There may be several reasons for this. When there is no glare from the sun, spotting the hawks is easier. Trees and shrubs are pretty bare, so they don't interfere with the view. And our most commonly seen hawk, the red-tailed hawk, is a migrant who moves south from more northerly climes. There may just be more of them here in winter.
The Observer, in a February 2011, article, took note of the large number of hawks in town. We have seen hawks on Liberty, on Wagner, and on Zeeb, and a pair along the Huron River. Hawks have been spotted, at one time or another, all around town.
The red-tailed hawk is the species most often seen locally, because unlike some of its cousins, it's comfortable in unforested areas. We have seen red-tails most often along roads bordering open fields, hunting from a perch such as a tree, a utility pole, a wire, or any other structure that provides an unobstructed view. Generally the birds soar high--they can see a mouse in the field from ten stories up--but we see them hunting from far lesser heights.
Red-tails eat small animals such as squirrels, chipmunks, and mice. They also hunt and eat other birds, although that's generally not a large part of their diet. The activity around our backyard bird feeders (which also attract chipmunks and squirrels) gets very quiet when a hawk shows up. And, while red-tails seem to prefer live food, the guidebooks say they will eat carrion. Because red-tails have a reputation as opportunistic eaters, it might be well to keep the Chihuahua close or leave it at home when hunting this hunter!