Identification pitfalls and assessment problems*

Transcription

1 [Brit. Birds 78: , February 1985] 97 Identification pitfalls and assessment problems* 8 Purple Heron Ardeapurpurea The Purple Heron is seldom seen away from large reed-beds and marshland vegetation where it spends much of the day concealed. It is, therefore, most frequently seen in flight, and then usually soon after dawn and shortly before dusk. As most species can appear dark-plumaged at these times, it is important that, without great emphasis on colour, a flying Purple Heron be clearly distinguished from possible confusion species: Grey Heron Ardea cinerea and Bittern Botaurus stellaris. 38. Juvenile Grey Heron Ardea cinerea, Kent, July 1981 (R.J. Chandler) This paper, like those earlier in the series (Brit. Birds 76: 26-28, 78-80, , , , ; 77: ), is a publication of the Rarities Committee, which is sponsored by Zeiss West Germany. continued..

2 98 Identification of Purple Heron Compared with a Grey Heron, it is smaller, obviously shorter- and slimmer-winged, narrower-bodied, smaller-headed and both longer- and thinner-necked. As with Grey Heron, the neck is coiled in flight, but, unlike that species, its more prominent bulge protrudes well below the line of the breast and belly, and the base of the neck meets the breast at a sharp angle. Accentuating the snaky appearance of the head and neck, the bill is very slender, proportionately longer than that of Grey Heron, and tapers evenly to a sharp tip. Although Purple Heron is shorter-legged than Grey Heron, it is also shorter-tailed and possesses much larger feet due to its extremely long toes. In flight these features of Purple Heron combine to produce a greater projection of the legs beyond the tail than on the other species. Compared with Bittern, Purple Heron is larger, longer- and slimmerwinged, narrower-bodied, proportionately smaller-headed and both longer- and thinner-necked. In flight, Bittern, like Grey Heron, lacks the angled, downwards-protruding, retracted neck of Purple Heron. A Bittern's bill is considerably shorter and proportionately thicker than that of Purple Heron, and its legs, tail and toes are shorter than those of the other species, combining to give Bittern less projection of the legs beyond the tail in flight. The ageing terminology used here for Purple and Grey Heron follows BWP I: the nestling's down is followed by juvenile plumage, immature plumage being gradually acquired during the first autumn and winter and sub-adult plumage being acquired during the second autumn. A Purple Heron can reach adult plumage during its third autumn, whilst a Grey Heron can attain adult plumage during its fourth autumn, though individuals of both species retain traces of sub-adult plumage for longer. The plumages of all three species are well described and illustrated in several books, including BWP I and The Popular Handbook of British Birds. In all three species, the sexes are alike, and there is little seasonal variation. The plumage and bare-part coloration differences between juveniles and adults are minimal in Bittern and fairly subtle in Grey Heron, but marked 39. Adult Purple Heron Ardea purpurea, Kent, April 1981 (David M. Cottridge)

3 Identification of Purple Heron First-winter Purple Heron Ardeapurpurea, Netherlands, December 1973 (P. Mumterman) in Purple Heron. Sub-adult and adult Purple Herons are basically dark grey and black, thus most confusable with a Grey Heron; and juvenile and immature Purple Herons are basically brown, thus most confusable with a Bittern. Given good views, however, all three species are readily identifiable on colour alone. The following adult Purple Heron plumage characters separate that species from adult Grey Heron, whose equivalent characters follow in brackets: crown dark (white); neck chestnut, striped with black (greyishwhite streaked with black); wing-coverts dark slate-grey, showing comparatively little contrast with the primaries and secondaries (pale powder grey, contrasting with black); area at base of primaries rich buff (white); underwing very dark (blue-grey); centre of breast and belly dark vinaceous (white); and undertail-coverts black (white). All these features, except the crown and, to a lesser extent, the wing-coverts, can also be used to separate adult Purple Heron from juvenile and immature Grey Heron. The crown, wing-coverts, primary-base and underwing points can be used to separate

4 100 Identification of Purple Heron 41. Adult Purple Heron Ardea purpurea, France, July 1975 (Pierre Petit) 42. First-winter Grey Heron Ardea cinerea, Kent, January 1983 {R.J. Chandler) sub-adult Purple Heron from adult Grey Heron, and the latter two features can be used to separate sub-adult Purple Heron from juvenile and immature Grey Heron. The following juvenile and immature Purple Heron plumage characters separate that species from Bittern, whose equivalent characters follow in brackets: bill yellow (yellow-green); mantle and scapulars brown, edged with tawny, but appearing fairly uniform (black boldly edged with buff); back, rump and uppertail-coverts dark grey (buff with dark spotting);

5 Identification of Purple Heron First-winter Grey Heron Ardea cinerea, Kent, January 1983 (R.J. Chandler) 44. Adult Purple Heron Ardea purpurea, Mallorca, April 1979 (Brian Thomas) 45. First-winter Purple Heron Ardea purpurea, Romania, September 1979 (Jan Sevcik)

6 102 Identification of Purple Heron 46. Bittern Botaurus stellaris, Cornwall, September 1981 (S. C. Hutchings) primaries and secondaries uniform blackish (blackish barred across with buff); and legs and feet yellow (yellow-green). The position is unfortunately complicated by the occasional occurrence of apparently melanistic Grey Herons which superficially resemble juvenile Purple Herons. Three instances have been published (Brit. Birds 70: 76, ; 71: 416) and unpublished records include one photographed by R. F. Porter and another observed by D.J. Britton (in lift.). There is also a published account thought to refer to a hybrid Purple X Grey Heron (Bull. Brit. Orn : 1-4). Structurally, melanistic Grey Herons are invariably of normal size for the species, and thus larger than Purple Herons, though this may not be helpful if no other herons are available for comparison. In flight, they lack the large feet and bulging neck of Purple Herons. Some, however, exhibit minor structural differences from normal Grey Herons, perhaps due to poor feather condition associated with the apparent melanism. For example, the neck may be thinner and the flight heavier, on more bowed wings. Their plumage is variable, but characteristically is a dirty or oily brown, more uniform than on any age of Purple Heron, and lacking darker streaks on the head and neck. Bare-part colour is not exceptional for the species, but some individuals curiously combine the bright yellow bill of an adult, though with black upper edge and tip, with the complete, neatly demarcated dark cap of a juvenile. Such individuals may strongly recall Purple Heron, especially if the neck is sleek. Melanistic Grey Herons are undoubtedly rarer in Britain than are Purple Herons, except perhaps in winter. Thus, the problem needs to be kept in perspective, but a poorly described brown heron with little supporting structural evidence, though most likely to be a Purple Heron, is perhaps best regarded as unproven. A total of 423 Purple Herons has been accepted as having occurred in Britain and Ireland (up to the end of 1982), and the species is currently occurring at the rate of about 20 a year. Almost all have appeared during April to October, with a peak from late April to late May, and the great majority have been seen in England and Wales, mostly in the coastal counties from the Isles of Scilly to Norfolk. I am grateful to D. J. Britton for supplying the paragraphs concerning melanistic Grey Herons. PETER LANSDOWN 197 Springwood, Llanedeym, Cardiff CF26UG