Birds' tails as signaling devices: markings, shape, length, and feather quality.

While exaggerated length and ornamental shapes are confirmed sexually selected tail traits in birds, the signal function of tail markings has received less study. Signal roles for tail markings as amplifiers of length, shape, and feather quality are discussed, and the role of tail markings as feather-quality handicaps is proposed: absence of melanin increases damage and abrasion. Predicted correlations of tail markings with other tail traits are derived for these signal roles. A comparative study of the relationships between these tail traits in an entire avifauna, the western Palearctic, tested the predictions. Tail displays were present in nearly 80% of species, associated with greater long-tailedness, but not all displayed tails had markings or ornamental shape. The incidence of marks across tail shapes and the combinations of marks indicate that tail markings act as handicaps or amplifying handicaps of tail feather quality. Tail elongation and ornamental shapes could act as additional handicaps of feather quality-that is, they could be multipurpose signals. Incorporation of revealing indicators such as feather damage and associated handicap and/or amplifying traits may allow a reduction in the cost of signaling while maintaining signal reliability and, hence, influence sexual selection in complex signaling systems.