Plumage aberrations have been recorded in many bird species (Sage 1962). These can be restricted to single or multiple feathers, or it may concern the whole body, which then radically changes the appearance of an individual bird (e.g. Graham et al. 2005, Vittery 2005, Ciach & Ściborski 2008, Ciach 2009). Plumage aberrations in birds are rarely recorded, given the fact that they usually attract the observer's attention. In most birds of prey, records of abnormal coloration are exceptional (Ciach et al. 2011).

On 2 February 2012, a second calendar-year female Northern Goshawk Accipiter gentilis with atypical coloration collided with a building in Sędziszów, a town in southern Poland bordered by farmland and abandoned fields (50°34′N, 20°03′E). The plumage was beige (Figure 1). The upperwing coverts were a rather uniform beige except for the marginal coverts which were darker. Scapulars were uniform beige-white. The uppertail coverts and rump had a brownish tinge. The head was beige. Underparts were beige with a carrot-orange tinge, and a distinct brownish streaking on the breast. The underwing coverts, which were darker than the upperwing, showed a dark streaking pattern, and along with the undertail coverts were carrot-orange tinged. All primaries and secondaries were light coffeecoloured and had a brownish banding. The rectrices on the upper and under side had a distinct brown banding. As a result of the coloration, the bird appeared darker from below. Bare parts were unchanged: yellow legs with blackish claws, grey bill with blackish tip and yellow cere. Body measurements (see Svensson 1992) were: wing (maximum wing chord) — 350 mm; tail — 256 mm; bill length — 32.3 mm; and tarsus — 92 mm. The female was sexed by dissection. The specimen is preserved in the collection of the Natural History Museum at the University of Wrocław, Poland (register number 201588).

The described Northern Goshawk presents a brown aberration. It is defined as a qualitative reduction of eumelanin (phaeomelanin not affected). As a result of an incomplete oxidation of eumelanin, black feathers become dark brown (van Grouw 2006). Those feathers, being sensitive to sunlight, bleach quickly, which explains the contrast between under and upper parts. Our bird differs from several other aberrant Northern Goshawks mentioned in the literature. Aberrant offspring of Northern Goshawk with a genetic defect were reported in the city of Hamburg, where individuals with a pale silver-blue plumage displayed severe locomotor disorders (Rutz et al. 2004). Clark (1999) mentioned leucistic (reported as partial albino, which is impossible: a bird is an albino, or not) and dilute plumages of Northern Goshawk, not giving, however, any details. Brown mutation has been recorded recently in Common Buzzard Buteo buteo in Poland (Ciach et al. 2011).

Figure 1.

(A) Dorsal and (B) ventral view of a juvenile female Northern Goshawk with brown mutation (2 February 2012, Sędziszów, southern Poland).

Across Eurasia, the Northern Goshawk shows a tendency to become progressively paler from south to north, and from west to east. The dark grey plumage of European A. g. gentilis shifts into the paler forms of the eastern races A. g. buteoides and A. g. albidus (Cramp & Simmons 1980). The latter subspecies, occurring from north-east Siberia to Kamchatka Peninsula, is conspicuously pale (Kretchmar & Probst 2003). The nominate Northern Goshawk in aberrant plumage, as described in this note, may be confused with individuals of the Eastern Palearctic subspecies A. g. albidus.

We are grateful to Alojzy Przemyski, who provided the specimen, and to Ludwik Maksalon, who measured the specimen. We also thank two anonymous referees for comments on this note.