Bird brains are really brain boxes say scientists who have studied their complex tunes
The term "bird brain" may take on a whole new meaning as scientists have found a link between the relative sizes of songbirds' brains and the complexity of the tunes they sing.
The study looked at nearly 50 species of songbird and found that those with larger higher brain areas in relation to lower brain areas have a greater capacity for learning songs.
Higher brain areas control more cognitive and learning functions, while lower brain areas control more motor functions.
The study, at the University of Bath and Cornell University in the US, is the first to prove that the capacity for learning in birds is closely related to brain structure as opposed to overall brain size.
The research shows that when a bird's higher cortex-like brain area, called the high vocal centre (HVC), is larger relative to a lower brain area called RA, or if the RA is large relative to an even lower area called N12, the species is able to learn dozens of different notes.
Some species show these relative size differences in both sets of areas. These birds, such as mockingbirds, blackbirds and warblers, can learn hundreds of notes.
Professor Tamas Szekely, from the University of Bath's Department of Biology and Biochemistry, said: "This research is not only an extremely complex and interesting study of songbirds, it also gives us a unique insight to how brain development may contribute to human linguistic capabilities.
"The research gives us an example of the neural biology involved in language learning.
"It is possible to draw parallels between the ways in which bird brains have developed to learn complex songs to the way human brains have evolved to allow language."
Humans are able to speak and to set and achieve complex goals because of prolonged development of higher brain areas, such as the cortex and frontal cortex.
In bird species with great capacities for song learning, higher brain areas are likely to have been built up over lower areas as a result of sexual selection, where females mated with males that had more elaborate songs.
In birds, and perhaps in humans, selection for increased learning capacity may have acted by prolonging the development of the last parts of the brain to grow.
The areas of the brain responsible for language are the last to mature and do not fully develop until humans are in their early 20s. The human frontal cortex is larger than other animals' with respect to the cortex as a whole, and the human cortex is larger with respect to the rest of the brain than in other animals.
To complete the study, researchers looked at the brains of three males each from 49 common songbird species representing an extensive variety of songbirds from the United States, Europe and South Africa, where each bird was actively singing to attract females as part of his reproductive cycle.
The study is published this week in the Proceedings of the National Academy of Sciences USA.