Patterns of morphometric variation in birds' tails: length, shape and variability

In Palaearctic birds, tail length was more variable than wing and tarsus, but not bill lengths. Tails of the ornamental shapes pin, lyre and graduated varied more in length than did non-ornamental shapes. Log coefficient of variation (C V) tail length showed an overall U-shaped relationship with longtailedness, but although the CV for most tail shapes increased in short-tailed species, only in ornamental shapes was C V also high in long-tailed species. C V of fork depth was lowest at a fork depth of 2, and considerably higher in shallow forked tails. CV streamer lengths were similar to CV deep fork depths. The more deeply forked tails thus seem ornamental. Phylogenetically independent contrasts confirmed in males, but not females, that long-tailed species had greater CV than medium-tailed species, and the greater CV of graduated than square tails, but the CV of short- and medium-tailed species did not differ. These comparisons, however, did not control for tail shape. The greater elongation and CV of tails with ornamental shapes are consistent with an influence of sexual/signal selection on these tails, and the increase in CV with longtailedness suggests that Weber's law applies to the perceptual threshold for tail length. Sexual selection may have a widespread, but moderate, influence on tail traits in birds.