Burning the candle at both ends: two sympatric thrushes are equally detectable and sing at similar rates at dawn and dusk

Many birds sing prolifically during the dawn chorus and a subset of these species also sing at dusk. How vocal activity varies between these two periods has not been examined in many species. In this study, we explored rates of detection and song rate of two sympatric passerines, Hermit (Catharus guttatus) and Swainson’s (Catharus ustulatus) thrushes, at dawn and dusk. At three intervals in the breeding season, we examined eight 1-min intervals from 60-min before to 60-min after both sunrise and sunset. We recorded whether each species was detected, the total number of songs sung, and the number of individuals singing. We found that both Hermit and Swainson’s thrushes were equally likely to be detected at all time periods except that Hermit Thrush singing was reduced and Swainson’s Thrush song was absent 60-min after sunset. We also detected both species singing less often earlier in the season. We found that song rate was not strongly related to either time of day or time of season, but the song rate/individual detected was higher for Hermit Thrush when more individuals were singing. Our results suggest that the dusk chorus ceases at a higher light level than the dawn chorus starts and that neighbors’ singing may increase the song rate. Our study adds to our general understanding of the daily and seasonal singing phenology of Hermit and Swainson’s thrushes and suggests breeding bird surveys for these two species could occur in both morning and evening with equal likelihood of detection.