likelihood arrival

Likelihood of Arrival in Britain

Atlantic Yellow-legged Gulls are not so well-established visitors to Britain as Mediterranean Yellow-legged Gull. It appears that the north Iberian population (Cantabrican Atlantic Yellow-legged Gull) is quite sedentary with only 1.7% of birds moving over 1,000km from Galicia, mainly to the south and no birds from the Cantabrican colonies moving further than 500km (Munilla, 1997). Bermejo et al (1984), from a census of wintering gulls in Spain, thought that the total number of all forms of Herring Gull (at that time including Yellow-legged Gull) found on the Atlantic coast of 102,160 corresponded well with recent estimates of the breeding population in the area, also suggesting a mainly sedentary population. However, one ringed in Galicia, Spain, as a pullus in July 1989, was found freshly dead on the Isle of Wight in February 1992 (I. Munilla, pers. comm.). It has been thought that the Dark Atlantic Yellow-legged Gulls of Macronesia are sedentary (Paterson, 1997) and indeed, in the current study, no Atlantic Yellow-legged Gulls have been seen following ships in the Canaries. However, a bird ringed as a pullus at Fuerteventura was seen at Porto, Portugal, on 1st April 2001 to confirm that some movement does take place (Hoogendoon, in prep).

In addition, Moore (1996) observed a total of five birds, all juveniles, moving east from the Azores and Madeira in August over eight seasons from 1987-94 ranging between 100 and 1,120 km. Identification of such birds is not easy. Moore argues on grounds of an indistinct primary window and a two-tone underside for these birds being atlantis rather than Lesser Black-backed Gulls. Both are good features for atlantis. Further supporting features that could have been mentioned are the heavy blunt-ended bill, the bulky chest and the broad wings shown in the photograph accompanying the article. It could be argued that the observation, of only five juveniles and no older birds over eight years, indicates that the birds of Macronesia are indeed mainly sedentary. Hamer et al (1994) found that the diet of birds in the Azores consisted mainly of mesopelagic fish which has been interpreted (Garner 1998) as suggesting that they have a wide-ranging pelagic style of existence. However, there is no evidence from Hamer's work that the birds actually leave the vicinity of the Azores.

A number of authors have suspected that atlantis wanders from Macronesia to the Atlantic coast in Morocco but this is not verifiable, in the absence of ringing, as there is a resident breeding population of Yellow-legged Gulls in south Morocco, which is quite similar to that in the Canaries, to confuse the picture. Overall, there is no compelling evidence that the Macronesian birds are great wanderers and their likelihood of being vagrants in, say, northern Europe and North America is not high.

However, the likelihood of occurrence does not appear to be zero. Wilds & Czaplak (1994) considered that a specimen of a Yellow-legged Gull found in 1973 in Quebec, Canada was of the atlantis form. The detailed description certainly indicates an Atlantic Yellow-legged Gull with its dark mantle colour lacking any brown tinge, the early moult, the large white mirror on P10, the absence of a mirror on P9 and the extensive black on the wing-tip. Interesting features of this bird are its heavy head streaking and the slightly reduced area of black on P7 and P8 compared to atlantis specimens taken mainly from the Azores. Such head streaking is a feature of birds from the Azores (Garner, 1998) but is also a feature of birds from Atlantic Iberia. The slightly reduced area of black may simply reflect variation (Wilds & Czaplak, 1994) but perhaps more strongly points to a Portuguese origin as birds from here can show reduced areas of black on the wing-tip. With evidence that Atlantic Yellow-legged Gulls occasionally reach North America, we should classify them as a potential vagrant to Britain.

Dark Atlantic Yellow-legged Gulls do clearly make limited movements southwards outside the breeding season to west Africa. In Gambia for instance, the Yellow-legged Gull is classified as:

‘rare visitor to the coast, most presumably being the Atlantic Is. race L. a. atlantis’ (Gore, 1990)

L. c. atlantis: ‘uncommon Palearctic migrant to the coast, recorded in all seasons with highest frequency of observations December – January. Possibly under-recorded. Recorded both to the south and to north in Senegal’ (Barlow, Wacher & Disley, 1997).

The descriptions given in the latter text do indicate that the visitors are Dark Atlantic Yellow-legged Gulls which could originate from either Macronesia or from the colonies on the mainland of Africa in Mauritania, Western Sahara and Morocco. It should be noted that the remark in this book that the head and neck show ‘little or no streaking in non-breeding plumage’ is not strictly correct. What appears to be meant is that in mid-winter Dark Atlantic Yellow-legged Gulls have completed their moult and show little or no head streaking in contrast to say graellsii. The accompanying plate shows in fact a Dark Atlantic Yellow-legged Gull with some head streaking as may be found in autumn in some populations. One adult at Kololi in Gambia on 12th August 1999 (N. Rossiter, pers. obs.) showed characteristics of Dark Atlantic Yellow-legged Gull.

From the point of view of Britain, perhaps the main period in which Atlantic Yellow-legged Gulls may be tempted north is in spring when vast numbers of Lesser Black-backed Gulls, with which they freely associate, return north from their wintering areas in north-west Africa, Iberia and Macronesia. It appears that Portugal, Spain and Morocco are particularly important wintering areas for Lesser Black-backed Gulls with BTO-ringed grand totals for recoveries up to 1996 (Toms & Clark, 1998) of Portugal 1,096, Spain 564 and Morocco 510. Much smaller numbers were found in the Canary Islands (15), north Atlantic Ocean (14) and Madeira (1). Further south in Africa 25 were found in Western Sahara, 18 in Mauritania and 7 in Senegambia. Some abmigration might be expected under these circumstances. Garner & Quinn (1997) report that a Yellow-legged Gull ringed at Berlengas in Portugal was recovered in Gloucester in May 1996. Close examination of descriptions of spring Yellow-legged Gulls in Britain needs to be performed to test this idea further.

Another possibility is that Atlantic populations adopt a northern post-moult movement. Hume (1978) noted Yellow-legged Gulls in a Midlands roost which arrived typically in November and stayed until February. The description given of narrow wings, rounded head, darker mantle colour than Common Gull L. canus, sharp contrast between wingtip and inner wing, completion of moult by November, thick shortish bill with blunt tip, large red gonydeal spot just extending onto upper mandible, extensive black on wingtip and one mirror only (P10) matches well Dark Atlantic Yellow-legged Gull. The long legs are associated more with those from the Mediterranean/Atlantic intersection zone but it is probably unsafe to be too precise about the origin. The author has also seen adult Yellow-legged Gulls in Northumberland in winter and early spring (January-March) which show many characteristics of Dark Atlantic Yellow-legged Gull.