Blanks are fake/prop bullets basically, loud bang (though if you put one to your head you'll still die). And the comic is saying the animals found out he's just making a loud noise and can't shoot them so they're spreading the info around.
Someone who's ever actually shot a gun feel free to chime in.
Pretty much, though blanks aren't actually bullets, they're rounds or cartridges. The main difference between blanks and regular rounds/cartridges is that blanks don't have bullets (the metal bit which gets shot out of the gun)
They still have the gunpowder though, so like can@sh.itjust.works said, they are still dangerous due to the explosion that no longer has to propel a bullet.