It is not surprising to find that birds made their first appearance in
the Mesozoic era, for of all animals they are most like the reptiles as
a class. Feathers are about the only dependable characteristic of the
entire group, nearly every other feature being matched by some reptilian
creature of great antiquity. The nesting habit, which includes care of
the young as well as the eggs, is a matter of progress which relates to
flight and to warmer body temperature. It appears to have been developed
by forest-dwelling types living among trees and nesting there in
comparative safety from enemies prowling on the ground.
The oldest known prehistoric birds were found in lithographic stone of
Upper Jurassic age. _Archaeopterix_ was discovered in 1861 at
Solenhofen, Germany. Sixteen years later a similar bird in a better
state of preservation was found in Bavaria. The latter was named
_Archaeornis_. These Jurassic fossils are regarded as true birds by some
authorities, while others believe them to be more nearly related to the
reptiles, the opinions being based on careful studies of the skulls and
other skeletal features. Both birds had teeth of reptilian character,
and it is evident that there was no beak, for the jaws were covered with
scaly skin. The bony construction of the long tail would suggest lizards
rather than birds, were it not for the presence of feathers which were
attached at each side. Head, neck, and parts of the body were covered
with scales. Wings were well provided with stout feathers but the
skeletal framework indicates that the birds were gliders rather than
true flyers. Claws on the wings served like fingers to aid in climbing
among the branches of trees, a practice which is occasionally noted
among the young of living birds. In adult birds of today, however, the
claw-like appendages of the fore limbs are greatly reduced and of little
service.
The next fossil birds of importance have been found in Cretaceous rocks
of Kansas, both of them fishers of the seas instead of forest
inhabitants. _Ichthyornis_ was a small bird, standing about eight inches
in height, a powerful flyer with reptilian jaws and teeth. _Hesperornis_
was built for diving and swimming, like the loon, but was somewhat
larger and provided with teeth. Its wings were too poorly developed to
be of use in flying.
Toothed birds became extinct with the close of Cretaceous time, and the
ancestors of modern types were in existence before the Age of Mammals,
but fossil remains are few and poorly preserved. Large ostrich-like
birds, however, are known to have lived in North America during the
Eocene period. One of these, named _Diatryma_, stands nearly seven feet
tall in the reconstructed skeleton. Its legs are heavy, wings greatly
reduced, beak massive. In its relation to modern birds it is possibly
nearer to the cranes than the ostriches.
Flightless birds of large size are known from many parts of the world
and seem to have been prominent throughout the Cenozoic era, as they are
today in the southern hemisphere. _Aepyornis_ lived in Madagascar during
the Pleistocene period and may have become extinct quite recently. Its
eggs are the largest known among fossils, several times the size of an
ostrich egg. Also in this period the moas were living in New Zealand
where their remains are still abundant. One of the largest, known as
_Dinornis_, had about the same form as _Diatryma_ but the neck was
longer, head and beak smaller, legs better fitted for running, height
about eleven feet.
A much smaller flightless bird, the dodo, became extinct in modern time.
This former inhabitant of Mauritius and other islands of the Indian
Ocean was related to the doves and pigeons, and had lost its power of
flight through disuse of the wings. It was a clumsy, defenseless bird
weighing possibly as much as fifty pounds. Actual remains are few and
incomplete, and descriptions published by the explorers who knew the
bird two centuries ago are not entirely trustworthy. In the Pleistocene
Rancho la Brea beds of California the largest of all prehistoric flying
birds has been found, a vulture bearing the name of _Teratornis_.
Re-assembled skeletons show them to be slightly bigger than existing
condors.