Known from subfossil bones and the writings of Dubois published in 1674, this bird was larger than its relative F. punctatus on Mauritius, being about the size of a common kestrel, or around 35 cm from head to tail, with males being noticeably smaller than females.

This trait, while present in most birds of prey, is most pronounced in the larger, bird-eating species and reduces between-sex competition by niche differentiation.

It can be assumed that the bird was of the same generally brownish coloration as its closest relatives, with a lighter underside and darker spots or stipples, the tail, brown or more probably grey, being banded and tipped black.

Dubois mentioned three kinds of birds of prey extant on Réunion in the early 1670s: in order of decreasing size, papangues (the Malagasy harrier, Circus macrosceles, which still exists), pieds jaunes ("yellow-feet") and émerillons (a term for small falcons like the merlin, Falco columbarius).

Certainly, they were considered a pest as they fed on poultry, but hunting is unlikely to have been able to reduce their population much at such an early time, as evidenced by the continuous survival of the marsh harrier, which was heavily persecuted for centuries for the same reason.