This field had only recently been ploughed when my wife took this photo. The disturbance of the soil most likely delivered lots of grubs and goodies to the soil's surface, all in easy reach of birds. But why the circle?
I have a theory:
This particular part of the field - where the pigeons are assembled - is the first and last part of the field where water gathers in heavy rainfall. Although the field was quite dry on the day this photo was taken - I don't think it's a coincidence that the pigeons are formed in the shape of a small pond. It might not be that clear from the photo above, but the field runs downhill to this pigeon gathering, and I think the pigeons have figured out that this area is more fertile for them and produces something we can't neccesarily see...
I think what we can't see is the minerals that have been washed steadily into this pond shape. Rainwater steadfastly making its way past rocks, stones and pebbles may gradually produce a puddle of, for example, water that's richer in calcium (Ca). When the water evaporates, these minerals are still available in the soil and this could be producing more invertebrates, all in this pond shape.
It's just a theory, though, and it gave me a good excuse to share this interesting photo with you.
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