Male birds have many features in their arsenal to attract healthy female mates, including ostentatious plumage, complex dances and elaborate vocal performances. However, Roderick Suthers, from Indiana University, USA, and Eric Vallet and Michel Kreutzer, both from the Université Paris Ouest Nanterre La Defense, France, explain that it is unclear exactly how a male bird's vocal performance communicates his fitness. According to the team, female domestic canaries are particularly willing to mate with males that incorporate a specific wide bandwidth two-note syllable – the so-called ‘sexy’ A syllable, executed at a high 15 syllables s–1 repetition rate – into their repertoire. So, Suthers and his colleagues decided to analyse the males' vocal tract gymnastics to find out how difficult the syllable's trills are to coordinate and how they convey information about male fitness (p. 2950).
Explaining that the two sides of the bird's syrinx are capable of producing sound independently – with the left side producing low frequency notes while the right side produces high frequency sounds – Suthers and his colleagues surgically implanted two thermistors, one in each side of the male canary's syrinx, to measure airflow through the organ and to determine which side of the syrinx produced the pair of characteristic sexy syllable notes. The team also inserted a cannula into one of the bird's airsacs to determine the airflow direction. Repeating the surgery on two other birds, the team then recorded the birds' songs and determined which side of the syrinx produced each note.
Having identified seven sexy syllables in the birds' recitals and analysed the airflow producing them, the team found that each sexy syllable was produced as the birds pulsed their breath while exhaling. Also, each syllable was separated by minibreaths – when the birds inhale swiftly after a syllable to replace the exhaled air – allowing the bird to sustain a lengthy recital. The team says, ‘The combination of these two respiratory strategies permits high note repetition rates’, allowing the males to hit the 15 syllables s–1 repetition rate required for a syllable to be deemed ‘sexy’ by the female. In addition, Suthers and his colleagues found that despite repeating the syllables at an extremely high rate, each syllable still had a wide frequency bandwidth, which is a key feature of sexy syllables, achieved by sequentially alternating two notes, one from either side of the syrinx.
The team explains that as each half of the syrinx is controlled by one brain hemisphere, it is the ability of the males to coordinate the two brain hemispheres (bilateral coordination) to alternate the pair of sexy syllable high and low frequency notes – resulting in a wide bandwidth spectrum – that allows the female to assess her suitor's coordination and quality. They also suggest that listening for two-note sexy syllables allows females to rule out cheats with poor bilateral coordination that could produce similar effects by only using one half of the syrinx.
So, it appears that females have evolved a preference for sexy syllables because they reveal how well coordinated males are. The team says, ‘Females who prefer these traits should have a higher probability of choosing a male with excellent bilateral coordination than females that choose based on only one or a few of these traits’.