The common hazel (Corylus avellana) is one of many of our native trees that are wind pollinated. It is monoecious, meaning that each plant carries male and female flowers, both known as catkins. It flowers very early and the catkins are out now. The male catkins, pale primrose yellow and up to 12cm long, are also known as lambs' tails. I read that bees can collect the pollen but only with difficulty and in small amounts - because it is carried by the wind the pollen is not sticky.
The female catkins are tiny and mostly concealed within the buds that will form the hazelnuts. Only the red styles protrude.
This shows the male and female catkins together. The female flowers have to be fertilised by pollen from other trees.