Characteristics of Adults
The characteristic features are given in a top-down manner from all Atlantic Yellow-legged Gulls together, through other possible forms to differences at the regional level. Eight features common to most adult Atlantic Yellow-legged Gulls have been described. They have:
Features of all Atlantic forms
Looking at the earlier accounts and literature and the features in tables 2-4 (Appendix I), there is considerable similarity between the birds of Macronesia, the Atlantic coast of Morocco and southern Portugal. These forms are collectively termed Dark Atlantic Yellow-legged Gull as they are darker than Mediterranean michahellis. The form in north Iberia, known as Cantabrican Atlantic Yellow-legged Gull, is clearly different in many respects from that further south including its mantle shade which is typically paler than Mediterranean michahellis. Those in west Portugal and north-west Spain appear to be intermediate between the Dark Atlantic and Cantabrican forms and may be regarded as forming a cline between them. Those in north Morocco and Andaluica form an intersection between the Dark Atlantic and Mediterranean forms. A tentative classification is therefore as:
The classification rests on many factors as discussed below but, as the mantle colour is the most obvious distinction, a pictorial summary of the mantle colours found is shown in
Comparison of Mantle Shades. Assessing the colour of any object is far from simple as it will vary according to illumination, angle at which object is viewed, background and observer sensitivity (Chamberlin & Chamberlin, 1980, pp.46-49). Thus in very strong sunlight, many atlantis look silvery-grey and, in very subdued light, many cantabricans look as dark as graellsii. The observations here were made from photographs taken either with relatively subdued light at midday (overcast but not dull sky) or about two hours before sunset in direct sunlight. The colour determinations were made by comparing Photo CD on-screen images with Dulux colour cards based on the Natural Colour System (NCS). The comparisons show that the Atlantic Yellow-legged Gulls occupy a continuous spectrum between argenteus (south-west France) and the darkest atlantis. Compared to Mediterranean Yellow-legged Gull, the Cantabrican form is usually paler and the Dark Atlantic form darker. The latter has both blue-grey (for example, Canaries) and less frequently purplish-grey (for example, south Portugal) components. Perhaps the most important observation from Figure 2 is the clinal nature of the variation in mantle colour, indicating how difficult it is to specify precise boundaries between taxa based solely on this property.Other researchers have also found that atlantis is darker than Mediterranean michahellis. Jonsson (1998) quotes 6.0-7.0 (mean 6.3, eastern populations highest) on the Kodak Grey Scale, where higher numbers mean darker plumage, compared to 7.0-7.5 for atlantis. Bermejo (1999) quotes 5.5-6.0 for Mediterranean michahellis, 5.25-5.50 for Iberian Atlantic michahellis and 4.25-5.0 for atlantis on the Munsell Index where lower numbers mean a darker plumage. Bermejo (pers. comm.) also considered the birds in south Morocco and western Sahara to have very similar mantle shades to those of Atlantic Iberia. Dwight (1925) rated atlantis as deep neutral grey compared to light neutral grey (two shades paler) for Larus cachinnans and dark neutral grey (one shade darker) for graellsii.
Characteristics of adults of each form:
Dark Atlantic Yellow-legged Gull
Cantabrican Atlantic Yellow-legged Gull
West Portugal/North-west Spain cline between Dark Atlantic and Cantabrican
Atlantic-Mediterranean Intersection
Identification of Dark Atlantic adults from:
graellsii, argenteus, michahellis, cachinnans
Separation of Cantabrican Atlantic adults:
© Copyright Nick Rossiter 2000-2001.