There's always so much to remember when you're packing for a long journey, and when migrating species begin their preparations for departure they focus on building their muscles and fat stores in readiness. However, when breeding grounds are the migrants’ final destination, the females must also anticipate the conditions that will greet them and ensure that they will be in tip-top reproductive condition when they arrive. In contrast, species that stay put have more information about the environment in which they will lay their eggs. ‘Scientists who study how animals pick when to breed hypothesize that migrant and resident birds use different kinds of cues from the surrounding environment to get their bodies ready to lay eggs’, says Helen Chmura, from the University of California, Davis, USA. Although cues such as increasing day length are known to trigger migrant preparations for departure, Chmura, John Wingfield and Thomas Hahn wondered whether other factors may fine-tune bird breeding preparations and whether some cues may be more significant for species that stay put than species that migrate before breeding.
Focusing on white-crowned sparrows that live in Northern California, Chmura and Hahn collected females from two closely related members of the family: Gambel's white-crowned sparrows, which migrate north to breed in Northern Washington, Canada and Alaska; and their home-loving relatives, Nuttall's white-crowned sparrows, which spend the entire year in California. Back in the lab, the duo and a large team of assistants separated the migratory Gambel's sparrows from the Nuttall's sparrows. Then they simulated the dawn chorus each day from early February until late April, playing recordings of the migratory males’ serenades to their females, while ensuring that the non-migrating resident females listened to recordings of males from their own neck of the woods: ‘Resident sparrows have regional song dialects’, says Chmura. Meanwhile, two other groups of the birds experienced a silent simulated sunrise. Then, Chmura, Hahn and Simone Meddle monitored the birds’ reproductive hormones, ovarian development and the condition of the birds’ plumage – in case they moulted – to find out whether the males’ melodies had any effect on the breeding preparations of the migrants and their stay-at-home cousins.
However, when the team compared the development of the birds’ reproductive systems, they were surprised that listening to recordings of the males’ songs had no effect on the females’ reproductive preparations. Even though the preparations of the resident Nutall's sparrows for breeding were more advanced than those of the migrating Gambel's sparrows, listening to the male's serenades had not fine-tuned the Nuttall's sparrows’ preparations further. However, when the team compared plumage of the migrants that had been serenaded with those that had experienced a silent sunrise, it was clear that a morning recital accelerated the migrants’ moulting process.
So it appears that recordings of the males’ arias didn't make either migrant or resident birds prepare to breed more quickly: either the birds weren't listening to the recordings, or they were listening, but didn't pay attention to the information. However, listening to the dawn chorus can help migrants replace their plumage more quickly in preparation for a swift departure. And Chmura adds, ‘Understanding the similarities in how migrants and residents use and respond to environmental cues is important, as frequently these differences are used to underpin predictions about how they will respond to climate change’.
