The most similar species is the Sooty Falcon which has a more rounded head, long wings extending past the tail and less yellow around the eye.
From reviewing the evidence, the bird was most probably the émerillon, with the pieds jaunes being either migrant falcons (the only species that might occur in the area, the Sooty Falcon, Eleonora's Falcon and the Peregrine Falcon, are larger than F. duboisi was, if not in overall length, then at least in wingspan) or the juveniles of the marsh harrier, which are distinctly colored and also have yellow feet.
It is shaped like a large Hobby or a small Eleonora's Falcon, with its long pointed wings, long tail and slim body.
Peregrine Falcon | Robert Falcon Scott | Falcon Crest | General Dynamics F-16 Fighting Falcon | Falcon | Rolls-Royce Falcon | Ford Falcon | Falcon 9 | Falcón | The Maltese Falcon (1941 film) | The Maltese Falcon | Bat Falcon | Sooty Tern | Millennium Falcon | Ford Falcon (Australia) | falcon | The Falcon | Sooty Shearwater | Sooty Falcon | Orange-breasted Falcon | Kevin Falcon | Eleonora's Falcon | Sooty Oystercatcher | Sooty Gull | Peregrine falcon | Ford XR Falcon GT's | Falcon Heights, Minnesota | Falcon 9 v1.1 | Dassault Falcon 900 | Brown Falcon |
Also there may be some superficial resemblance to Sooty Falcon and Grey Kestrel, but those two species both have yellow feet and cere.