The results also show that the shape of grit remaining in the gizzard does not reflect the initial shape of ingested grit, in contrast to previously published ideas. The recovered behavioral flexibility of grit use might reflect the omnivorous nature of Gallus gallus domesticus and may aid smooth dietary shifts. The use of larger sizes and amounts of grit by herbivorous groups may be a response to the needs of digesting hard, coarse materials.

These results show that diet affects the characteristics of ingested grit, grit in the gizzard, and excreted grit, whereas gizzard muscularity affects the characteristics of grit in the gizzard and excreted grit.

Excreted grit in all groups was smoother than the offered grit. Grit in the gizzard was much smoother than the offered and ingested grit, especially in the herbivorous, muscular gizzard groups. Grit in the gizzard tended to be larger in herbivorous groups, but the grit in excreta was smaller, whereas the size of excreted grit was larger in groups with less muscular gizzards. Larger, rougher grit was selectively ingested by all chicks, but size preferences were especially pronounced in the herbivorous groups. The amounts of grit ingested and remained in the gizzard were larger in herbivorous groups, but these groups excreted less grit.

All animals ingested more grit than they excreted, possibly because excreted grit was below the detection size limit of 0.5 mm of the present study. Within each muscularity group, two different diets were provided (herbivory and non-herbivory) to test whether diet and gizzard muscularity affect grit characteristics including amount, size, and shape (circularity, roundness, and solidity) at different stages of digestion (ingested grit, grit in gizzard, and excreted grit). We conducted an experiment to test the factors that affect chicken grit use in 68 male layer chicks of Gallus gallus domesticus, which were divided into two different groups in gizzard muscularity (high and low). Many birds use grit in the gizzard to enhance mechanical processing efficiency. The gizzard is the only gastrointestinal organ for mechanical processing in birds.

Chicken grit