lusitanius Yellow-legged Gull

(last update: 14-12-2010 )

Coordinators:
Gabriel Martín
Antonio Gutierrez
Rui Caratão
Mars Muusse

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Below you find a copy of the article "Sex Differentiation of Yellow-legged Gull (Larus michahellis lusitanius): the Use of Biometrics, Bill Morphometrics and Wing Tip Coloration".

"we" in the text below refers to the original authors. The text has been added with most care, but if any errors occur, please let me know and mail to marsmuusseatgmaildotcom.

Sex Differentiation of Yellow-legged Gull (Larus michahellis lusitanius): the Use of Biometrics, Bill Morphometrics and Wing Tip Coloration

JUAN ARIZAGA, ASIER ALDALUR, ALFREDO HERRERO & DAVID GALICIA

IN: Waterbirds 31(2): 211-219, 2008.

A number of biological processes in birds, such as diet and foraging (Holmes 1986; Durell et al. 1993; Clarke et al. 1998), parental care (Pierotti 1981) or migration (Swanson et al. 1999; Rubolini et al. 2004; Cristol et al. 1999), differs between the sexes. Accordingly, sex-identification is basic to understanding adequately all these processes which, overall, give us key clues about the life history of species. The Yellow-legged Gull
(Larus michahellis) is a circum-Mediterranean gull, breeding from Iberia to the Black sea (Olsen and Larsson 2004). In Iberia two subspecies currently breed (Bermejo and Mouriño 2003; Olsen and Larsson 2004): L. m. michahellis, occurring along Mediterranean coast, up to central Portugal in W Iberia, and L. m. lusitanius, in Atlantic coasts from northwest Iberia, up to south central Portugal (Pons et al. 2004). L. m. atlantis, present in Macaronesia and the northwest coasts of Africa, do not breed in Iberia (Bermejo and Mouriño 2003; Pons et al. 2004).
Among the large gulls (largest of Larus spp.), sexes differ in their size with males being larger (Ingolfsson 1969; Coulson et al. 1983; Bosch 1996), and there are a number of studies dealing with discriminating methodologies used to distinguish between sex classes. Discriminating functions vary not only between species, but often also among populations (Evans et al. 1993). A number of studies have focused on biometrics of a number of Mediterranean Yellow-legged Gull populations (Carrera et al. 1987; Bosch 1996) in relation to sex, whereas studies on the Cantabrian Yellow-legged Gull are scarce and analyses on sex-differences are virtually lacking.
In addition to biometrics, the wing-tip coloration patterns in several gull species have been described to vary not only between species, but among age and sex classes and populations (Coulson et al. 1982; Allaine
and Lebreton 1990; Saks and Rattise 2006). Although in a number of large gulls these patterns are thought to be independent of sex (Mierauskas et al. 1991; Snell 1991), we have no data on the Cantabrian Yellow-legged Gull populations. Bill size in gulls is highly dimorphic between sex classes (Ingolfsson 1969; Coulson et al. 1983; Bosch 1996). However, it is virtually unknown if this dimorphism is also observed in relation to bill morphology (shape), as it is observed in other seabird species (Kaliontzopoulou et al. 2006). Our aim was to obtain useful criteria to distinguish sex in a population of Yellow-legged Gull L. m. lusitanius in the eastern Bay of Biscay, in relation to
(1) classical biometric variables,
(2) wing-tip patterns of coloration (black and white areas at the wing tip) and
(3) bill morphology.

MATERIALS AND METHODS

Sampling Area and Data Collection

Data on 155 dead adults (EURING code A, birds with more than five years) of Yellow-legged Gull collected in a dump in Zarauz (43°17'N, 02°10'W, N Spain), in the eastern Bay of Biscay, were used as a part of a government culling program. After labelling the specimens, they were kept frozen (see for a similar method Bosch 1996) and, before taking measurements, they were thawed. Only data on adults collected from early April to early July were used to guarantee that measured gulls were local. Thus, birds from other non-local populations, such as the Mediterranean Yellow-legged Gull (L. m. michahellis), a relatively common winter visitant in the Bay of Biscay (Yesou 1985; Martínez-Abraín et al. 2002) were avoided.
Within each gull, measures were taken of
(1) 25 feathers and skeletal-associated measurements (Table 1, Fig. 1),
(2) 14 records associated with the size and features of white and black areas at the tip of the outermost
primaries (Table 2, Fig. 2; feather areas were taken with a mesh to 1.0 mm2 accuracy, and lengths were recorded with a digital calliper to ± 0.5 mm) and
(3) bill morphology, for which seven landmarks were established (Fig. 1). Within each landmark, two variables were recorded: the x and y coordinates, so 14 records related to bill morphology were obtained. In the first two sets of measurements, data on disarranged or growing feathers
were omitted. All measurements were recorded by a single author (AA). Most of the measurements had a low
proportion of fleshy tissue, so shrinkage should be minimal after freezing (Bosch 1996).