We describe ten phenotypic characteristics and moult commencement in an inland colony of Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in The Netherlands. All features were checked for difference between sexes and, when possible, known-aged birds were described. Both taxa exhibited wide variation in single characteristics, most apparent in the range of upperpart grey tone, iris speckling and the pattern on P10 and P9. Herring and Lesser Black-backed Gulls in this colony showed considerable overlap with birds from Scandinavia and from Britain, which justifies the name ‘Dutch intergrade’, sometimes used for breeding Lesser Black-backed Gulls in The Netherlands.

Gulls with a large geographic distribution commonly show variation in plumage or biometrics across their range, which is often the basis for delineating subspecies. In the western Palaearctic, the Larus argentatus-fuscus complex has been the subject of many debates over species and subspecies boundaries. Based on recent molecular work, the most distinct populations are now classified as full species (Yésou 2002; Liebers et al. 2004; Collinson et al. 2008).

Full species status has been given to a number of closely related taxa, and against a background of identification problems and possible further taxonomic revisions, interest in inter- and intra- taxon variation has increased. Birds breeding in overlap zones have become a specific focus of interest as they may show intermediate features. Phenotypic characteristics (upperpart grey tone, primary patterns, biometrics, and soft part colour) of Herring and Lesser Black-backed Gulls have been compared across different European locations (Voous 1959; Barth 1968, 1975), and more locally on the German Friesian Islands (Goethe 1961; Noeske 2008). Mierauskas et al. (1991) compared yellow-legged Herring Gulls from the eastern Baltic with Caspian Gulls L. cachinnans from southern Ukraine using wingtip pattern, vocalisations and bill dimensions, while Buzun (2002) compared West Siberian Gulls L. heuglini with other taxa using primary patterns, wing length, iris colour, orbital ring colour and primary moult. Timing of moult in Herring and Lesser Black-backed Gulls has been described using different approaches: various European locations (Stresemann & Stresemann 1966), western England (Verbeek 1977; Stewart 2006) and Finland (Hario 1984). Herring Gulls L. a. argenteus and Lesser Black-backed Gulls L. f. graellsii breeding in France, Belgium and The Netherlands show upperpart grey tone and biometrics which are intermediate between the British L. a. argenteus and L. f. graellsii and Scandinavian subspecies L. a. argentatus and L. f. intermedius (Barth 1968). Upperpart grey tone is the prime subject in most studies; nothing has been published to date on the moult pattern or detailed phenotypic traits of these intermediate birds.

The industrial complex Moerdijk in The Netherlands holds a mixed colony of Herring and Lesser Black-backed Gulls. Here we describe phenotypic variation and commencement of primary moult in both species breeding there, and discuss the implications of the variability found for field identification of their various subspecies.

This paper is a contribution of the Gull Research Organisation. We thank Tetra Pak Moerdijk and Shell Moerdijk for granting access into the colonies. Many thanks for a helping hand in ringing to: Stef Waasdorp, Job ten Horn, Jean-Pierre Leys, Fred Cottaar, Harry van Vugt, Jan Benoist, Johan Siemons, Camille Duponcheel, Corne Balemans, Patrick Balemans, Greetje Hiemstra, Charlotte Sanders, Suzanne Halters, Esther van Loenen, Senne van Eekelen and students of Prinsentuin College Breda. Thanks to the ringing groups: Eric Stienen & Norman van Swelm. Special thanks to Mr. J. A. de Vries (Zwaagwesteinde) and Prinsentuin College (Breda) for financial support of our research. An earlier draft of this paper was commented on by Visa Rauste, Martin Reid, Christiaan Both and Peter Adriaens. We are very grateful for their suggestions.

Adriaens, P. & Mactavish B. 2004. Identification of adult American Herring Gull. Dutch Birding 26: 151–179.

Allainé, D. & Lebreton, J.-D. 1990. The influence of age and sex on wingtip pattern in adult Black-headed Gulls Larus ridibundus. Ibis 132: 560–567. [Crossref]

Barth, E. K. 1968. The circumpolar systematics of Larus argentatus and Larus fuscus with special reference to the Norwegian populations. Nytt Magasin for Zoologi 15 Suppl. 1: 1–50.

Barth, E. K. 1975. Taxonomy of Larus argentatus and Larus fuscus in north-western Europe. Ornis Scandinavica 6: 49–63. [Crossref]

Buzun, V. A. 2002. Descriptive update on gull taxonomy: ‘West Siberian Gull’. British Birds 95: 216–232.

Chabrzyk, G. & Coulson, J. C. 1976. Survival and recruitment in the Herring Gull Larus argentatus. Journal of Animal Ecology 45: 187–203. [Crossref]

Collinson, J. M., Parkin, D. T., Knox, A. G., Sangster, G. & Svensson, L. 2008. Species boundaries in the Herring and Lesser Black-backed Gull complex. British Birds 101: 340–363.

Drost, R., Focke, E. & Freytag, G. 1961. Entwicklung und Aufbau einer Population der Silbermöwe, Larus argentatus argentatus. Journal für Ornithologie 102: 404–429. [Crossref]

Goethe, F. 1961. Zur Taxonomie der Silbermöwe (Larus argentatus) im südlichen deutschen. Nordseegebiet. Die Vogelwarte 21: 1–24.

Hario, M. 1984. Onset and pattern of primary moult in the Lesser Black-backed Gull Larus f. fuscus – a comparison with the Herring Gull L. argentatus. Ornis Fennica 61: 19–23.

Howell, S. N. G. & Dunn, J. L. 2007. Gulls of the Americas. Houghton Mifflin Harcourt, Boston & New York.

Jonsson, L. 1998a. Yellow-legged Gulls and yellow-legged Herring Gulls in the Baltic. Alula 3: 74–100.

Jonsson, L. 1998b. Baltic Lesser Black-backed Gull Larus fuscus fuscus - moult, ageing and identification. Birding World 11: 295–317.

Liebers, D., de Knijff, P. & Helbig, A. J. 2004. The Herring Gull complex is not a ring species. Proceedings of the Royal Society of London B. 271, 893–901. [Crossref]

Malling Olsen, K. & Larsson, H. 2004. Gulls of Europe, Asia and North America. Christopher Helm, London.

Mierauskas, P., Greimas, E. & Buzun, V. 1991. A comparison of morphometrics, wingtip pattern and vocalizations between Yellow-legged Herring Gulls (Larus argentatus) from Eastern Baltic and Larus cachinnans. Acta Ornithologica Lituania 4: 3–26.

Monaghan, P. & Duncan, N. 1979. Plumage variation in known-age Herring Gulls. British Birds 72: 100–103.

Noeske, A. 2008. Mantelfärbung und taxonomische Stellung der Herringmöwen Larus fuscus auf Amrum. Vogelwelt 129: 379–394.

Sibly, R .M. & McCleery, R. H. 1983. Increase in weight of Herring Gulls while feeding. Journal of Animal Ecology 52: 35–50. [Crossref]

Stewart, P. 2006. The primary moult of the Lesser Black-backed Gull L. fuscus. Special Publication No 1, The Severn Estuary Gull Group. Available online http://www.gull-research.org/indexelements/LBBGUMoult.pdf.

Stresemann, E. & Stresemann V. 1966. Die Mauser der Vögel. Journal für Ornithologie 107: 224–232.

Teixeira, R.M. 1979. Atlas van de Nederlandse broedvogels. Natuurmonumenten, ‘s Graveland. Verbeek, N. A. M. 1977. Timing of primary moult in adult Herring Gulls and Lesser Black-backed Gulls. Journal für Ornithologie 118: 87–92. [Crossref]

Vercruijsse, H. J. P. 1999. Zilvermeeuwen uit de duinen van Schouwen. Alterra publication IBN-DLO, Wageningen.

Voous, K. H. 1959. Geographical variation of the Herring Gull Larus argentatus in Europe and North America. Ardea 47: 176–187.

Yésou, P. 2002. Systematics of Larus argentatus-cachinnans-fuscus complex revisited. Dutch Birding 24: 271–298.