Catkins are tubular flower clusters that lack showy petals and often rely on the wind for pollination. Many shrubs have catkins, but here in eastern Massachusetts only two (both in the genus Alnus) have both male and female catkins present in winter.
The above photo shows clusters of small female catkins and conspicuous male catkins of Speckled Alder (Alnus incana). In closely related Smooth Alder (Alnus serrulata), the male catkins hang similarly, but the female catkins are oriented upward (rather than pendent).
After flowering, female catkins of both species develop into brown cone-like structures containing seeds. One of the cones in the following photo has some scraggly growth which sticks out and, according to Donald Stokes' The Natural History of Wild Shrubs and Vines (1981), is a distortion caused by a fungus.
Alders grow quickly and can form dense stands that help prevent erosion in vulnerable wet areas. They are favored by Beavers for both food (inner bark) and building materials (wood), and finches – including American Goldfinches, Pine Siskins, and Common Redpolls – are known to consume the seeds of these native shrubs.