Nocturnal vocal behavior of the diurnal Undulated Tinamou

Some diurnal birds vocalize at night but the reason for this has seldom been studied. We monitored the nocturnal vocal activity of the Undulated Tinamou Crypturellus undulatus from June 2015 to May 2016 at three recording stations in the Brazilian Pantanal. We tested whether an index of nocturnal vocal output (number of vocalizations automatically detected) of the species varied over the monitored annual cycle and whether it was related to night temperature and moon phase. The annual pattern of nocturnal vocal output was unimodal, with high vocal activity during September and October (49.9% of the total calls detected) and very low vocal output from January through June (8.5% of the total). The unimodal pattern found suggests that nocturnal vocal activity might be related to mate attraction rather than to territorial defence, for which a more constant pattern would have been expected. The percentage of the moon illuminated was positively associated with nocturnal vocal activity, probably related to the elevated light level on moonlit nights. Nocturnal vocal activity was not associated with minimum air temperature. Nocturnal vocal output was positively associated with vocal output during the preceding night, but not with vocal output during the preceding day, suggesting that it was not directly associated with diurnal calling. This is the first study showing a complete annual cycle of nocturnal vocalizations for a diurnal bird species. Our study provides new evidence regarding the function of and the reasons for nocturnal vocalizations in a diurnal non-passerine bird and has important implications for continued study of reproductive behaviour in tinamous. Further research should examine whether courtship display is associated with nocturnal vocalizations in this and other species.