Abstract: Long-tailed manakins (Chiroxiphia linearis) have a puzzling social system in which teams of two males display cooperatively in
dispersed lek arenas, but only the alpha partner mates with visiting females. One benefit of performing as a nonmating partner
might be to gain experience as an ��apprentice�� to improve the performance of the complex duet song and joint dance. We
examined the relationship between the age of singers and two measures of singing performance: song variability and sound
frequency matching. Singing performance improved with age; variability in four song characteristics of males less than 3 years
old was greater than that in their older partners, and frequency matching increased with the age of the younger partner.
Randomization tests of song samples from seven well-established teams showed that males did not track the song-to-song variation
in their partners� singing. Another randomization test showed that frequency matching by these teams was higher than that of
randomly paired partners. We considered three alternative hypotheses for the congruent songs: (1) short-term accommodation
to the partner�s song; (2) active choice of partners with similar intrinsic frequencies; and (3) long-term development of congruent
song through either practice or song copying. Our results and evidence from long-term monitoring of banded birds
best support the hypothesis that frequency matching develops over several years during the complex and protracted process of
partner formation. Nonmating males may benefit from increasing their competence at display, eventually enjoying increased
mating success when they inherit display sites from older males. Key words: age, Chiroxiphia, cooperation, learning, lek, manakin,
Pipridae, sexual selection, song, suboscine, vocal development. [Behav Ecol 13:65?69 (2002)]