Catkins are a familiar sight in late winter and spring. A catkin is a spike of flowers albeit hanging in most cases. Generally wind pollinated though by insects in some species (where the catkins tend to be shorter and fatter). Most appear before the leaves which as an evolutionary trait increases their chance of success by wind pollination.

 Catkins are single-sex flowers; typically (though not always) 'male' which disperse pollen to 'female' flowers or catkins. Plants with catkins can be monoecious (i.e. have both male and female flowers) or dioecious (i.e. have either male or female flowers).

 Hazel is a common catkin at this time of year (as seen above). Each one has up to two hundred male flowers which are wind pollinated. Being a monoecious species Hazel also has female flowers. I didn't think to take a close up but note what looks like a red bud on top of the twig that bears the Catkins, that is a female flower (resembling a tiny red tassel).

 Moreover Hazel avoids self-fertilisation because the female flower becomes fertile after the catkins on that tree have ripened and shed their pollen. Thereby the female flowers are more likely to cross-fertilise with pollen from another tree. The resulting fruit is the good old Hazelnut, up to four from each bud.