The difference between a mammal skull and a bird skull is simple, a bird skull has a beak. Even if you happen to find a bird skull with its beak broken off, the thickness of the bone should be enough to tell you that it is a bird skull. Bird bones are thin and light weight, this enables them to fly. Mammals don't need light bones because, with the exception of bats, they don't fly. Reptile and amphibian skulls are different from mammal skulls in that their teeth are all of a similar peg-like shape, whereas the teeth of a mammal are more specialised.
picture courtesy of Kris Byrant
caiman (Alligator mississipiensis)
magpie (Pica pica)
Identification Through Elimination
Identifying a skull is done through a process of elimination. This is done by identifying specific attributes of the skull and eliminating any potential candidate that doesn't share those attributes. The skull can tell you a lot about the animal to which it once belonged providing you know what to look for and what questions to ask.
One of the first things you should be asking yourself when identifying the skull is where was it found? You should be thinking about both the country and the type of environment in which it was found. From this information you can illiminate all species that aren't found in the same country and all the species that don't inhabit that type of environment. For example, supposing your skull was found in a field somewhere in the middle of England , you can illiminate cougars, arctic foxes, kangaroo's and all those other delightful species that do not inhabit England. You can also illiminate marine species- the chances of finding a whale skull in a field in the middle of England is unlikely.
You are now left with all the species that are found both in England, and in the type of environment in which your skull was found. Now you can look at the size of the skull. A skull that is smaller than your hand is unlikely to be that of a hippotomus etc. thus you can eliminate all species that don't share the same size range as your skull. Remember to bare in mind that baby animals will be smaller than the adults of the same species, however, baby animal skulls are unlikely to remain intact once the flesh has decomposed (see about skulls).
The positioning of the orbits should tell you a little bit about the the skull. Herbivorous animals generally have their eyes located at the sides of their head. Herbivores spend long periods of time feeding, this makes them suseptable to predation. With their eyes positioned at the side of their heads they can see further about them, and are less likely to be caught in a surprise attack. Carnivorous animals generally have their eyes postioned at the front of their heads (humans included), this gives them greater binocular vision which is necessary for judging distances.
To help you eliminate the realistic possibilties check out the skull key on 'Will's skull page' (see links)