VOOUS KH (1961) A diver-hybrid or the occurrence at Gavia arctica viridigularis in Belgium and the Netherlands. LIMOSA 34 (3): 185-188.

Attention is drawn to the only record of an inter-specific hybrid of the diver family in ANNIE P. GRAY'S check-list of bird hybrids (1958) which refers to a paper by VAN HAVRE (1931). It is an alleged hybrid of the combination Gavia immer and G. arctica, ;?;, 27 november 1927, Heerle-Minderhout, Belgium. The careful description and the illustrations show a mounted specimen which do not differ from the Black-throated Diver (aretica) unless for its large size: wing 316, bill (from anterior edge of nostril to tip) 51, middle toe (without claw) 92 mm. However, its measurements are surpassed by those of two winter specimens from the Netherlands, recently described as belonging to the Siberian form G arctica viridigularis: (I) ;?;, 12 march 1937, Den Helder. Wing, in quill moult, bill 55.5, middle toe 97.5 mm (Voous 1955). (2) ;?;, 14 february 1940, Winsum. Wing 340, bill 49, middle toe 87 mm (HEKSTRA & Voous 1961). Similar large-sized Black-throated Divers have been reported from Japan (AUSTIN & KURODA 1953, Voous 1955). There is therefore no reason to consider VAN HAVRE'S specimen as an inter-specific hybrid. Instead, it could as well be treated as the first known specimen of the race G, a. viridigularis in Belgium. However, from the breeding time in central Sweden a Black-throated Diver has been reported which in the length of its wing equals the largest known specimen of the race viridigularis (EDBERG 1957): ;?;, 3 may 1956, Laxarby, Dalsland; wing 356, bill 52 mm. The publication shows two males in full breeding plumage, a large one and a small one, both of which have according to the author and taxidermist (EDBERG in litt.) a bluish-purple gloss on the chin and throat. Hence, in spite of its large dimensions, but in accordance with its presence in Sweden in the breeding time, the giant specimen cannot be referred to the green-throated form viridigularis. At the same time this specimen defeats the use of large dimensions as a criterium for the subspecific identification of winter specimens of Black-throated Diver as viridigularis. According to EDBERG (in litt.) similarly large specimens of this species have been more frequently found on Swedish lakes than very small ones. This situation in itself implies a new problem in the pattern of the individual variation of the Black-throated Diver in general. Therefore, Gavia arctica viridigularis can no longer be maintained to have occurred in the Netherlands or Belgium. Gavia arctica viridigularis

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