiency, nor is the inability to fly a disadvantage. Even with-
out this ability, they are perfect living things endowed with a flawless anato-
my.