Michael Beecher and Eliot Brenowitz, University of Washington professors of psychology and biology, say that while a great deal of knowledge has been gleaned by studying songbirds over the past three decades, a narrow focus on just a few species only provides a fragmentary picture of how the brains of nearly 4,000 songbird species function.
Writing in companion papers in the March issues of the journals Trends in Neurosciences and Trends in Ecology and Evolution, the two UW scientists argue that there is much greater diversity in how and when birds learn to sing than is generally recognized. They say the value of the birdsong system as a model for studying how the brain controls the learning of language would be greatly enhanced by taking into account the diversity seen among different bird species.
"We are interested in comparative approaches," said Beecher, who is an animal behaviorist. "There are many patterns of learning, but most studies are on zebra finches or white-crowned sparrows, in which song learning is restricted to the first year of life. People are not taking advantage of the wide spectrum of bird species. There probably are more species learning songs into their third and fourth years than those who only learn in the first few months or first year."
"One of the great things about songbirds is there is great variety in the manner in which different species learn to sing," said Brenowitz, a neurobiologist. "They are great models, but we should take advantage of the diversity of what they have to offer."
Brenowitz noted that the often-studied zebra finch is sexually mature in just 90 days.
"They learn song quickly so it is hard to say this change in the brain is related to this aspect of song development. We can understand these kinds of things better in other species that mature more slowly. We can learn, for example, how the brain controls learning new songs as an adult, or to mimic the songs of other species. With different species, you get to ask all kinds of questions and get all kinds of answers that you can't with any single species."
The researchers said scientists need to be cautious about regarding the behavior of one or two species as typical of all songbirds in general.
"Settling on one species is risky," said Brenowitz, "because it depends on which species you began with. If you started with the starling, which learns throughout its life, rather than the zebra finch, our view of the basic or norm for birds would be very different."
The UW scientists said that previous research tended to label songbirds as either closed-end or open-ended learners, depending on when they learned their song repertoires. The assumption has been that most species, with a few exceptions, learned their songs early in life. More recent research, Beecher and Brenowitz said, has shown that there is a continuum of learning, with some species acquiring a fixed repertoire early in life, others whose song changes over the course of a year, others that add new songs from year to year and still others who learn an entirely new group of songs each year.
"We want to set an agenda for the next generation of studies and focus on comparative work beyond the standard species that have been examined," said Beecher. "Researchers would benefit from looking at species that do things differently because there are very different learning patterns. There is no one typical way in which songbirds learn."
"Some birds stick with what they learned the year before, others change," added Brenowitz. "There is a pool of plasticity in the bird brain that such species as mockingbirds and starlings take advantage of but white-crowned sparrows don't. There is a parallel in human language learning - factors that limit most people in learning a second language to childhood, while a few have no problem, even as adults."