One summer when I was a kid, we had a family of Great Horned Owls take up residence in our backyard, which meant that we became pretty friendly with our local raptor centre. One time they called us and said "Hey, we have this orphaned Great Horned Owl chick here, and we know you guys have some chicks in your yard, can we bring this little guy over and just put him in one of your trees?" My mom replied "Why? Won't it just starve here? There are already 3 chicks." The woman on the phone just responded "Oh don't worry, owls can't count! :) "

To be fair, trouble counting is a problem with even smarter birds. You can put a ball in front of a nesting goose and it will roll it into the nest thinking its an egg.