available statistics

Available Statistics

One aspect of Atlantic Yellow-legged Gulls which is reasonably well-established from the literature is their small size. Table 1 shows that all the forms of Atlantic Yellow-legged Gull are some 5-10% smaller in their measurements than Mediterranean Yellow-legged Gull. These are average measurements of course but it means that in general a male Atlantic Yellow-legged Gull will be the same size as a female Mediterranean Yellow-legged Gull and an Atlantic Yellow-legged Gull will be some 5-10% smaller than a Mediterranean Yellow-legged Gull of the same sex. Indeed, the Atlantic Yellow-legged Gulls are basically similar in size to graellsii or argenteus while Mediterranean Yellow-legged Gull are similar to argentatus. It can also be seen that the west Iberian females are quite small, being considerably smaller than Macronesian atlantis and even smaller than female graellsii on wingspan. The north-west Iberian (Galician) birds were directly compared to Mediterranean Yellow-legged Gull in the study of Carrera et al (1987) and found on average to be about 17% lighter in weight, 6% shorter in the wing, 4% shorter in the bill and 6% shorter in the tarsus. The biometrics of breeding populations suggest a fairly sharp division between the forms with Carrera et al (1987) also finding that the Mediterranean Yellow-legged Gulls breeding in the Balearic Islands, Spain, are very similar in size to those breeding on the Chafarinas Islands, near the Mediterranean coast of Morocco.

Table 2a shows the biometrics from various studies in Macronesia. These show generally lower figures than Cramp (1983). The measurements for the Azores and the Canaries are very similar. It is thought that Bannerman's figures will relate to the eastern Canaries where he was most active. Table 2b shows the biometrics from various studies in the western Mediterranean. These michahellis, thought to be the largest Mediterranean Yellow-legged Gulls, are typically 10% longer in wing, bill and tarsus than birds from the Atlantic Islands.

Recent population estimates, taken from Paterson (1997) unless otherwise stated, indicate a total of 98,000 pairs of Atlantic Yellow-legged Gulls with Azores 6,000, Madeira 4,000 (Skornik, 1997), Canaries 4,300, Morocco 4,400 including 2,500 in the south, south Portugal 400, south-west Portugal 22,000 (Moore, pers. comm.) and northern Spain 57,000 including Galicia 46,000 (Munilla, 1997), Asturias 2,900, Cantabria 5,000 and Basque Country 2,900. Cramp (1983) indicates a southern limit for regular breeding of Laayoune in Western Sahara with occasional breeding down to Mauritania. Where the Atlantic meets the Mediterranean, there are 1,500 pairs of Yellow-legged Gulls in Cádiz Bay, 3,000 in Gibraltar (Skornik, 1997) and 5,000 at the Chafarinas Islands. For all Atlantic Yellow-legged Gulls, incubation is typically in April and early May though Bannerman & Bannerman (1966) report that the Azores breeding season runs one month later (in May) than in the Canaries (Bannerman, 1963; Moreno, 1988). Some evidence for a later Azores breeding season is provided by Chavigny & Mayaud (1932) who collected eggs there from 1st-12th May. Incubation in north Spain also is reported as occurring mainly in the first half of May (Teyssèdre, 1983).

Table 1: Biometrics (in mm) of Herring L .a. argenteus, Yellow-legged and Lesser-black Backed Gulls L. f. graellsii.

Feature	argen-teus	Medi-terranean Yellow-legged Gull micha-hellis	Atlantic Yellow-legged Gull				graell-sii
			NW Iberia	W Iberia	NW Africa	Macro-nesia
Source	Cramp (1983)	Cramp (1983)	Carrera et al (1987)	Cramp (1983)	Urban et al (1986)	Cramp (1983)	Cramp (1983)
wing	404-425	438-460	419	401-431	420-439	423-437	406-427
tail	160-170	169-179	-	158-169	162-170	165-173	152-164
bill	48.9-53.2	51.5-56.6	54.8	49.5-53.6	49.4-55.7	49.1-51.5	47.7-52.1
tarsus	60.5-65.2	64.5-70.6	65.6	60.3-64.4	61.6-65.0	62.7-64.7	60.3-65.2
toe	56.7-61.6	59.2-65.4	-	55.2-59.3	-	58.7-60.3	52.5-56.3

Where a range is given, the first number refers to the average for females and the second to the average for males. A single number is an average over both sexes.

Table 2a: Further Biometrics (in mm) of Atlantic Islands Yellow-legged Gulls.

Feature	Atlantic Islands Yellow-legged Gull
	Macro-nesia	Azores	Azores	Canaries (East?)	Canaries - Tenerife	Azores
Source	Cramp (1983)	Dwight (1922)	Dwight (1925)	Banner-man (1963)	Volsøe (1951)	Vaurie (1965)
wing	423-437	401-423	402-421	401-418	417-440	418-438
tail	165-173	162-170	162-171	162-170	-	-
bill	49.1-51.5	50.3-55.1	50.2-54.3	46.5-54	-	-
tarsus	62.7-64.7	61.7-66.6	61.6-66.3	62.5-65.5	-	-
toe	58.7-60.3	52.3-56.0	52.2-55.2	-	-	-

Where a range is given, the first number refers to the average for females and the second to the average for males. A single number is an average over both sexes.

Table 2b: Further Biometrics (in mm) of Mediterranean Yellow-legged Gulls.

Feature	Mediterranean Yellow-legged Gull
	Medi-terranean	Camargue	Balearic Islands	Islas Charafinas
Source	Cramp (1983)	Isenmann (1973)	Carrera (1987)	Carrera (1987)
wing	438-460	440-465	447	457
tail	169-179	-	-	178
bill	51.5-56.6	56-61	57.4	56.6
tarsus	64.5-70.6	67-72	70.3	68.5
toe	59.2-65.4	-	-	-

Where a range is given, the first number refers to the average for females and the second to the average for males. A single number is an average over both sexes.