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Many animals produce coordinated signals, but few are more striking than the elaborate male–female vocal duets produced by some tropical songbirds. Yet, little is known about the factors driving the extreme levels of vocal coordination between mated pairs in these taxa. We examined evolutionary patterns of duet coordination and their potential evolutionary drivers in Neotropical wrens (Troglodytidae), a songbird family well known for highly coordinated duets. Across 23 wren species, we show that the degree of coordination and precision with which pairs combine their songs into duets varies by species. This includes some species that alternate their song phrases with exceptional coordination to produce rapidly alternating duets that are highly consistent across renditions. These highly coordinated, consistent duets evolved independently in multiple wren species. Duet coordination and consistency are greatest in species with especially long breeding seasons, but neither duet coordination nor consistency are correlated with clutch size, conspecific abundance or vegetation density. These results suggest that tightly coordinated duets play an important role in mediating breeding behaviour, possibly by signalling commitment or coalition of the pair to mates and other conspecifics.

Figure S1: Wind drift propensity across distance to coastline and latitude at 19 weather surveillance radar stations during spring migration (1995-2018). Slope of α represents drift propensity; 0 is complete compensation for wind, 1 is complete drift with wind. Predictions of propensity of drift are shown from 0 to 325 km distance to coastline at 35°N, 40°N, and 45°N. The fitted lines and 95% confidence bands are from a least squares linear model with an interaction between latitude and distance to coastline.; Figure S2: Four sites showing overcompensation, a rare flight behavior. Spring (March 1st to June 15th, a-b) and fall (August 1st to November 15th, c-d) distributions of migrant track (pink), heading (blue), and wind direction (gray) at weather surveillance radar (WSR) stations summarized from 1995 to 2018 between sunset and sunrise. Black arrows denote preferred direction of movement and the gray arrows the average wind direction. PDM arrows’ lengths are scaled to average migrant groundspeed and wind arrows’ lengths are scaled to the average wind speed. Note, all distributions are scaled to the same size and number of observations but exhibit differing maxima. Rose diagrams are summarized in 5 degree sectors.

Three supplementary figures with legends

Figure S1, spatial and temporal distribution of mean wind profit. Figure S2, migrants' wind profits versus maximum and minimum wind profits. Table S1, radar antenna heights. Table S2, mean differences between migrant flight height and height of wind profit quantiles.