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Morphometric measurements of birds are the first data to be really considered as biometric in this discipline. Baldwin et al. (1931) depicted and explained in detail the external measurements used in ornithology. Currently, many of these measurements have been forgotten or are rarely used both in books dedicated to bird taxonomy (Cramp & Simmons, 1977) and in field guides on different geographical areas or on large bird groups such as shorebirds, raptors, passerines, etc. (Svensson, 1992; Baker, 1993). Old biometric analyses used measurements performed on birds preserved in natural history museums. An appropriate representation of specimens is generally found in these museums, both in numbers (which allows for a large sample size) and in geographic origin (which enables the establishment of comparisons between birds of different areas) (Jenni & Winkler, 1989; Winker, 1993, 1996). Body mass was another one of the data used in the initial biometric analyses. Its objective was to determine the presence of daily or seasonal variations, or variations linked to other specific periods: breeding, rearing and migration. The next step was the establishment of a link between metric differences and the sex of birds. In some species, these differences were very visible and therefore statistical analyses were not required to support the distinction between males and females as in some raptors such as the Merlin Falco columbarius (Newton, 1979; Wiklund, 1990), owls and skuas (Andersson & Norberg, 1981). Similarly, marked biometric differences between bird populations of the same species found in different geographical areas were recorded (Svensson, 1992). This resulted in the identification of subspecies when these populations were geographically isolated, not sharing potential hybridization areas. Thus, for example, 10 subspecies of the Bluethroat Luscinia svecica have been identified throughout Europe, Asia and Alaska (Collar, 2005), a further 10 subspecies of Southern grey shrike Lanius meridionalis have been identified (Lefranc & Worfolk, 1997; Klassert et al., 2007), etc. Substantial databases were created as a result of the routine collection of a minimum number of measurements when a bird was captured, this information being used for specific purposes. Possibly, the existence of these data and the ability of observation lead researchers

Usefulness of Biometrics to Analyse Some Ecological Features of Birds