The dangly catkins on birch trees are the male flowers.  They will elongate and produce pollen.

Meanwhile the female catkins are developing.  You can see them top right in the picture above.  They are very small at the moment but when the male catkins drop off on to the floor, the fertilised females will fatten up with trillions of seeds.

Birch trees produce so many seeds in the hope that some of them will find a nice place with bare soil and lots of bright light.  Birch is one of the first species to colonise cleared land.

Meanwhile birch seeds are a great food source for birds like coal tits and siskins.