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
need real audio player to play most of the examples provided here
Bird song facts:
- There are about 4000 species of song birds each of which usually
produce 1 to many bird songs.
- In all studied cases adult song has been shaped by prior learning.
- In most cases it is the male of the species that produces
song, whereas females typically produce very little or no sound/song.
- 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.
- 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:
- What genes/proteins/hormones are involved in song
- What are the brain structures involved in song production
and how do they develop?
- What are the developmental differences between males and
females that produces male singers?
- How do within species dialects develop what are the
characteristic differences in song pattern?
- Are there regional preferences among females for local
Questions about ultimate cause of bird song variation:
- Why do birds appear to have to learn the details of their
songs as juveniles?
- Why do dialects exist in nature?
- Do dialects provide an adaptive advantage?
- Are they a product of local genetic differences?
- Are they the results of an accidental mispronunciation
that is learned
Proximate Cause Case Study: The White Crowned Sparrow
Permission to use this material generously supplied by
Doug Neilson Director
of the Borror Lab at OSU.
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
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.
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.
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.
Song learning facts:
- Juveniles can sing but need adult examples from which to learn to produce
- Juveniles have a preference/learn songs of their species better than they
learn other species songs.
- There is variability across individuals
Stages of song development
Normal song development proceeds through a series of stages:
- 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
- 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.
- 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.
- 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
of songs from the overproduced plastic song repertoire, guided by hearing the
song of another bird.
Proximate cause of Dialectic differences:
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