One of the reasons mice infestations are so tricky to deal with is that a pair of mice can rapidly form a thriving nest of individuals. Mice breed at a prodigious rate; a single breeding pair can theoretically produce thousands of offspring each year. And remember, every time a mouse gives birth to a litter of pups, half of those will be female mice capable of giving birth to more offspring after around four to six weeks. Each female can complete five to ten pregnancies every year, giving birth to litters of between three and fourteen pups each time. That adds up to an average of 32 to 56 pups from every female mouse each year.
Interestingly, the breeding behaviour of mice changes according to whether they live indoors or outdoors. Mice that live outdoors have to contend with more environmental pressures, meaning they have shorter lifespans and don’t breed during the colder months, conserving their energy for survival instead. Indoor mice, on the other hand, have it made. Mice that live indoors and have access to as much food, shelter, and warmth as they need don’t need to spend as much time or energy scavenging and building their nests. Because of this, mice that are living in your home can breed even faster and more efficiently than those in the wild.
If a pair of mice find their way into your home, they can build a nest and start breeding before you’re even aware of their presence. This is one of the reasons that mouse infestations need to be dealt with as soon as they are detected. The longer you leave it to investigate strange noises coming from your attic or walls, the more chance there is of mice establishing a thriving colony before you can stop them.