The Hazels are well into flower, spring must be here. Hazel trees are monoecious, with both male and female flowers on the same tree. However, the female flowers on one tree can only be pollinated by pollen from other hazel trees. The yellow male catkins appear before the leaves and hang in clusters, from around February. The female flowers are tiny and bud-like with red styles; they always make me think of Triffids. Once pollinated by the wind, the female flowers slowly develop into the familiar hazel-nuts with a woody shell surrounded by a cup of leafy bracts.
According to the Oxford English Dictionary, the term catkin is derived from the Dutch katteken meaning kitten, and presumably relates to a kitten's tail.