There are few things more frustrating than finding kills and puffs of feathers where hen birds have been taken off the nest. When the grass is short and the crops are relatively low, catching up with the fox or foxes responsible is pretty straightforward. A shine around with the lamp or a sit with a thermal sight or image intensifier will usually bring results; maybe not immediately but certainly within a couple of nights. Foxes are creatures of habit, after all.
When the cover is up and the crops are waist-high, catching up with them is less straightforward