Envision a beautiful spring morning: expectant flower buds on the trees, morning dew coating the grass, a bright sunrise peeking up over the horizon, and the melodious sound of birdsong all around. In the Northern Hemisphere, the sound of birds singing is a well-known sign of springtime.
Singing is typically performed only by male birds, and is closely associated with their breeding season – the time of year when birds engage in courtship, mating, and nesting. The typical songbird breeding season coincides with our spring and summer months, and birds often migrate to warmer locations for the remainder of the year. Song in birds is known to have two primary functions: 1) mate attraction (typically with males soliciting females), and 2) defense of breeding territories (‘claiming’ land for nesting and feeding). Given these key functions, singing is essential for reproductive success in male songbirds, and we therefore consider song a sexual signal relevant only during breeding.
Surprisingly, though, we find that many migratory songbirds sing during the non-breeding season. This is unexpected for a sexual signal like song. One reason it is unexpected is because these birds have migrated very far away from their breeding sites to separate wintering grounds where they do not mate. Additionally, these birds have physiologically lost the ability to mate – their sex organs shrink and become inactive during the non-breeding season and are regrown the following spring. Furthermore, song production is energetically costly, time-consuming, and it increases predation risk for the singer, so we expect birds not to sing outside of the specific breeding context. The function of winter singing has been discussed and debated, but until now, it remained poorly understood. Biologists Marjorie C. Sorenson, Susanne Jenni-Eiermann, and Claire. N. Spottiswoode, however, recently tested three hypotheses to explain the function of winter singing by migratory birds, and their findings have shed light on the phenomenon .
Sorenson et al. looked at winter singing behavior across bird species that migrate between Eurasia and Africa, many of which have been documented singing on the wintering grounds in sub-Saharan Africa. They took a comparative approach to look across these groups (57 species), and they also performed focused field tests on a single species, the Great Reed Warbler (Fig. 1).
Fig. 1 A Great Reed Warbler sings. Photograph by Pat Hayes (http://www.realbirder.com).
The first possible explanation for winter singing that the researchers tested was that winter song might function in maintaining feeding territories – while birds are not building nests or feeding offspring on their wintering grounds, they still must feed themselves and thus need access to resources. If this was the case, they expected to see clear spatial separations between individuals. On the breeding grounds, Great Reed Warblers sing one song for courtship, and another song for territorial defense. If defending feeding territories, the winter singing should match the territorial song rather than the courtship song.
The second potential explanation for winter singing was that the birds might be ‘rehearsing’ their songs in preparation for the breeding season. We know that males exhibiting higher quality singing are more likely to successfully reproduce during the breeding season, so perhaps the winter singing is just practice for the ‘real deal’? Sexual selection tends to push song complexity higher and higher, so such practice may become necessary in order to pull off certain elaborate note sequences. If correct, the scientists expected to see more winter singing behavior in species with stronger sexual selection and higher song complexity, circumstances in which singing practice is more likely to pay off during the breeding season.
Thirdly, the researchers considered the explanation that winter singing was simply a triggered behavioral response to elevated testosterone levels leftover from the breeding season. Testosterone circulation increases dramatically during the breeding season, and may carry over into the winter or begin ramping up prior to spring, both of which could stimulate singing. Support for this hypothesis would be increasing winter testosterone levels aligned with increasing winter singing.
After analyzing their data, only one of these three possible explanations was supported by evidence from their study – the “rehearsal” hypothesis won out! Birds who sing in the winter – both the Great Reed Warbler (focused field study) and many of the 57 migrant species examined (comparative analysis) – seem to be practicing their songs, attempting to improve their own song quality to win the best territory and successfully attract females when they return to Europe to breed. This song-practicing behavior is most evident in species with highly complex songs and presumed strong sexual selection acting on song.
This fascinating result demonstrates the important influence sexual selection can have on a species, not just for driving elaborate male traits, but for shaping year-round behavior and energetics. Additionally, this work expands our understanding of the song learning process in birds, a hot topic in animal biology today.
As the saying goes, “practice makes perfect”!
 Sorensen, Marjorie C., Susanne Jenni-Eiermann, and Claire N. Spottiswoode. “Why Do Migratory Birds Sing on Their Tropical Wintering Grounds?.” The American Naturalist 187 (2016): E65-76. doi: 10.1086/684681.
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