BOSTON -- As leaves begin to fall from the trees, and cooler temperatures send animals looking for winter shelter, Boston-area homeowners may notice unwelcome company scurrying through their homes.

Boston has been ranked by independent pest control analysts as the seventh worst city for rodent infestation.

The 2009 Rodent Risk Report said that an estimated 21 million homes nationwide could be invaded by mice or rats.

The city's main drawbacks are its age and dense population. Older buildings tend to have more openings where rodents can sneak in and nest, said researchers. The higher the population, the more buildings there are, which requires more constant maintenance to keep pests out.

Although Boston is still in the top 10, researchers said that the city has made improvements in infrastructure that have moderately reduced the risk that rodents will have ready access to buildings and homes. In 2007, Boston was ranked No. 2, behind New York City, for rodent risk. This year, New York remained No. 1, while Boston dropped five spots.

Colvin and Kaukeinen began conducting the study because rodents pose a greater problem than being unseemly house guests. They spread disease, cause structure damage and pose a fire danger by gnawing on electric wires.

The study was created using U.S. Census Bureau data, and regional information of 30 metropolitan areas Colvin and Kaukeinen reviewed.

Researchers compared the size, density and age of each city. They also reviewed population demographics and income levels. Researchers were careful to reflect the current economic condition in the 2009 report, reviewing foreclosure and unemployment data supplied by the U.S. Bureau of Labor, Kaukeinen said. City infrastructure data was the most important index, because that told them the overall condition of city buildings, said researchers.

City revenue and infrastructure expenditure on things such as streets and highways, sewerage, solid waste management, structures, and utilities played a large role in determining a city's infestation risk. Records of rodenticide sales were also considered, as were climatic factors such as average rainfall, that can affect rodent populations.

The report was funded by the Rickett Benckiser Company.

In addition to publishing the report, Colvin and Kaukeinen offered tips to help reduce the risk of infestation:

1) Keep all trash in sealed containers and remove junk piles.
2) Avoid leaving food out on counters overnight and limit access to pet food and birdseed.
3) Seal up any hole or cracks in foundations -- rodents can fit through a hole the size of a dime.
4) Cut brush and bushes back at least a foot from the home or building.
5) Stock up on baits and traps before the appearance of rodents.
6) Avoid products with misleading effectiveness claims.