Chestnut-flanked sparrowhawk has blackish grey upperparts with a very distinctive pattern on the underparts distinctive; the breast and belly are heavily barred grey and brown, with chestnut colored flanks. The throat is white and the head is rather broad compared to similar species. The cere is yellow as is the thin eyering which surrounds the red eye. Females and juveniles are browner. They sit 25 cm (9.8 in) tall and have a wingspan of 60 cm (24 in).
The chestnut-flanked sparrowhawk occurs in west central Africa from southern Nigeria through Cameroon and Gabon to Democratic Republic of Congo. It is said to occur in the Upper Guinean forests west of Nigeria but this has not been confirmed.
The chestnut-flanked sparrowhawk is found mainly in lowland tropical rainforest, mainly in the middle storey but it can adapt to dense secondary growth and will approach habitation in the forest. It is found up to 750 m (2,460 ft) above sea level.
The habits of the chestnut-flanked sparrowhawk are poorly known but it is known to lay eggs during January to April in Gabon. It probably feeds mainly on birds but has been recorded catching fruit bats. As it is rarely seen in the open it is presumed to be a still hunter which sits in the cover of foliage and sallies out to catch prey. Has been known to enter houses after poultry and to follow driver ant columns to ambush the attendant small birds.
Closely related to the African goshawk Accipiter tachiro and the red-chested goshawk Accipiter toussenelii, albeit smaller than either of those species. It is normally regarded as monotypic, although some authorities recognise the smaller birds in the Congo Basin as the subspecies Accipiter castanilius beniensis.
- BirdLife International (2012). "Accipiter castanilius". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Kemp, Alan; Kemp, Meg (1998). SASOL Birds of Prey of Africa and its Islands. New Holland. pp. 174–175. ISBN 1 85974 100 2.
- Ferguson-Lees, James; Christie, David A. (2001). Raptors of the World. Christopher Helm. pp. 523–524. ISBN 0-7136-8026-1.
- "Chestnut-flanked Sparrowhawk (Accipiter castanilius)". Handbook of the Birds of the World Alive. Lynx Edicions. Retrieved 31 October 2016.