De herkenning van duikers Gaviidae in de hand

Red-throated Divers are the most numerous diver species in Dutch coastal waters, while c. 12% of the wintering population may consist of Black-throated Divers (beached bird survey results, experienced observers only; n = 108). Winter plumage divers are often mis-identified, even in the hand. Most fieldguides are remarkably poor in quality regarding this group of birds and are useless when the birds are actually held in the hand. A notable exception is the field guide of Lars Jonsson, published in the early 1990s, which should be consulted for its plates. With this manual, the identification of divers during beached bird surveys should be both more easy and accurate. There is considerable overlap in size between the four species, and biometrics should therefore only be used as ’additional’ information unless clearly stated in this manual. Four important measurements of the bill are described (figure 1); feathers to tip (A-C), nostril to tip (B-C), nostril (x-z) and nostril to feathers (x-y). Important differences between Red- and Black-throated Divers are a white area in the secondaries of the latter (all plumages, figure 6), regular white spotting of the upperparts in the former (winter- and juvenile plumage), dark upper parts with light edges on the feathers in juveniles or with some remains of summer plumage in adults in the latter. Old corpses of winter or juvenile plumage (small) divers should be examined on the rump (never white specks in Black-throated Diver, always white specks in Red-throated Diver). If feathers are dirty and wet, a sample should be collected for examination at home. The main difference between Black-throated and Great Northern Diver (all plumages) are bill length (table 2) and the presence or absence of a dark streak over the greater underwing coverts (figure 8). The rump may show remains (white specks) of summer plumage in Great Northern Divers, but not in Black-throated Divers. The pattern of the feathers of the upperparts in both adults and immatures are quite different (figures 5 and 7), and Great Northern Divers are obviously a lot larger in overall size (be aware of exceptionally large Black-throated Divers). Important differences between Great Northern and White-billed Divers are the number of tail feathers (20 in Great Northern, 18 in White-billed), the gonys (smooth in Great Northern, distinct bump in White billed; move the tip of your finger from the throat-feathers towards the tip of the bill and a small bump at the gonys cannot be missed), the amount of feathers on the bill (figure 1): (a xy)/(A xz) < 0.5 in Great Northern Diver, (a xy)/(A xz) >0.5 in White-billed Diver. The colour of the bill is quite unreliable, although a summer plumage individual with a huge, banana shape and colour bill is obviously a White-billed Diver.