Calabrese (broccoli) heads

[KC Sunshine]

Help - new to veg growing. We checked our broccoli and the heads were starting to produce florrets about an 10cm across. We checked them two weeks later and something had started to eat the florrets. We covered them in netting as we have a lot of pigeons. Now they have all nearly been eaten. There is no sign of slug damage to leaves, in fact the leaves are looking good. It is like they have literally been skimmed across the top with a knife taking just the flower tips from the florrets. What could be eating them?