Andrew P King

Meredith J West

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Reprinted from 
II March 1977, Volume 195, pp. 1002-1004 
Species Identification in the North American Cowbird: 
Appropriate Responses to Abnormal Song 
Andrew P. King and Meredith J. West 
Copyright© 1977 by the American Association fOT the Advancement oj Science 
• 
Species Identification in the North American Cowbird: 
Appropriate Responses to Abnormal Song 
Abstract. Female cowbirds raised in auditory isolation from males responded to 
the songs of male cowbirds with copulatory postures. The songs of males reared in 
isolation were more effective in eliciting the posture than the songs of normally 
reared males. The females did not respond to the songs of other species. These re­
sults indicate one mechanism of species identification for this parasitic species. 
The brown-headed cowbird, M 0-
lothrus ater, is reported to parasitize 
more than 200 species (1). Thus, unlike 
most vertebrates, young cowbirds are 
not exposed to stimulation by con­
specific parents during their earliest inter­
actions with adults or peers. Although 
much remains to be learned about the de­
velopmental impact of such stimulation, 
there is growing evidence to demonstrate 
its facilitative role in avian species identi­
fication (2). How, then, does the cow­
bird, naturally deprived of such experi­
ence, come to identify other cowbirds? 
This question has puzzled students of de­
velopment and evolution for many years, 
and, until now, only speCUlative solu­
tions existed (3). We report a series of ex­
periments detailing a mechanism of spe­
cies identification that allows us to under­
stand how cowbirds have evolved to 
overcome the fact that they were not 
reared by conspecifics. 
Two findings led to the discovery of 
this mechanism. First, we had raised a fe­
male cowbird in the laboratory in com­
plete auditory and visual isolation from 
other cowbirds. At 8 months of age, she 
was exposed to a recording of male cow­
bird courtship song. She adopted a pos­
ture we term the "copulatory response": 
her wings were lowered and spread 
apart, her neck and body were arched, 
and the feathers around the cloacal re­
gion were separated (Fig. 1). A series of 
playbacks confirmed the specificity of 
the response: she showed no response to 
the songs of a red-winged blackbird 
(Agelaius phoeniceus) or to the cowbird 
flight call. 
The second finding related to the de­
velopment of the male song. Male cow­
bird song, which can be transcribed as 
"burble burble tsee," occurs during 
. 
j 
courtship and is accompanied by a bow­
ing display. We collected data from a 
small group of males that suggested that 
male peer experience was sufficient to 
produce normal adult courtship song. 
However, males reared in isolation from 
both male peers and adults, developed 
songs that differed from the normal ver­
sion, particularly in the extent to which 
some of the notes are mod�lated (Fig. 2). 
The following experiments were de­
signed to elaborate upon these observa­
tions by testing other naive females. The 
songs of both normal and isolate males 
were included to help clarify the role of 
peer experience in song development. 
The subjects were six female cowbirds 
raised in auditory and visual isolation 
from adult cowbirds. Their prenatal audi­
tory experience was controlled in that 
eggs were obtained from a captive colo­
ny of cowbirds and incubated in isola­
tion. Their postnatal social contact was 
limited to a 1- to 5-day period during 
which the individual eggs hatched into 
barn swallow (Hirundo rustica) nests, 
and the young birds were fed by their 
hosts. After being returned to the labora­
tory, they were hand-reared as a group 
without contact with adult cowbirds. 
They were placed in soundproof cham­
bers between the ages of 35 and 60 days 
(4). The birds were housed in pairs to al­
low the opportunities for social inter­
action and auditory experience thought 
to be essential for the birds to come into 
breeding condition. Chambers 1 and 2 
each housed two female cowbirds; cham­
ber 3, a female cowbird and a female red­
winged blackbird; chamber 4, a female 
cowbird and a male cowbird; chamber 5, 
a male cowbird and a male cardinal 
(Richmendena cardinalis). The light 
phase of their photoperiods was gradu-
�'---'. --
" . 
Fig. 1. Female cowbird displaying the copula­
tory response. 
ally extended during the spring months. 
The playback period began in May 
1975, when the birds were 9 months old, 
and was conducted in two phases. In the 
first phase, the females were exposed to 
recordings of normal cowbird song ob­
tained from a wild-caught male and the 
control songs of male red-winged black­
birds, meadowlarks (Sturnella magna), 
and Baltimore orioles (Icterus galbula). 
There were four playbacks spread 
throughout each day; the order of pre­
sentation varied each day but was the 
same for each bird. In the second phase, 
• 
the procedure was identical except that 
... the playback series included the abnor­
'tal song of an isolate male. The re­
\......sponse measure was the presence or ab­
sence of the copulatory posture (5). 
During the first phase, four of the six 
female cowbirds responded more often 
to normal song than to any of the control 
songs (Table 1). One of these, female 
DB, showed a dual preference for cow­
bird and meadowlark song. The fifth cow­
bird, female W, did not come into breed­
ing condition and hence did not respond. 
The sixth, female CB, was housed with 
the male cowbird and also showed no 
response. She was observed, however, 
to copUlate frequently. The variability 
across the females was probably due to 
variations in breeding condition, which 
was indicated by the numbers of eggs 
laid. 
During the second phase, four females 
demonstrated a clear preference for the 
abnormal song (Table 1). For each of the 
four birds, the abnormal song produced 
the highest rates of response, the normal 
song the second highest and the control 
songs of the other species the lowest 
rates. 
� In order to pursue this finding in great­
/. '" detail, the birds were maintained in 
\,--tSolation for another year and retested in 
\ 
the spring of 1976. Several procedural 
changes were also made. Female CB, 
which had been reared with the male 
cowbird, was transferred to a chamber 
housing a female canary 3 weeks before 
the 1976 testing. Also, two samples of 
parts of the abnormal song were used: (i) 
the "burble-burble" phrase alone, (ii) 
the "tsee" alone, and (iii) a new sample 
of abnormal song from another isolate 
male. 
The abnormal song retained its superi­
or effectiveness, and abnormal, normal, 
and partial-abnormal songs were all 
more effective than the controls (Table 
1). The differences among the conditions 
were reliable (Friedman two-way analy­
sis of variance, Xr2 = 23.3, P < .001). 
Both samples of abnormal song were 
equally effective, and one of the partial 
songs was more effective than the full 
normal song (6). Female CB, despite her 
sexual experience and her lack of re­
sponding in the earlier experiment, re­
sponded as strongly as the sexually naive 
females (7). 
Another piece of evidence attests to 
the potency of the abnormal song as a 
sexual releaser. Two wild female cow­
birds were captured and placed in the lab­
oratory in the winter of 1976. While in 
these surroundings, they heard, but 
could not see, caged male cowbirds and 
male and female free-living cowbirds. Al­
though a full series of playbacks was not 
carried out owing to the birds' nervous­
ness in captivity, the first time the abnor­
mal song was played to them, one of the 
females immediately adopted the copUla­
tory posture. In subsequent exposures, 
she continued to respond to the abnor­
mal song but never to the normal. Thus, 
response to abnormal song was not re­
stricted to females raised in isolation. 
Furthermore, the response of this female 
and that of female CB suggest that audi-
16 
12 
8 
4 
16 
1 2  
8 
4 
NORMAL 
ABNORMAL 
-
r ,.. III � \��,  
r • '. '�. O� ____ � __ "L'��' �" I ________ _ - _ II R '" , 
1----112 II< -----I 
Fig. 2. These recordings were made on a sono­
graph (Kay) with a wide-band filter and set for 
flat response and linear display. The units on 
the ordinate are kilohertz. The most striking 
difference is the rapid modulation between 2 
and 8 khz present in the abnormal but not the 
normal song. This type of modulation was not 
present in any of a sample of 522 song spectro­
graphs obtained from a captive colony of nor­
mally reared male cowbirds (10). 
tory and sexual experience with males 
does not diminish the effectiveness of the 
abnormal song. 
In summary, these data indicate that 
neither male nor female cowbirds require 
experience with adult cowbirds in order 
for species identification to occur. Naive 
females respond adaptively to male 
song; naive males produce an acoustical­
ly atypical, but highly effective, song. 
Isolate song, then, appears to have quali­
ties of a "supernormal" releaser. Why 
normal song appears less effective is as 
yet unspecified, but it may reflect the 
fact that males sing in contexts other 
than courting. The songs of males, there­
fore, may contain more complex mes­
sages indicating not only the species, 
sex, or breeding condition of the singer, 
but agonistic or territorial information as 
• 
well. A closer assessment of the function­
al properties of the songs of normally 
reared males may help to clarify these 
speculations. In addition, these data sug­
gest the need to examine in more detail 
the developmental role of self-stimula­
tion. It may be that the self-generated 
sounds of the isolate males channeled 
their song development toward an em­
phasis on the acoustic properties that 
contain the sexual message in cowbird 
song. Males reared with other males, 
however, whether age mates or adults, 
may receive acoustically more varied 
stimulation, may learn to modify their 
song components as a result of auditory 
and behavioral feedback from their com­
panions (thereby leading to a dilution of 
the purely sexual message), or both. In 
any case, these data implicate self-stimu­
lation as possibly a very important form 
of species-typical experience for cow­
birds as has been hypothesized (8). Final­
Iy, although it is tempting to label the 
cowbird's response to isolate song as id­
iosyncratic or unrepresentative of other 
songbirds, this cannot be done, as little 
comparative information exists regard­
ing the responses of other species to iso­
late songs. Such information would be 
useful in species that, like the cowbird, 
have courtship songs. Songs of a primari­
ly territorial nature may be more difficult 
to assess. 
Although these results provide one an­
swer to the question of the mechanism 
by which cowbirds identify one another, 
they do not explain how young or juve­
nile cowbirds first come to recognize one 
another. Little is known about cowbird 
social behavior, yet data from our labora­
tory indicate that cowbirds have highly 
structured dominance hierarchies and 
complex intraspecific behaviors that may 
also facilitate identification or maintain 
social integration among acquainted cow­
birds (9). The mechanism we have de­
scribed might represent an independent 
system designed to ensure identification 
during the most important context, the 
breeding season, and to enhance repro­
ductive opportunities by inducing sex­
ually appropriate behavior in the female. 
Thus, although cowbirds may have 
multiple means to identify one another, 
the presence of a mechanism geared di­
rectly for reproduction may represent a 
critical adaptation for a parasitic species. 
ANDREW P. KING 
Department of Psychology, 
Cornell University, 
Ithaca, New York 14850 
MEREDITH J. WEST* 
Department of Psychology, 
University of North Carolina, 
Chapel Hill 27514 
References and Notes 
I. H. Friedmann, U.S. Natl. Mus. Bull. 233, 6, 
(1963). 
2. G. Gottlieb, Development 0/ Species Identifica­
tion: An Inquiry into the Prenatal Determinants 
o/Perception (Univ. of Chicago Press, Chicago, 
1971). 
3. D. Lehrman, in Ethology and Psychiatry, N. F. 
White, Ed. (Univ. of Toronto Press, Toronto, 
1974) pp. 187-197; E. Mayr, Am. Sci. 62, 650 
(1974). 
4. Each chamber consisted of two concentric 
boxes constructed of elywood and sheetrock. 
Wood and acoustic ule baffles between the 
boxes were designed to be most effective be­
tween 2 and 16 khz. Suppression was greater 
than 39 db at 1000 hertz, and it increased with 
higher frequencies to greater than 50 db between 
8 and 16 khz. The interior box was a I. l-m cube, 
fabric-lined to reduce sound reflection, lighted 
by two 40-watt Vita Lite tubes and continuously 
ventilated. White noise was broadcast in the 
room housing the chambers. 
5. The songs were recorded with a dynamic mien" / phone (Uher 5(7) and tape recorders (Uher--
4000). The recordings were played through a 
driver (JBL 2420) and hom (JBL 2340). Play­
back levels were determined with a sound pres­
sure meter (General Radio 1933). The same 
sound pressure levels (SPL's) \\ere used for 
playbacks of the normal and the abnormal 
songs. At 0.35 m from the speaker along the 
axis, the SPL was 86 ± 1.5 db slow reading and 
104 + 1.5 db impulse. Control songs of the other 
species were adjusted to the slow reading. The 
A-weighted SPL inside the chamber was 50 db 
slow reading. 
6. Differences between categories of song were 
tested with t-tests. Reliable differences were 
obtained for the following comparisons: abnor­
mal one or two versus normal song (abnormal 
one: t = 11.04, P < .001; abnormal two: 
t = 9.95; P < .(01); abnormal versus control 
songs (abnormal one: t = 8.85, P < .001; abnor­
mal two: t = 19.43, P < .(01); normal versus 
control song (t = 6.09, P < .(01); the "burble" 
phrase versus the "tsee" phrase (t = 3.17, 
P < .05); and either partial song versus the nor­
mal "burble" (t = 4.22, P < .(01). There was 
no reliable difference between the two samples 
of abnormal song or between the "tsee" phrase 
and normal s.ong. 
7. Female CB's lack of response the previous year 
probably resulted from her being housed with a 
male cowbird, who repeated his sorog to her 
many times each day, thus raising her threshold 
for song responsiveness. Furthermore, her com­
panion's song was undoubtedly of superior 
acoustic quality, a factor that might also have 
diminished her interest in the recorded songs. 
8. G. Gottlieb, Psychol. Bull. 79, 362 (1973). 
9. A. King, paper presented at the meeting of the 
Animal Behavior Society, Wilmington, N.t., 22 
to 27 May 1975. 
10. D. Eastzer, thesis, Cornell University (1975). 
11. We thank G. Wilcox who designed the isolatior 
chambers, R. Chu and D. Eastzer who helped i 
the field by collecting eggs and nestlings, R. E�  
Johnston for valuable suggestions during the 
initial stages of this - project, and P. Cabe, W. 
Dilger, D. Miller, P. Ornstein, and H. Rhein­
gold for helpful comments on the manuscript. 
We are especially grateful to G. Gottlieb for 
both technical and conceptual assistance. 
Request for reprints should be sent to M,J. W. 
15 September 1976; revised 15 November 1976 
" 


Species identification in the North American cowbird: appropriate responses to abnormal song.

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Species identification in the North American cowbird: appropriate responses to abnormal song.

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