I guess I should elaborate since I was in a bit of a rush before.

Basically with the influx of the youngsters of the year, the adults broaden
their territory in search of food aswell. You see more in the fall because
thats when the population is at its peak. Then you have the natural
mortality rate of the youngsters, and older individuals. which is why you
don't see as many later in the winter.

Cheers

Ryan Shaw