Physical traits of male Japanese bush warblers (Cettia diphone) in summer and winter: hyperactive aspects of the vocal system and leg muscles in summer males.

The Japanese bush warbler has a very distinctive song, shows marked sexual size dimorphism, and has a polygynous mating system. However, the physical traits of males and seasonal variation in such traits have remained unknown. Twenty-five anatomical measurements representing physical traits of males in the breeding (summer, n = 5) and non-breeding (winter, n = 5) seasons were examined morphologically and compared statistically. Differences were evident between summer and winter (P < 0.05, t test) in the following seven items: body mass (19.8 ± 0.7 g vs. 15.6 ± 1.2 g [mean ± SD]), mass of male reproductive organs (184.0 ± 25.7 mg vs. 6.0 ± 1.4 mg), hind limb (3789.2 ± 346.2 mg vs. 3003.4 ± 226.8 mg), leg muscles (883.0 ± 63.5 mg vs. 581.4 ± 33.2 mg in either side), skin around the neck/throat (1280 ± 34.9 mg vs. 287.2 ± 84.7 mg), and syrinx (35.8 ± 2.39 mg vs. 25.0 ± 3.24 mg), and circumference of the neck/throat (52.1 ± 2.3 mm vs. 38.3 ± 2.6 mm). In contrast to winter males, summer males had thickened flabby skin prominently in the neck/throat area and an inflatable esophagus, perhaps a morphological basis for the throat sac as a vocal resonator. Also, the remarkable development of the flexor muscles of the legs of summer males suggests that perching and movement using the legs increases during the breeding season. These distinct characteristics of summer males may be related to the polygynous mating system of this species.