available statistics june 2002

Available Statistics on Biometrics

One aspect of atlantis which is reasonably well-established from the literature is their small size. Tables 1 and 2 shows that all the forms of atlantis are some 5-10% smaller in their measurements than michahellis. These are average measurements of course but it means that in general a male atlantis will be the same size as a female michahellis and an atlantis will be some 5-10% smaller than a michahellis of the same sex. Indeed, the atlantis are basically similar in size to graellsii or argenteus while Mediterranean Yellow-legged Gull are similar to argentatus. The north-west Iberian (Galician) birds were directly compared to michahellis 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 michahellis breeding in the Balearic Islands, Spain, are very similar in size to those breeding on the Charafinas Islands, near the Mediterranean coast of Morocco.

Table 2a shows the biometrics from various further studies in Macronesia and Portugal. 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 of this form, are typically 10% longer in wing, bill and tarsus than birds from the Atlantic Islands.

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

Feature	argen-teus	micha-hellis	atlantis				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 2: Further Biometrics (in mm) of atlantis.

Feature		atlantis
	Macronesia		Azores	Azores	Canaries (East?)	Canaries (Tenerife)	Port-ugal	Azores
Source	Cramp (1983)		Dwight (1922)	Dwight (1925)	Banner-man (1963)	Volsøe (1951)	Van Swelm (2002)	Vaurie (1965)
wing	423-437		401-423	402-421	401-418	417-440	425.1	418-438
tail	165-173		162-170	162-171	162-170	-	169.7	-
bill	49.1-51.5		50.3-55.1	50.2-54.3	46.5-54	-	51.0	-
tarsus	62.7-64.7		61.7-66.6	61.6-66.3	62.5-65.5	-	61.7	-
toe	58.7-60.3		52.3-56.0	52.2-55.2	-	-	61.2	-

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 4: Further Biometrics (in mm) of michahellis

Feature	michahellis
		Medi-terranean	Camargue	Camargue	Balearic Islands	Islas Charafinas	Medi-terranean
Source		Cramp (1983)	Isenmann (1973)	Van Swelm (2002)	Carrera (1987)	Carrera (1987)	Faber et al (2001)
wing		438-460	440-465	420.5	447	457	454.6
tail		169-179	-	172.2	-	178	-
bill		51.5-56.6	56-61	55.5	57.4	56.6	57.0
tarsus		64.5-70.6	67-72	69.8	70.3	68.5	69.6
toe		59.2-65.4	-	66.5	-	-	-

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.

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