Birds are beautiful creatures of God. They are bipedal, warm-blooded, egg-laying vertebrates, characterised by feathers, toothless beaked jaws, having a strong yet light weight skeleton. These are just the common features of a bird. But there are still some features that most of the people do not know about birds. Many birds swallow sharp pebbles and grit and hold these rocks in a muscular part of their stomach called the glizzard. Stones are natural substance or a solid farm of one or more minerals or mineraloids which do not react to a digestive fluids sufficiently fast. So why does a bird swallow grits or pebbles? Lots of animals eat rocks, or at least they swallow rocks, they do not eat them in the sense of digesting them. This is true for many birds and reptiles that are alive today, and a few mammals such as seals and whales. This is also true for extinct animals, including herbivorous dinosaurs and marine reptiles. Rocks that have been in the digestive system of an animal are called gastrolith, which literally means ‘stomach stones’.
Among living animals, gastroliths are most commonly found in birds. Many birds that eat fruits, cereals, food grains or insect, swallow grit. To know why they do this we need to understand the digestive system of birds.
Birds do not have tooth to chew the food. Some have, but are poorly developed and are only used for holding the food firmly, which cannot connect the food into a pulp. Generally, after the food enters the mouth, it passes successively through the oesophagus, crop, proventricular and lastly to the glizzard. The seed-eating and insect-eating birds such as the chestnut-headed Bee-eaters crush their food in the glizzard and connect it into a pulp after they swallow the food whole.
The glizzard is actually a chamber with tough muscular walls where the food is ground. And this is where the grits play an essential part. The harder parts of an insect (such as the legs, heads wings etc.) are crushed when they come in contact with the grits and are connected into pulp. So more specifically we can say that the glizzard contracts and grinds the gastroliths against each other and against the food that the bird has swallowed. The rocks or grits grind down the food—essentially the bird is using the gastroliths to chew the foods in its glizzard—and the rocks grind each other down, too.
Eventually the sharp, jagged chunks of rock become smooth rounded pebble, and they are not much good for grinding anymore. So the bird will vomit them out and find new, sharp rocks to swallow. For the past few decades, many palaeontologists have assumed that the rounded rocks are gastroliths. And birds occasionally regurgitate the rocks that have become smooth and rounded, and then they swallow new, sharp ones.
But for grinding the food in the stomach it is not only grits that the bird swallow. According to a researcher named Verbeak, who made observation in California in 1969 on a female Anna’s Humoring bird which ate sand particles, some birds also eat crumbs from the dried and burnt tree trunks and some also eat the outer shells of dead molluscs. As per scientists, who believe that many birds increase their grit intake during their breeding seasons, because grit contain a good amount of calcium which play an important part in increasing the thickness of the eggshells during the breeding season.
Some birds eat grits according to the shape and colour of the stones. Birds species were significantly consistent in colour of grit and grit of different colours varied in prevalence among species. Some scientists concluded that according to their analysis they found positive correlations between lilac and red colour grit in the glizzard of birds. Male birds prefer more lilac and red grits than females, with their effect differing among species.
These findings are consistent with the sensory bias hypothesis that birds express preferences for grit of specific colours and a high diversity of colour related to sensual colouration of the body, even when the colour of such grit is only visible to the individual at the moment of ingestion. So birds are specific and consistent in their choice of grit colour, which are positively related with some body-colours. Even the colour of stones which contains some chemicals might be the reasons for the pigments of particular colours of feathers.