Male flowers.
If your courgettes are stressed, either from being planted to close together, or from poor quality soil you will get a predominance of male flowers.
We lost our first sowing of courgettes to the cold snap but the second sowings are going daft.
We always leave a few later in the season to grow into marrows.
Oh! And you can eat the flowers. Lovely dipped in batter and deep fried, and there are recipes out there for stuffing the flowers.