New Research into Bird Song
While zebra finch females utter single note, low-pitched calls, males have the ability to sing in a variety of frequencies, even producing a whistle that goes beyond a piano keyboard’s high end. Male birds make use of song to attract mates and to protect their territory. It is believed that the varied frequency of songs may be more attractive to females, as well as providing greater and more precise information.
The two variables affecting the pitch of a bird’s song are air pressure and muscle activity. Recent research has revealed that muscle activity plays the larger role in this respect. This study was conducted by Tobias Riede of the National Center for Voice and Speech (under the administration of the University of Utah), as well as Franz Goller, and John H. Fisher. Funding for the research was provided by the National Institutes of Health.
The zebra finch’s vocal organ is called the syrinx, and measures a mere one-eighth of an inch on either side. It was already known, through past studies, that male zebra finches had larger vocal muscles controlling the syrinx than did the females. In this study the cartilage scaffold, which supports the bird’s syrinx, as well as the “labia” (the part that oscillates when air moves through it) in the syrinx, were investigated. This revealed that the male finch’s cartilage scaffold is larger, while the labia are a different shape to that of the female. Riede concluded that this must be so that the labia can be tensioned tightly by the muscles that pull the scaffold, so as to reach the high-frequency notes.
The researchers sought to study whether lung pressure or vocal muscle strength was the more important factor in the control of the male zebra finch’s pitch. They began by recording the sounds of six male finches and six female finches for a period of two weeks. Tubes containing air pressure sensors were implanted into an air sac. Specially designed equipment ensured that the birds could continue to fly and sing freely whilst measurements were taken and their sounds recorded again. The results showed that higher air pressure lead to higher pitch, indicating that lung pressure does affect song frequency.
Following this experiment, the researchers cut the nerves that control the birds’ vocal muscles. They then recorded the birds’ sounds as they sang and flew about. It was noted that the pitch of all birds dropped to approximately the same level and males were unable to produce high frequencies. The fact that they could no longer put sufficient tension on the labia showed that the vocal muscles play a key role in bird song pitch.