This is a good time of year for those who want to know what pollens are in
their honey to collect a sample of Hazel pollen.  The reason is that it is a
recognisable shape under the microscope and always 25 microns across.  With
hazel pollen for a reference you can compare other pollen grains and decide
that they are bigger, smaller or about the same size as hazel.  This can
considerably narrow the range of possibilities when identifying what you are
looking at.
Has anybody seen bees working hazel for pollen?  The books say they do so
sometimes but I have never seen it.  I once identified Yew pollen (also wind
distributed) in the gut of a bee.
Chris Slade