Assuming this to be the red-footed falcon (falco vespertinus), I found this photo. Leigh Fermor spots them around the remains of an impressive bridge built by Trajan.
‘It was the remains of Trajan’s amazing bridge that we had come to see, the greatest in the Roman Empire. Apollodorus of Damascus, who built it, was a Greek from Syria, and two great stumps of his conglomerate masonry still cumbered the Romanian side; a third stood across the water in a Serbian meadow. Swifts were skimming over the water and red-legged falcons hovered and dived all round these solitary survivors of twenty massive piers.’
Source: Patrick Leigh Fermor, Between the Woods and the Water (London: Penguin Books, 1986), p. 225