Birches produce separate male and female catkins, which are thin, pendulous, conelike structures that are most conspicuous in fall after leaf drop. They break apart in your fingers (or the elements) into hundreds of tiny scales that disperse in the wind carrying pollen or seeds.
You can find some photos of both catkin and seed here
http://www.british-trees.com/Oldsite/seeds/p9.htm
Regards,
Matt