Flowering hazel

There's lots of hazel in flower at the moment, and the catkins really make the bush stand out at a distance. However the catkins are the male flowers, if you look a bit closer you may see the diminutive fiery red headed female flowers.

Plants have various way to stop self-pollination, but I'm not sure which method hazel uses. What I can say is that on only one of the bushes I saw today were the female flowers open like this. All of the others were closed. Maybe on each individual bush the female flowers only open after the males have shed their pollen. If each bush sheds it's pollen at slightly different times then that would avoid self-pollination but would allow the female flower to be fertilised,