The Malagasy harrier (C. macrosceles) of Madagascar and the Comoro Islands was previously treated as a subspecies of this bird but is increasingly regarded as a separate species.

Today its diet includes many introduced mammals (rats, mice and tenrecs) but it originally fed mainly on birds and insects.

[3] It has been evaluated as endangered by BirdLife International and it is threatened by destruction and disturbance of its habitat and by poaching, deliberate persecution and accidental poisoning by rodenticides.

He named it in honour of Louis Maillard, a French botanist and engineer who mentioned the bird in a book about the island.

Depending on accepted mutation rates the two species diverged between 300 000 and 100 000 years ago, and Reunion Harriers evolved features enabling capture of birds and bats in the forested habitat.

A molecular phylogeny of the harriers (Circus, Accipitridae) indicate the role of long distance dispersal and migration in diversification.