The saying “as rare as hens’ teeth” implying that something is so rare that it is effectively non-existence, sprang to mind last week when I spotted a small flock of Rooks on our gravelled yard strutting about on baggy-trousers feathered legs as they carefully selecting small pebbles.
It is generally accepted that birds evolved from reptiles. Some fossil reptiles dating from 150 million years ago have teeth and have wings and so these creatures could presumably fly or at least glide. Archaeopteryx, discovered in Germany, is the most famous of these fossils. Some view Archaeopteryx as a flying reptile while others claim it to be the first bird.
Mutations are random changes that happen when genes are being made. They happen all the time and are understood as just natural mistakes in the process of gene replication during reproduction. They can be harmful, beneficial, or have no effect. Evidence is now accumulating that suggests that birds evolved from a toothed reptile, possibly a flying reptile, that suffered a mutation some 116 million years ago that caused it to lose the gene responsible for making enamel on its teeth.
Loss of enamel, the tough outer shell that protected the teeth, left the teeth open to very rapid ware and therefore became life-threatening. Loss of enamel was therefore possibly the driver that caused the eventual loss of teeth, the development of gizzards and beaks, and the evolution of modern birds.
However it evolved, the gizzard is a remarkable organ. Some birds have serrations along the sides of their beaks, on their tongues or on the roofs of their mouths but these are not teeth; they are merely aids for holding and carrying food.
Most birds swallow food whole and grind it in a gizzard before passing it on to the stomach. Some, like pigeons and game birds have a crop whereas others like ducks, swans, geese and owls don’t have a crop.
To do the grinding the birds swallow stones and store these in their gizzards to aid the breakdown of tough material. Over time, these gizzard stones get worn down from the constant grinding and their remains are either passed or regurgitated. Obviously they need to be replaced; hence the small flock of Rooks on our gravelled yard.