Birdshot and buckshot are two very popular shotgun loads.
The main difference between birdshot and buckshot is pellets in a buckshot shell are much bigger, 2.5mm vs 6mm. The smaller pellets do less damage and are better for smaller game.
Birdshot is made mainly for dropping birds at sometimes quite wide distances while still leaving the body intact. This means that they have to hit a fast-moving, flying target that is far away without destroying it.
Inside of the shell, there are tiny round steel or lead pellets (often a mixture of both, depending on the birds that are being hunted) that all have the same size. Birdshot shells contain the smallest pellets of all shotgun ammunition.
When shot, these pellets spread outwards, which means you do not have to be extremely accurate to land a hit. A large number of pellets inside the shell also increases the chances of hitting a target. This is necessary because birds are comparably small targets with a comparably wide range of movement.
Birdshot pellets strike their targets with less momentum than buckshot does. This is why they are used to hunt smaller animals – birds like ducks, geese, turkeys, quails, and pheasants, but also other small game like squirrels and rabbits.
Birdshot does not destroy small targets but is still strong enough to kill them. Hunters that are mainly looking for trophies sometimes even fill their shells with fewer pellets to make totally sure that their target does not get damaged and can still be displayed. Some critics consider this practice inhumane, though, since it takes longer for the animal to die and is also more painful.
When purchasing birdshot, a higher number on the ammunition means that the pellets are smaller. The smaller the pellets, the more of them are inside a shell.
This means that the pellets have more impact on the target and are used to hunt larger game like deer. If you use buckshot to hunt smaller animals, you would destroy them completely and leave little meat or pelt to harvest.
Just like birdshot, buckshot pellets move outward when shot. Because they are heavier and fewer in number, the resulting pattern is tighter, though. This means it is more important to aim correctly when you want to hit your target. The farther away the target is, the wider the shot spreads before reaching it.
Since buckshot can also do serious damage to humans, most seasoned hunters wear orange vests rather than camouflage. Due to the fanned-out shot pattern, you do not even have to be in direct line to get shot. Other than birdshot, buckshot is also able to penetrate some kinds of walls like for example drywall. This makes buckshot more dangerous to bystanders or other hunters.
Buckshot sizes are also designated by numbers or letters. Most of them use the “aught” designation, like #0 (“single aught”) or #00 (“double-aught”). The more aughts, the bigger the pellets. For smaller buckshot, numbers are used.
Like with birdshot, a higher number designates smaller pellets. Experienced hunters also buy shells and pellets separately and compile them according to their needs.
So use birdshot for birds and small animals and buckshot for larger game.