Why Mouse Infestations Persist Into February in Seattle
In Seattle, seeing mice scurrying through a house or hearing them in the walls well into February is a common — and frustrating — surprise for many residents. Unlike regions with harsh, bone-chilling winters that put wildlife into torpor or push them into deep hibernation, the Pacific Northwest’s mild, wet winter climate and the city’s dense, warm urban environment create near-ideal living conditions for commensal rodents. As a result, mouse activity doesn’t stop with the first cold snap; in many homes the problem actually intensifies in late winter when the animals take advantage of steady indoor heat, dependable food sources, and protected nesting sites.
Biology helps explain why. House mice (Mus musculus) breed very effectively in hospitable environments: gestation is short (about 19–21 days), females can produce multiple litters a year with an average litter size of several pups, and juveniles reach reproductive maturity in just weeks. That means a small overwintering population can expand quickly in the sheltered confines of a building. February also sits at a biological tipping point — as days begin to lengthen and conditions stabilize indoors, mice increase foraging and nesting activity and offspring disperse, making infestations more visible to homeowners.
Human behavior and the built environment compound the problem. Seattle’s older housing stock, many multi-unit buildings, and common winter practices — storing firewood near foundations, keeping garages and basements cluttered, continuing to feed birds, or leaving pet food accessible — all create entry points and sustenance for mice. Urban factors such as the heat-island effect, frequent deliveries and landscaping waste, and changing precipitation patterns (including milder winters linked to climate variation) further reduce the seasonal pressures that would otherwise limit rodent survival. In short, biological resilience plus plentiful shelter and food means that, for Seattleites, mouse problems often persist — and even worsen — through February rather than petering out. The rest of this article will examine how to identify active infestations, common entry routes, sanitation and exclusion strategies, and effective control options for tackling mice before spring breeding accelerates the issue.
Seattle’s mild winter climate
Seattle’s winter climate is comparatively warm and wet rather than severely cold, and that moderating influence keeps many outdoor microhabitats suitable for mice throughout the season. Temperatures rarely plunge into prolonged deep-freeze conditions that would otherwise increase rodent mortality or force all activity into deep hibernation. Combined with the urban heat‑island effect, sheltered areas around buildings, heated basements, and insulated attics provide relatively stable, above‑freezing refuges where house mice and deer mice can remain active, forage, and shelter from the occasional cold snap.
Biologically, mice exploit those favorable conditions by maintaining breeding and foraging cycles that don’t shut down the way they would in harsher climates. House mice can breed year‑round if food and shelter are available; gestation is short and multiple litters per year are common, so even a small surviving population can rebound quickly. Mild winters mean higher survival of juveniles and adults, lower winter mortality, and continued reproductive turnover into February, so infestations started in fall often persist and increase rather than dying back with the season.
Those climatic and biological factors explain why mouse problems still show up in February in Seattle: steady shelter and food availability, continued reproduction, and the protection offered by buildings keep local populations active and growing. Because outdoor conditions seldom produce a hard reset of rodent numbers, infestations are more likely to be a continuous urban pest challenge rather than a strictly seasonal one—making ongoing exclusion, sanitation, and monitoring necessary rather than occasional winter‑only responses.
Year-round urban food sources
Urban areas like Seattle supply a steady, diverse array of foods that mice can exploit throughout the year: overflowing dumpsters and unsecured restaurant waste, curbside garbage, backyard compost piles, bird seed and suet from feeders, fallen fruit from ornamental trees, pet food left outdoors, and indoor crumbs and storage in homes and businesses. Many of these sources are predictable and replenished regularly, so mice learn where to forage and can remain in or close to human structures rather than dispersing. The proximity of food to warm shelter (buildings, basements, wall voids) lowers the energy cost of foraging and makes even scattered resources sufficient to support local populations.
Because these food resources are available year-round, house mice can maintain reproductive activity and population growth well into the winter months. Under favorable conditions mice breed rapidly—gestation is roughly three weeks and young reach sexual maturity within weeks—so a steady winter food supply can sustain successive litters even when outside conditions would normally limit reproduction. Indoor heat and accessible nutrition mean that February, which is still mild in Seattle compared with colder regions, often does not impose the same seasonal population bottleneck; instead, mice remain active, breeding, and reinforcing local infestations.
In Seattle specifically, the combination of abundant urban food with the city’s relatively mild, maritime winter climate and urban heat islands means infestations commonly persist into February. Residents and businesses that leave composts, trash, or bird feeders accessible—along with connected infrastructure like alleys and utility corridors—create a continuous mosaic of foraging opportunities. When pest management and exclusion are inconsistent, those year-round food sources allow mouse populations to rebound quickly after any control efforts, so the underlying availability of food is a primary reason infestations linger into late winter.
Building structural vulnerabilities and entry points
Older and even relatively new buildings often have an abundance of small gaps, cracks, and voids that mice can exploit. Mice can squeeze through surprisingly small openings around utility penetrations (pipes, cables, vents), gaps under doors, torn window screens, holes in soffits and eaves, and deteriorated foundation joints. Attic and crawlspace access points, unsealed chimneys, and missing or degraded weatherstripping provide direct pathways into insulated, warm interior spaces. Once inside, mice use wall cavities, insulation, and cluttered storage areas as nesting sites and runways, making their presence difficult to detect and their movement through the structure systematic rather than random.
In Seattle specifically, the combination of frequent rain, older housing stock, and tightly connected urban structures amplifies these vulnerabilities and helps explain why infestations often persist into February. Moisture accelerates the deterioration of building materials, creating or enlarging entry points; multi-unit buildings and attached structures share utility chases and interstitial spaces that let rodents move between units even if one unit is well sealed. The city’s relatively mild winters also mean less cold-related mortality and more comfortable conditions for mice to remain active and reproduce indoors. Because mice breed year-round under favorable indoor conditions, populations that take refuge in February can continue to grow unless the underlying structural access points are identified and sealed.
Eliminating persistent infestations therefore requires more than a quick trap-and-remove approach: the structural vulnerabilities themselves must be systematically addressed. Many entry points are hidden or only apparent from both inside and outside inspections, and piecemeal fixes on a single unit often fail when connected neighbors or exterior gaps remain unsealed. Effective, lasting control typically involves coordinated building-wide inspection and exclusion work (sealing gaps with appropriate materials, installing door sweeps and vent covers, repairing damaged flashing and foundation joints), combined with sanitation and monitoring. Without that comprehensive approach, mice will continue to exploit weak points and reestablish infestations through February and beyond.
Connected green spaces, sewers, and rodent corridors
Connected green spaces, sewers, and other urban corridors form an interconnected network of habitats that allow mice to move, shelter, and reproduce across large areas of the city. Parks, ravines, tree lines, hedgerows and continuous landscaping provide cover and nesting material, while storm drains, culverts and sewer lines offer sheltered, thermally stable pathways below street level. Because these features link residential, commercial, and industrial zones, a population reduction in one building or block can be quickly offset by immigration from adjacent green corridors or underground passages.
Sewers and culverts are particularly important in Seattle’s mosaic. They maintain more constant temperatures than the surface, protect animals from predators and weather, and often contain organic debris or invertebrates that mice can exploit as food. The same underground systems frequently connect to building foundations, utility conduits, and access points in basements, enabling repeated incursions into structures that have already been treated. Above ground, parks and landscaped strips supply nesting materials, seeds, berries and insects; these resources let mice survive lean periods and sustain local breeding, so corridors function both as movement routes and as reservoirs of animals.
Those landscape and infrastructure linkages help explain why mouse infestations commonly persist into February in Seattle. The city’s mild, maritime winter means surface conditions rarely become harsh enough to cause a deep winter die-off; combined with continuous food and shelter in connected corridors, mice can keep breeding or maintain stable populations through late winter. Even where individual homeowners or businesses do exclusion or control, nearby green spaces or sewer-connected populations can quickly repopulate treated sites. Breaking that cycle typically requires coordinated, landscape-scale prevention and exclusion to interrupt corridor-driven reinvasion rather than isolated, single-building responses.
Inconsistent pest control and exclusion practices
Inconsistent pest control and exclusion practices mean sporadic or reactive treatments, incomplete sealing of entry points, and a lack of coordinated, long-term strategy. When homeowners or property managers only address mice after sightings instead of maintaining ongoing prevention, populations can rebound quickly; mice reproduce rapidly and any surviving individuals or immigrants will refill a cleared space. Partial measures — for example, setting traps in living areas but leaving attic or wall voids unmonitored, or using baits that are not refreshed or positioned correctly — often reduce visible activity temporarily without eliminating nesting sites or pathways, allowing colonies to persist and become harder to control over time.
In Seattle specifically, that inconsistency is a major reason infestations linger into February. The city’s mild winter climate and dense urban housing create conditions where mice can remain active year-round; when pest control is inconsistent across neighboring units or buildings, infestations easily spread through shared walls, sewers, and outdoor corridors. Many residents assume cold weather will curb rodent activity and scale back prevention in late fall, so by February — when indoor breeding and movement continue and outdoor food sources remain available — mice already established in a building face minimal resistance. Additionally, DIY fixes and short-term contract treatments without thorough exclusion work or follow-up allow survivors and new entrants to repopulate treated spaces.
To prevent persistence into February and beyond requires an integrated, consistent approach: regular inspections, systematic sealing of gaps and holes (including small cracks and utility penetrations), persistent sanitation to remove food and shelter, and coordinated treatments with follow-up monitoring. In multi-unit properties, coordination among tenants and managers is critical because isolated remediation in one unit is undermined by neighboring infestations. Finally, applying year-round vigilance — not only reactive measures when mice are seen — breaks the cycle of re-infestation and addresses the structural and behavioral drivers that allow mice to survive Seattle winters and remain a problem in late winter months.
