By Peter Adriaens
peter.adriaens AT dutchbirding.nl
Photographs by: Laurent Raty, Koen Verbanck, Alain Fossé, Roland François and Leo Janssen
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Plates 1-21 | Plates 22-46 | Acknowledgements
The gull colonies in Zeebrugge, Belgium, have expanded dramatically in recent years. Herring Gulls Larus argentatus argenteus first bred in this area in 1987 (two pairs), and numbers have increased ever since, reaching 1 409 pairs in 2001 and c 1 300 - 1 400 pairs in 2003. Numbers of Lesser Black-backed Gull Larus fuscus graellsii/intermedius are now even higher; the species first bred successfully in 1991 (two pairs), and is now well-established, with e.g. 2 820 pairs in 2001 and c 3 500 in 2003 (Stienen et al 2002; Wouter Courtens pers comm). Since 1996, a few Yellow-legged Gulls Larus michahellis have also colonized the harbour, pairing with either Herring or Lesser Black-backed Gull. It was not until 2002 that the first pure breeding pairs (2 in total) were seen (Vercruijsse et al 2002).
These large colonies also produce and attract a number of hybrids. At least 11 adult or near-adult hybrid large gulls have been observed and studied since 2000. In this article, 9 of them are presented. One was colour-ringed. Most were paired with either Herring or Lesser Black-backed Gull, and some successfully reared young. At the end of this article, a juvenile hybrid from Zeebrugge and another colour-ringed hybrid (photographed at Oostende) are also included.
A lot has been written about hybridization in the Herring -- Lesser Black-backed -- Yellow-legged Gull species complex (see extensive list of references at the end of this article), but many papers are in Dutch, German or French, and very little attention has gone to describing the characters of such hybrids. This article aims to illustrate the appearance of Belgian hybrid gulls.
Mixed breeding in western Europe
All three species may interbreed, as has been observed quite regularly in western Europe. The following are just examples, not an all-inclusive list.
Belgium There is very little information on interbreeding between Herring and Lesser Black-backed Gulls. One mixed pair reared three young in Zeebrugge in 1998 (Didier Vangeluwe pers comm). It is possible that interbreeding was more regular in the 80s and early 90s, when few pairs of Lesser Black-backed Gull were present in rapidly growing Herring Gull colonies; the following table compares the total number of pairs of each species in Belgium in a few years, starting with 1985 (the year Lesser Black-backed Gull was first recorded as a breeding bird):
Nowadays, mixed breeding with Yellow-legged Gull seems to be more regular. Interbreeding has been recorded yearly since 1996, both with Lesser Black-backed Gull and Herring Gull, but only in very small numbers (1 - 3 mixed pairs). In 1998, one pair Yellow-legged x Herring Gull successfully reared two young (Didier Vangeluwe pers comm). In 2002, one pair of Yellow-legged x Lesser Black-backed Gull and one of Yellow-legged x Herring Gull were found. In both pairs, the Yellow-legged Gull was a male (Vercruijsse et al 2002).
The Netherlands There is very little or no published information on interbreeding between Herring and Lesser Black-backed Gulls in recent years, though it does occur. On the other hand, the number of mixed pairs with Yellow-legged Gull is higher than in Belgium, with e.g. 15 - 25 Yellow-legged Gulls paired with either Lesser Black-backed or Herring Gull in the Port of Rotterdam every year since 1986 (Van Swelm 1998), and 7 mixed pairs at Ijmuiden in 2002, three of which succesfully produced young (Cottaar F. in press). Yellow-legged Gulls have been found breeding at Ijmuiden regularly since 1987, pairing almost exclusively with Lesser Black-backed Gulls. Other places in the Netherlands where Yellow-legged Gulls have been found interbreeding, include Neeltje Jans (Zeeland), Workumerwaard (Friesland), and Lelystad (Flevoland) (Van Swelm 1998).
In addition, hybrid offspring have also returned to the breeding colonies of Ijmuiden: one hybrid (Yellow-legged x Lesser Black-backed Gull) was paired with a Lesser Black-backed Gull in 2001, and produced two young (backcrosses). Another two hybrids were found in 2002, one of which reared two young (Cottaar F, in press). For more information on breeding of Yellow-legged Gulls in the Netherlands, see Van Swelm (1998).
Britain and Ireland (Peter Stewart pers comm) For the time being, the situation is different from the continent in that interbreeding is mostly found between Herring and Lesser Black-backed Gulls. A few mixed pairs have been recorded from e.g. Gloucester since 1993, when one pair raised two hybrid young. In 2003, there were 14 hybrid Herring x Lesser Black-backed Gulls reported from various locations in Britain, but no actual breeding pairs. For more information, see also Harris (1970), Hillis (1975), and Harris et al (1978).
Mixed breeding of Yellow-legged Gull and Lesser Black-backed Gull has been reported very recently though. The first record of a mixed pair was in Worcestershire in 1999; this pair produced two chicks. Interbreeding of Yellow-legged and Herring Gull is also said to occur (Olsen & Larsson 2003).
Furthermore, mixed breeding of Herring and Lesser Black-backed Gulls has been recorded from France and Denmark (Olsen & Larsson 2003), Yellow-legged x Herring Gull from Germany (Gloe 1993), and France (Yésou 1994), and Yellow-legged x Lesser Black-backed Gull from France (Lefèvre 1998), Germany (Olsen & Larsson 2003), and Spain (Basque Country; Rafael Saiz Elizondo in litt).
Characters of Belgian hybrids
First of all, I would like to emphasize that the parent species of these hybrids were usually unknown. Only in two, colour-ringed birds (Birds A and J) were the parents known, but the morphological characters of bird A did not entirely match those of the parent species, oddly enough (see below). Surprisingly, there was one consistent character in all nine Zeebrugge birds: the very broad, white scapular and tertial crescents. I am at a loss to explain where these come from; none of the possible parent species show them to this extent. Interestingly, a presumed hybrid Herring x Lesser Black-backed Gull from Britain (photo 139 on page 381 in Garner & Quinn 1997) shows the same obvious crescents.
Other characters were more variable. The upperparts were usually darker grey than in any michahellis Yellow-legged Gull, with a slight bluish tinge, but two birds were paler grey, like a pale Yellow-legged Gull. The head shape was normally more rounded than in that species. The legs were dull yellowish to almost bright yellow. The (pale) red gonys spot did not reach onto the upper mandible, except in one or two birds. The colour of the eyering was usually difficult to record in the field, but on two birds it looked reddish-orange. The primary pattern was usually characterized by a large amount of black on the outermost three to four primaries -- except for the rather large white mirror(s) on p10(-9) -- but comparatively little black on p6-5 (i.e. no more than in argenteus Herring Gull). The grey tongue on p10 was normally very short. In most birds, primary moult occurred rather late in the season (sometimes not before late June - early July!), as in Lesser Black-backed Gull. See photos on the next page for more details >>
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Plates 1-21 | Plates 22-46 | Acknowledgements