Girl songbirds like a guy who can really sing

New research reveals that the evolution of song learning in male birds may be driven by female bird choice. By selecting males with the longest and most complex song repertoire, females are influencing male songbirds to evolve towards a strategy that involves life-long learning of songs. 

This is a critical piece of the puzzle of song learning evolution as it reveals evolutionary action not just on the features of the song itself, but on how songs are learned, say researchers Cristina Robinson, Kate Snyder and Nicole Creanza from Vanderbilt University, US. 

The time taken to learn birdsong is known as the song learning window. In some species, often with a relatively simple repertoire, the song is set while still young, before the first breeding season.  In others, the song becomes static or stable during the first breeding season.  

And, in more accomplished songsters, we see open-ended learning where new sounds and syllables are learned well beyond reaching adulthood. 

“We were curious as to why some birds learn throughout their lives and why others only learn when they’re juveniles,” Creanza says. “Researchers have thought about this question for a while, but usually linked their findings back to those other environmental aspects of the birds’ lives. We had a hypothesis that sexual preference for songs could also be a factor.”

To carry out the study, published in the journal eLife, the team compiled data on 67 species. As in a recent study comparing song learning in various mating systems, they took a comparative, computational approach to examine the evolutionary history of particular traits across the songbird lineage. {%recommended 6875%}

They classified birds as having either song stability or song plasticity, and then assessed the interactions between the length of the song-learning window, mating system type and the evolution of song characteristics, such as syllable repertoire, and song duration. 

One might assume that a bird with a complex song would need a longer time period to master it, but some birds with simple songs, such as the North Island Saddleback (Philesturnus rufusater) from New Zealand, continue to learn and add new sounds and syllables throughout their lives. 

Conversely, the Marsh Warbler (Acrocephalus palustris), with a very complex song, has a relatively short song learning window of a single year. 

Despite these examples, the team’s analysis found that open-ended learning or song plasticity in birds does indeed correspond to significantly larger syllable and song repertoires. This finding concurs with Creanza’s previous study, which examined a smaller dataset. 

Lifelong learning in birds requires a degree of neuroplasticity, a quality with inherent metabolic costs. In terms of evolutionary pressure, these costs would theoretically favour the shorter learning window.

But if female choice favours certain characteristics such as longer, more complex songs, then female selection could be regarded as another evolutionary pressure acting on birdsong. 

“Sexual selection for large repertoires could indirectly favour individuals with longer learning windows,” the researchers write, driving the evolution of increased lifelong song learning. 

This study is the first to link song learning length directly with sexual selection.  Previously, hypotheses have linked temporal variation in song learning windows with seasonal factors – such as environmental variation, and breeding season length. 

The findings raise interesting questions about song learning in female birds. The study dataset had some species where female birds are known to sing, but there is very little information available on song learning windows in female birds.

There is increasing recognition of the previously overlooked importance of female birdsong, and Creanza and colleagues say that further studies are needed to determine whether life-long learning affects the evolution of female song in the same way it affects male song, and indeed whether species with adult song plasticity in males also occurs in females. 

As in recently reported studies on song learning in zebra finches, these findings may have applications to our own species.

“As we learn more about these time-windows for learning in birds and what causes them to evolve and lengthen, we may be able to apply those findings to how and why human learning windows may have evolved over time,” Creanza explains.  

“One day, if researchers understand what happens in the brain when a bird maintains its ability to learn, it might shed new light on how to help the brain repair itself in humans.”