Why do seed-eating pigeons have strong gizzards and long narrow intestines?

Seed-eating pigeons possess strong gizzards and long, narrow intestines as specialized adaptations for their diet.

The gizzard is a muscular part of the stomach that functions in the mechanical breakdown of food. Because seeds are tough and require significant physical processing to be digested, the gizzard acts as a grinding organ, often utilizing swallowed stones (gastroliths) to crush the seeds, effectively serving the role that teeth perform in other animals.1

In contrast to fruit-eating pigeons, which typically have broad, short intestines to allow them to void fruit stones intact, seed-eating pigeons require a longer, narrower intestine to facilitate the more complex and time-consuming process of extracting nutrients from seeds.2