ScienceWeek

ANIMAL BEHAVIOR: SONGBIRDS AND RELATIVE PITCH

The following points are made by R.G. Weisman and L. Ratcliffe (American Scientist 2004 92:532):

1) Modern ornithologists believe that songbirds first appeared some 40 million to 50 million years before human beings. That people derive pleasure from birdsongs and recognize their musical features suggests that despite the vast evolutionary gulf between birds and mammals, songbirds and humans share some common auditory perceptual abilities.

2) Relative pitch is the ability to produce or recognize a note based on a reference note. The ability to hum a simple tune when given a set note with which to start, as most people can, shows relative pitch in action. A sense of relative pitch is common among non-musicians even if they never use the term, but musical training develops this knack. Trained singers have an advanced appreciation of pitch change: They can produce precise relative frequency changes from one note to the next in a melody.

3) To compare the appreciation for relative pitch among humans and songbirds, it is necessary to understand how musicians and bioacousticians measure pitch change. Musicians describe the change between any two notes on the musical scale as an interval. The interval between any two adjacent notes is defined as a semitone. The ratio between the frequencies (measured in cycles per second, or hertz [Hz]) of the higher and lower of two notes separated by a semitone is always 1.059, regardless of where the two notes are located on the scale. Musicians speak of semitones and intervals; bioacousticians use the terms frequencies and frequency ratios.

4) Songbirds belong to the class Aves, which includes all birds, the order Passeriformes, consisting of perching birds, and the suborder Oscines, the true songbirds. The true songbirds are distinguished from other perching birds by the anatomy of their vocal organ, the syrinx, and by the fact that they need to learn their songs while juveniles from adult members of the same species. Oscines are an evolutionary success, comprising about half of the more than 9,000 species of birds and filling niches in almost every environment on the planet. Because songbirds are so diverse in appearance and behavior, no single species can represent the whole group.

5) Black-capped chickadees are familiar and abundant year-round in the northern two-thirds of the United States and in much of Canada. These chickadees are acrobatic, noisy and highly social birds. Their name derives from their call, which sounds like "chick-a-dee". This call is given by male and female, young and old, to keep in touch with members of the flock. The chick-a-dee call is complex, and some of the notes include "buzzy" harmonics. During the breeding season and less frequently throughout the year, males sing the "fee-bee" song, used to hold territory and to attract and arouse females.

6) The male chickadee's fee-bee song consists of two or three clearly whistled notes. The first note, "fee", is sung at a higher pitch than the second one, "bee". Sometimes the bee note appears to be sung as two separate notes at the same pitch. The New England naturalist and composer F. Schuyler Mathews suggested in the early 1990s that chickadees separate the fee and bee notes in their songs by a clearly perceptible pitch interval. Recall that musical intervals are simply precise discrete frequency ratios. approximately 70 years later, Steward Hulse restated Mathews's hypothesis in modern bioacoustic terms when he suggested that chickadees might hold the ratio between the frequencies of their fee and bee notes constant across birds.[1-5]

References (abridged):

1. Christie, P. J., D. J. Mennill and L. M. Ratcliffe. 2004 Pitch shifts and song structure indicate male quality in the dawn chorus of black-capped chickadees. Behavioral Ecology and Sociobiology 55:341-348

2. Hill, B. G., and M. R. Lein. 1987. Function of frequency-shifted songs of black-capped chickadees. Condor 89:914-915

3. Horn, A. G., M. L. Leonard, L. Ratcliffe, S. A. Shackleton and R. G. Weisman. 1992. Frequency variation in the songs of black-capped chickadees (Parus atricapillus). Auk 109:847-852

4. Mennill, D. J., L. M. Ratcliffe and P. T. Boag. 2002. Female eavesdropping on male song contests in songbirds. Science 5569:873

5. Mennill, D. J., and L. M. Ratcliffe. 2004. Overlapping and matching in the song contests of black-capped chickadees. Animal behavior 67:441-450

American Scientist http://www.americanscientist.org

--------------------------------

Related Material:

NATURAL HISTORY: ON THE MAGNETIC COMPASS OF SONGBIRDS

The following points are made by W.W. Cochran et al (Science 2004 304:405):

1) Billions of songbirds migrate between continents twice each year, but their orientation capabilities are almost exclusively studied in the laboratory. The authors presented birds with experimentally altered orientation cues and followed their subsequent migratory flights in the wild. Avian navigation capabilities are very precise (1), with many individuals returning to the same breeding sites year after year (1-3) after a voyage of up to 25,000 km (4, ).

2) Migratory songbirds can orient on the basis of compass information from the sun and its associated polarized light patterns, the stars, the earth's magnetic field, and the memorization of spatial cues en route. However, the interactions and relative importance of these cues remain unclear and a source of much debate. Our knowledge about the orientation mechanisms of songbirds relies almost exclusively on data from cue-manipulated captive migrants tested in various orientation cages, on vanishing bearings based on the first few hundred meters of flight, and to a much lesser degree on field data (ringing and radar and visual observations) from unmanipulated natural migrants.

3) On clear evenings, the authors fitted Catharus thrushes with radio transmitters and placed them in outdoor cages in an artificial eastward-turned magnetic field from about sunset until the sun was 11 deg or more below the horizon when they were set free. The authors then radio-tracked the birds in flight to obtain heading data. Because Catharus thrushes do not compensate for wind drift but individuals maintain nearly constant preferred headings from night to night, the authors used measured headings for orientation analyses.

4) In summary: Night migratory songbirds can use stars, sun, geomagnetic field, and polarized light for orientation when tested in captivity. The authors studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds. The authors exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. The authors suggest that birds orient with a magnetic compass calibrated daily from twilight cues, and that this could explain how birds cross the magnetic equator and deal with declination.

References (abridged):

1. P. Berthold, E. Gwinner, E. Sonnenschein, Eds., Avian Migration (Springer, Berlin, 2003)

2. J. P. Hoover, Ecology 84, 416 (2003)

3. P. O. Dunn, D. W. Winkler, Proc. R. Soc. London Ser. B. 266, 2487 (1999)

4. D. C. Outlaw, et al., Auk 120, 299 (2003)

5. W. L. Engels, Biol. Bull. 123, 94 (1962)

Science http://www.sciencemag.org

--------------------------------

Related Material:

BIRDSONG AND HUMAN SPEECH: COMMON THEMES AND MECHANISMS.

The following points are made by A.J.Doupe and P.K. Kuhl (Annu Rev Neurosci 1999 22:567):

1) Human speech and birdsong have numerous parallels. Both humans and songbirds learn their complex vocalizations early in life, exhibiting a strong dependence on hearing the adults they will imitate, as well as themselves as they practice, and a waning of this dependence as they mature. Innate predispositions for perceiving and learning the correct sounds exist in both groups, although more evidence of innate descriptions of species-specific signals exists in songbirds, where numerous species of vocal learners have been compared.

2) Humans also share with songbirds an early phase of learning that is primarily perceptual, which then serves to guide later vocal production. Both humans and songbirds have evolved a complex hierarchy of specialized forebrain areas in which motor and auditory centers interact closely, and which control the lower vocal motor areas also found in nonlearners. In both these vocal learners, however, how auditory feedback of self is processed in these brain areas is surprisingly unclear.

3) Finally, humans and songbirds have similar critical periods for vocal learning, with a much greater ability to learn early in life. In both groups, the capacity for late vocal learning may be decreased by the act of learning itself, as well as by biological factors such as the hormones of puberty. Although some features of birdsong and speech are clearly not analogous, such as the capacity of language for meaning, abstraction, and flexible associations, there are striking similarities in how sensory experience is internalized and used to shape vocal outputs, and how learning is enhanced during a critical period of development. Similar neural mechanisms may therefore be involved.

Annual Review of Neuroscience http://www.annualreviews.com

ScienceWeek http://scienceweek.com