Perceived wintering latitude determines timing of song output in a migratory bird

Abstract

Migratory bird populations frequently consist of individuals that overwinter variable distances from the breeding site. Seasonal changes in photoperiod, which varies with latitude, underlie seasonal changes in singing frequency in birds. Therefore, migratory populations that consist of individuals that overwinter at different latitudes with large overwintering ranges could experience within-population variation in seasonal production of song. To test the influence of overwintering latitude on intrapopulation variance in song production in the spring, we subjected two groups of Eastern Song Sparrows (Melospiza melodia melodia) from the same partially migratory breeding population to different photoperiodic schedules associated with a 1,300-km difference in overwintering location. One group remained on the natural photoperiodic schedule of the breeding site (resident group) while the other group experienced a nonbreeding photoperiod that mimicked a southern migration in the fall followed by a northern migration back to the breeding site in the spring (migratory group). We compared song output between the two groups in three different stages (nonbreeding, prebreeding, and breeding). Little singing occurred during nonbreeding stage sample dates (20 November, 6 December) for the resident group, and no singing occurred for the migrant group. During the prebreeding stage (27 January, 7 February), significantly more singing occurred in the resident group than in the migrant group. During the breeding stage (21 March, 4 April), after a simulated migration for the migrants, song output was similar in both groups. These results suggest that within-population variation in wintering latitude may contribute to variation in seasonal changes in singing behavior, which may covary with readiness to breed. Studies utilizing confirmed migrants and residents, rather than merely simulated migrants and residents, are also needed to better understand these processes. K E Y W O R D Sbirdsong, connectivity, partial migration, photoperiod, timing | 749 BREWER Et al.

Stopover singing is common in migrating male New World warblers (Parulidae) but differs from song produced during breeding

“…For example, migrating Wilson's Warblers had variable song rates (0.76-3.30 songs/ min) and may travel to the southern end of their breeding range (Ontario, Canada), to more northerly sites (Alaska, USA), or sites in between these extremes (Ammon and Gilbert 2020). Furthermore, as the initiation of song is mediated by photoperiod (Smith et al 1997), overwintering latitude may also influence intraspecific singing rates, with populations from the northernmost latitudes of the overwintering range producing breeding levels of song output earlier on migration than their southern counterparts (Brewer et al 2020). Additional research that geographically links singing migrants at stopover sites to their overwintering and breeding latitudes, such as via stable hydrogen isotopes (Covino et al 2017), would provide a more thorough understanding of changes in song rates along migratory pathways.…”

“…As breeding singing functions to establish territories and attract mates, the advantages of expressing song upon arrival to the breeding grounds may favor males that arrive prepared to produce song at rates attractive to females (Brewer et al 2020). At any given stopover site in the spring migratory pathway, both the onset of singing and rate of song output could vary across species and populations depending on the distance left to travel to breeding grounds, reflecting physiological readiness to breed (Brewer et al 2020).…”

“…We predicted that observed song rates would be higher in species with breeding ranges geographically closer to our study area and that the difference between species' breeding and migratory song rates would be larger in species that breed farther from our stopover region. Migratory singing is often overlooked in birdsong research, in which male song during the temperate breeding season has been a persistent, primary focus (Rose et al 2022), such that song rates of males on migration have not been quantified in wild populations (Brewer et al 2020). Our study contributes new data on singing on migration collected in a single stopover region and provides insight into the onset of breeding-related behaviors during migration.…”

“…As breeding singing functions to establish territories and attract mates, the advantages of expressing song upon arrival to the breeding grounds may favor males that arrive prepared to produce song at rates attractive to females (Brewer et al 2020). At any given stopover site in the spring migratory pathway, both the onset of singing and rate of song output could vary across species and populations depending on the distance left to travel to breeding grounds, reflecting physiological readiness to breed (Brewer et al 2020). In addition, migratory singing may function as practice for first-time breeders and an opportunity to improve song quality in preparation for breeding for returning ones (Janes 2017), as higher-quality songs provide important fitness benefits for males (Catchpole and Slater 2008).…”