Mouse and Rat Biology and Behaviour

Accurate identification and understanding of behavior can help you determine the best strategy for managing a problem with rats or mice. 


Identifying Mice and Rats

What Do Mice and Rats Look Like?

The common species of pest rodents in New York are house mice, white-footed mice, deer mice, and Norway rats. Roof rats are generally not found in NY. Adult mice and rats can be distinguished from each other by their size, some of their features, and fur patterns.

house mouse
mouse in leaves
a rat

House Mouse

Adult mice are 2.5 to 3.75 inches long (63-95mm), with a tail that ranges from 2.75 to 4 inches in length. Compared to young/juvenile rats, they have large ears and eyes, and a pointier nose. Their body is uniformly colored, meaning that their backs and bellies are roughly the same greyish-brown.

White-Footed Mouse and Deer Mouse

These two Peromyscus species with appearance and behaviors similar to the house mouse, have two-tone fur with a darker back (grey or tan) and white belly.


Norway Rats

Adult Norway rats can reach 11 inches (28cm) in length, with a slightly shorter tail for a total length of 16 inches. A field identification test used in areas where roof and Norway rats coexist is to pull the tail toward the head. If the tail extends past the snout, it’s a roof rat; otherwise, a Norway rat.

Evidence of Mice and Rats


In general, rodent droppings are cylindrical, with pointed to rounded edges. They are typically black, but can differ in color based on food that was eaten. For example, green or red droppings may indicate that the rodent fed on a rodenticide bait. There are no lines on rodent droppings (compared to the vertical lines found on American cockroach droppings). Mouse droppings are typically 1/8 to 1/4 inches long, Norway rat droppings are 3/4 to 1 inch long, and roof rat droppings are about 1/2 inch in length. The CDC offers a pictorial key to identifying droppings that can help determine which pest is present.


Rodents may leave footprints when walking through dirt or dust. Mouse footprints often look like a collection of dots, and the length of the actual footprint is 3/8 to 3/4 inches long.

Rat footprints look like an actual foot, measuring from 3/4 to 1 ¼ inches in length, depending on whether it’s the front or back paw. Adult rats may also leave a ‘tail drag,’ a wavy line between the paw prints.

Gnaw Marks

Impressions left behind after a rodent chews on something are often easy to see. For a pair of teeth (two grooves), gnaw marks from an adult mouse measure 1 to 2 millimeters wide, while adult rat gnaw marks are 3.5 to 4 millimeters wide.

Why do I have Mice and Rats? 

Rodent pests are defined as “commensal” because they live in close association with people. When food, water, and shelter are available, we have made a perfect environment for rodents (and other pests).

Food availability

Rodents feed on food items that we grow, harvest, process, store, and discard. Problems exist if people have inadvertently provided a nearby food source, ranging from fresh produce to food and even pet waste.

Water availability

Norway rats need to drink standing water every day. Water can be found in puddles or condensation on a pipe.

Mice will drink water too, but can obtain enough moisture from most food sources to remain hydrated.

Nest site availability

Norway rats prefer to burrow in soil, but will take advantage of voids near a food source. Mice can make nests inside walls, equipment, and other protected places.

rat burrow
rat drinking water from a puddle in a stone
file box with evidence of rodent damage