Chapter 2: Using bird singing behavior as a case study of how to apply the scientific method to produce and answer questions of proximate and ultimate causation.

you will need real audio player to play most of the examples provided here

Bird song facts:

  1. There are about 4000 species of song birds each of which usually produce 1 to many bird songs.
  2. In all studied cases adult song has been shaped by prior learning.
  3. In most cases it is the male of the species that produces song, whereas females typically produce very little or no sound/song.
  4. Males within any single species tend to sing highly similar sets of songs but may also have dialectic variations for a given song. These dialects are observable when comparing songs across different geographic regions.
  5. In some cases, dialectic variations of a song within a region will change over time (i.e. across generations).

These sources of variation in bird song (species, sex, dialectic and generational differences) provide us with an opportunity to generate hypotheses about how and why birds sing songs.

Questions about proximate cause of bird song variation:

  1. What genes/proteins/hormones are involved in song production?
  2. What are the brain structures involved in song production and how do they develop?
  3. What are the developmental differences between males and females that produces male singers?
  4. How do within species dialects develop what are the characteristic differences in song pattern?
  5. Are there regional preferences among females for local dialects?

Questions about ultimate cause of bird song variation:

  1. Why do birds appear to have to learn the details of their songs as juveniles?
  2. Why do dialects exist in nature?
    1. Do dialects provide an adaptive advantage?
    2. Are they a product of local genetic differences?
    3. Are they the results of an accidental mispronunciation that is learned


Proximate Cause Case Study: The White Crowned Sparrow


smpuget.jpg (31779 bytes)  

Permission to use this material generously supplied by Doug Neilson Director of the Borror Lab at OSU.

Song learning: Modern experimental studies of song learning in birds began in the 1950's in laboratory of W.H. Thorpe.  He and Peter Marler hand-reared from an early age and showed that young male song birds learn their songs from adults of the same species.   If a bird is reared without hearing the normal adult song of its species, a simplified "isolate" song is produced.

A song developed by a male white-crowned sparrow raised in the laboratory from the age of 5 days without hearing the song of an adult. 

 isolate2.TIF (33146 bytes)

A song developed by a male white-crowned sparrow that was tutored early in the first 2 months of life with a white-crowned sparrow song.

normal2.TIF (48716 bytes)

Here is the tape-recorded song he was tutored with.  Notice there are small differences between the tutor and the imitation, but overall the copy is quite good.   These small learning errors or improvisations introduce individual variation into the local populations of songs.

or0532.TIF (49902 bytes)


Song learning facts:

  1. Juveniles can sing but need adult examples from which to learn to produce normal song.
  2. Juveniles have a preference/learn songs of their species better than they learn other species songs.
  3. There is variability across individuals

Stages of song development

Normal song development proceeds through a series of stages: 

  1. In the first, the young male memorizes the songs of one or more adult birds.  In many species, males are most sensitive to memorize songs in the first few months of life, the so-called "sensitive phase." 
  2. Actual vocal production begins during or soon after the sensitive phase when the male begins subsong.  Subsong has been compared to babbling in human infants.  The function is still not clear, but only song birds go through this stage; birds that do not learn to sing do not produce subsong.   Here is a seven second long segment of subsong performed by a 240 day old white-crowned sparrow. In the laboratory, hand-reared birds perform subsong throughout their first autumn and winter.

subsong2.TIF (105132 bytes)

  1. As days lengthen in the late winter and early spring, subsong gives way to plastic song, in which the first evidence of imitations of tutors appears in the male's singing.   Shown below is a 9 second long segment of plastic songs by a 260 day old sparrow (the 5-10 sec long quiet intervals between successive songs have been shortened to 2 secs here) containing three different songs.   The three tutors that the male memorized during the sensitive phase are shown above, and are connected by arrows to the young male's imitations.  Note that in plastic song, imitations are often incomplete.  Also, note that the last song is a "hybrid" song composed of parts of two different tutor songs.  This male is "overproducing," he sings more imitations than eventually appear as his final crystallized song.  Click on each of the three tutors and the segment of plastic song to hear them, and compare the young male's imitations to his tutors.

plastic3.TIF (649374 bytes)

  1. After 35 days the same male is singing his single crystallized song type. The first song is the tape playback, 3 seconds later the male responds with his rendition of the matching song type.   He has stopped singing his imitations of Tutors 2 & 3, (above) and will retain his crystallized imitation of Tutor 1.  This is an example of selective attrition of songs from the overproduced plastic song repertoire, guided by hearing the song of another bird. 

crystal2.TIF (108555 bytes)



Proximate cause of Dialectic differences:

pugetdials3.JPG (1312728 bytes)Songs of the Puget Sound white-crowned sparrow can be grouped into about 12 dialects spanning the Pacific Northwest coast from northern California to British Columbia.  Shown here are sound spectrograms of 10 dialects along with their geographic distributions.  Click on each spectrogram to hear the song (.wav files).  Two small dialects, 8 and 9, on Whidbey and Camano Islands, Washington are not illustrated.  Dialects are numbered  in the approximate order in which they were discovered by Luis Baptista (1977), Mike Baker (1987), Glen Chilton and Ross Lein (1996) and our own work.  Dialect 4 is probably extinct.


















Text Box: Zebra Finch : The machine behind the mouth (or beak)


Text Box: The syrinx: The song machine 
located at the point where the trachea branches into the two primary bronchi. 
Sound is generated when: 
Air from air sacs is forced through the bronchi & syrinx 
the air molecules vibrate as they pass through the narrow passageways between the external labia & the tympanic membrane. 
Because there are 2 separate passageways and membranes, some birds are able to generate multiple sounds (harmonics) at the same time: 
Text Box: General facts about Zebra Finches:
The common and widespread in Australia (particularly drier areas), Timor and the Lesser Sunda Islands.
Live year round in social flocks of up to 100 or more birds. 
Feed on grass seed and insects.
One of the most common "caged birds" selectively bread for a number of traits.
Considered to be one of the best model systems for basic biomedical research of learning and memory. 
Also used as a model system to study many other thing (i.e. olfaction).

Text Box: Zebra Finch Song : Unique because of its harmonic characteristics and complexity