Roof rats (Rattus rattus) are also known as ship rats or black rats. They are native to Asia, but have been spread throughout much of the world through human activity and are common along most of the west coast of the United States.
Norway rats (Rattus norvegicus) are sometimes called brown rats or sewer rats. They have thicker bodies, shorter tails, and smaller ears than roof rats. It may be very difficult to distinguish Norway rats from other rodent species, particularly roof rats, based on appearance alone.
Their droppings have blunt ends and are approximately 3/4″ in length.
Norway rats use underground burrows and may be found on creek banks, shorelines, and sewer systems. They should be suspected if rats are seen entering burrows or tunnels in the ground.
House mice (Mus musculus) are smaller than rats (2-3.5″ with 3-4″ tail) but similar in appearance. Young rats are sometimes mistaken for mice. House mice are almost never seen outdoors.
House mice will readily infest homes and other occupied, man-made structures. Because they are very small, they are difficult to keep out of structures and may not be discovered right away. They may be active during the day or only at night.
House mouse droppings are pointed and approximately 1/4″ in length.
Like house mice, deer mice in the Peromyscus genus are smaller (2-3″ with 2-3″ tail) than rats. They can be distinguished from house mice by their large eyes and light-colored bellies. Deer mice commonly infest vacant structures or seldom-used outbuildings, but may occasionally be found in occupied homes, especially in rural areas.
Deer mice are the primary vector for hantavirus in western North America. Approximately 10% of deer mice in San Mateo County are infected with hantavirus, a potentially deadly viral respiratory infection.
Wood rats (Neotoma fuscipes, sometimes called pack rats or trade rats) are native to San Mateo County and frequently live in wooded areas near creeks and streams, where they make large, elaborate nests of twigs and leaves. These nests often provide shelter for other species, including deer mice.
Woodrats are similar in appearance to roof rats and Norway rats, but with larger ears and furred tail (though not fluffy, like the tails of squirrels).
Squirrels are common throughout San Mateo County, including urban and suburban areas. In the wild they nest in trees and feed on nuts, fruits, and buds. However, they may sometimes damage property by nesting in attics.
Squirrels will not usually create their own opening to a structure, but may enlarge an existing opening by gnawing. You can prevent this by ensuring that there are no gaps or holes that will allow a squirrel to enter your attic. You should also use a chimney cap to keep squirrels and other animals out of your chimney.