This is certainly true for kites. I decided to get up early yesterday and go find a kite roost that had been evading me for some time. Roosts can be a little tricky to find in the evening as it's near dark when the birds come in and it can be difficult to follow them. I had a good general idea where the roost was so when I got near the location and had a scan around with my binoculars I quickly spotted a good number of kites flying around, just having left the roost.
On closer inspection five kites were on the ground running around picking up worms, there were also two common buzzards in the field also feeding on worms and invertebrates. Kites while very graceful in the air look very clumsy and somewhat comical on the ground. The field the birds were feeding in was closely grazed by sheep, not where you would imagine there would be a good source of food for birds of prey. However the closely grazed grass makes small insects and worms accessible to kites and buzzards. In Wales it is well known that earthworms are an important part of the diet of the red kite.
As I still wanted to find the specific roost tree, I went to speak to the farmer to get permission to have a look around his land. He was aware of a number of kites on his land and had been enjoying regular views of them. A quick walk around the bits of forest on his land and I found a large tree with a lot of white splashes under it, a sure sign of roosting birds of prey. On closer inspection I found what I was looking for, a bumper haul of red kite pellets! Examing bird of prey pellets allows us to determine what they are eating. While I have only seen kites snatching at small invertebrates while hunting, a cursory glance at the pellets showed a lot of fur and some small bones, so they're obviously eating more than worms. Yesterday evening nineteen kites came into roost in the same area, so there will be plenty more pellets to collect and examine over the coming weeks.