TheHunter: Call of the Wild Wiki

"These 20-gauge shells are designed to be the perfect companion for upland game hunters.

Bird shot contains a huge amount of small pellets. This gives it short effective range and low damage. However, the wide and dense pattern generated by these properties also makes it ideal for hitting small fast-moving targets such as small game and birds.
― In-Game Description

The 20 GA Birdshot is an ammunition type available for the Strecker SxS 20G and Nordin 20SA families of shotguns.

This ammo is available as part of the Wild Goose Chase Gear and Duck and Cover Pack DLCs.