Quiet in the nest:The nest environment attenuates song in a grassland songbird

The nest environment may limit the ability of nest-bound birds to hear sounds from the outside world.In vocal learning species,such as humans and songbirds,it is vital for young animals to hear the voices of conspecific animals early in life.In songbirds,nest structure varies considerably across species,and the resulting impact on sound transmission may have consequences for vocal learning in nestlings.In this study,we tested the hypothesis that the nest environment attenuates song of Savannah Sparrows(Passerculus sandwichensis),grassland songbirds that build nests on the ground under cover of dense vegetation.We recorded live Savannah Sparrows singing at variable distances from 21 nests.We recorded songs using one microphone inside the nest(the typical position of a nestling)and another placed 1 m directly above the nest(a typical position of an adult).We found a substantial reduction in signal-to-noise ratio,where songs recorded inside the nest were an average of 11 dB lower than songs recorded directly above the nest.We estimate that the attenuation imposed by the nest reduced the maximum acoustic environment from 117.7 m(for recordings above the nest),to 78.6 m(for recordings within the nest),which is analogous to listening from a position 39.1 m farther away from the singer.Previous research estimated that song transmits up to 100 m in this species,so any adult male within 100 m of a young bird was previously considered a potential vocal tutor.By reducing the nestling acoustic environment from 100 m to 78.6 m,the number of male tutors available to nestlings is reduced by an average of 27%.Given the growing evidence that song learning begins very early in life,future research on vocal development should account for how the structural properties of the natal environment affect the songs that nestlings hear.

Diel and seasonal patterns of variation in the singing behaviour of Savannah Sparrows (Passerculus sandwichensis)

Background:The vocalizations of birds are dynamic traits that often vary in output with time of day and time of year.By quantifying patterns of diel and seasonal variation in vocal output,we can gain insight into the ecology and evolution of birds and the function of their vocalizations.In this investigation,we quantified diel and seasonal variation in song output by studying a breeding population of Savannah Sparrows (Passerculus sandwichensis).Methods:We used autonomous recorders to collect extensive recordings across the breeding season in a long-term, colour-marked study population of Savannah Sparrows in eastern Canada.We described diel and seasonal variation across five different breeding stages based on recordings of more than 50,000 songs from 34 males.Results:During the pre-breeding stage, prior to female arrival, males sang a pronounced dawn chorus with a peak in song output during the early morning.During the breeding stage,in contrast,the dawn chorus was diminished and males instead exhibited a pronounced dusk chorus with a peak in song output during the evening.Across the breeding season,the highest levels of song output occurred in late April and early May,soon after males arrived on the breeding grounds,and the lowest levels occurred in August,prior to the departure of birds for the wintering grounds.Conclusions:These patterns suggest that Savannah Sparrows' early-season dawn choruses are directed at males and serve a territorial defense function.Later-season dusk choruses,in contrast, appear to play a role in attracting mates and possibly acoustic mate guarding.