The reasons for the rise in rat complaints can be many, experts say. Warm weather not only brings rats out of their burrows, it also brings diners out to sidewalk cafes or to picnic in the park. They can find ready food sources in alleys and garbage cans. New construction, demolition and large-scale rehab projects can quickly disperse rats into a community, and adding to the problem are pet owners, or more exactly, those who don't pick up after their pets.