Hazel Flowering

Below images of the female and male Hazel flowers. The catkins in the bushes in our wood and hedgerows are at all different stages of flowering and led me to ponder how this diversity in flowering time is maintained in the populations. The male catkins release pollen before the red stigma flowers appear and so the latter are pollinated by adjacent bushes. It is obviously necessary for a wind-pollinated species where both sex flowers are produced on the same plant to prevent self-pollination.