A good guide to deciding whether the noise is caused by birds, rodents or other animals is the timing of the noises. If you are hearing the noise predominantly in the middle of the night, you almost certainly are sharing your home with rats or mice. If the noises are only occurring during the day or at dusk and dawn check around the eaves for birds entering and leaving.
Rats and mice have incisor teeth (front two teeth of each jaw) that are large and continually growing. The teeth have hard enamel on the front surface and softer dentine behind. As rats and mice gnaw they wear the teeth down and sharpen them into chisel like implements through uneven wearing. Rats and mice gnaw things other than food to keep their teeth worn down and sharp. Unfortunately for us they often choose to gnaw wiring, plumbing, joists and items stored where they are.
If rats and mice regularly use a hole or gap to pass through they will gnaw around the gap to open it up and make it easier to enter or exit. They will gnaw wood, plastic, metal and even concrete if they need to. The sound of rats or mice gnawing can can be distinguished by the rate of biting. Mice gnaw at about 4-5 per second, rats 1-3 per second.
A simpler way of determining which rodents you have in your roof is to enter the void (if possible) and look for evidence in the form of droppings. Mice produce small black-grey droppings approximately the size of a large grain of rice. Rat droppings are much larger, 1-2cm long.
In New Zealand the other animal that can be a cause of noises in the roof is the possum (Australian Brushtail Possum). This is a much larger animal than a rat or bird and it usually easy to tell its presence from the much heavier and slower footfalls and they don't gnaw wood like the rodents will.
Other causes of sounds in a roof can be tree branches rubbing on the roof or eaves; usually heard only when the wind is blowing, or expansion and contraction of timbers and gutters as the roof warms up during the day or cools down at night.
Why did the rats move into the roof?
Because they heard the people say the cheese was high.