Silver Birch (Betula pendula)
In fact several trees produce catkins as flowers and the Silver Birch is certainly one. Not quite the same as Hazel of course but nonetheless catkins is what they are.
Like Hazel, the Silver Birch flowers before the leaves come but flowers much later in the year. Silver Birch are in flower now, April and May whereas Hazel catkins were out as early as January and all over by the end of March.
As the Silver Birch flowers die off so the lovely fresh green leaves appear and there are very few prettier trees that a Silver Birch freshly dressed in delicate green.