Have you ever noticed that while it's pretty easy to tell your dog is sick, it's much harder to know how your cat or bird are feeling? Well, before you start asking your dog why it can't be more stoic like animals half its size, it's probably worth considering each animal's place in the food chain.
You see, the lower down the food chain you are, the more important it is for you to hide any sign of illness. Obvious illness or pain function as red flags for predators. You may as well wear a sign saying, "Easy meal, guys. Come and get me."
Now, even small dogs are genetically programmed to function like predators, which makes sense considering they are close relatives of wolves. And birds--you can probably figure out why the only way you'd find out your bird is sick is when it dies. The situation with cats, as compared to dogs and birds, is much more interesting. On the one hand, cats are predators, but on the other, they're small enough to be prey.
So how can you tell if your smaller pets need medical attention? For one thing, trust your hunches. No one knows your pets like you do, so if you notice even very subtle changes in habits and behavior, you may want to have a vet look them over. For a cat, other signs of illness might be that it has stopped grooming, is losing weight, vocalizing more or less than usual, or missing the litter box.