The Common Hazel is easy to identify in winter because it bears male catkins which open to release pollen at any time between late December and late April. The pollen is wind dispersed. All photos in this post are taken from the Tree Guide UK app. The catkin turns yellow and looks like this when it is open to release pollen. If you flick the catkin with your finger it will release a yellow dust consisting of fine pollen grains. Each grain contains male DNA.
In the photo (below left) if you look carefully at the top of the catkin on the left you can see the tiny brown, female flower bud with its red styles sticking out. The flower is ready to receive pollen from another tree.
The photo to the right is a close-up of the female flower bud. Inside the bud are 6 flowers. Each flower has 2 crimson styles that stick out at the top. They have areas (stigmas) that are receptive to the pollen, released from male catkins. When pollen grains land on the receptive style, pollination takes place. After pollination the flowers develop into a cluster of from 1 to 4 Hazel nuts. The red styles appear a few days after pollen has been released by the male catkins on the same tree. This avoids self fertilisation.