The Wilson Journal of Ornithology 124(1 ):73-8(), 2012 
MALE COMMON LOONS SIGNAL GREATER AGGRESSIVE 
MOTIVATION BY LENGTHENING TERRITORIAL YODELS 
JOHN N. MAGER III .' 4 CHARLES WALCOTT . 2 AND WALTER H. PIPER' 
ABSTRACT—We examined two critical predictions of the hypothesis that male Common Loons (Gavin irnmer) 
communicate greater aggressive motivation by increasing the number ol repeat syllables within their territorial yodels. We 
observed (from >3.500 hrs of field observations of 58 males) the probability that territorial interactions escalated from 
territorial flyovers by intraders to stereotyped 'social gatherings' to escalated tights between residents and intruders was 
positively correlated to the number of repeat syllables given by individually-banded males. Males yodcling during these 
escalated contests often assumed the upright 'vulture' posture rather than the usual 'crouch' posture, reflecting an escalated 
aggressive motivational state. Territorial pairs responded sooner and with more threat and alarm vocalizations to playback 
yodels that contained more repeat phrases. This reflected a greater willingness to attack by residents to perceived intrusions 
by males of higher aggressive motivational slate. Our study demonstrates the ability ol loons to communicate greater 
aggressive motivation by lengthening acoustic territorial threat signals, which not only may be important toi conveying 
imminent attack, but may also reflect important tactics for individuals of poorer lighting ability to deter territorial es ictions. 
Our results also raise questions regarding what receiver-dependent and receiver-independent selective factors are 
responsible for maintaining signal honesty in this non-osdne bird. Received M January 2011. Accepted 15 July 2011. 
Most threat signals communicate information 
regarding an individual's inherent, or condition- 
dependent fighting ability (Parker 1974) and/or 
willingness to attack (or aggressive motivation) 
(Maynard Smith 1982, Bradbury and Vchrencatnp 
1998. Hurd and Enquist 2001). Features that 
communicate fighting ability often reflect stable 
physical attributes, like physical size, that predict 
success in aggressive encounters (Parker 1974. 
Archer 1988). An animal’s willingness to attack is 
Men influenced by ephemeral factors such as 
health and motivational state of both the animal 
and its competitors (Maynard Smith 1982, Hurd 
ai >d Enquist 2001), Many birds benefit from 
communicating varying levels of aggressive moti¬ 
vation wuthin territorial signals to avoid conflicts 
‘hat consume energy and can cause serious injury. 
B ' r <L especially oscines. often communicate 
heightened motivation to attack by modulating 
lre queneies of their songs (Morton 1977, 1982). 
1 ‘‘hers do so by matching or overlapping songs of 
'heir competitors (e.g., Dabelsteen et al. 1997, 
v ehrencamp 2001). Recent studies indicate some 
birds - particularly non-oscines. add or remove 
Phases from their vocalizations to communicate 
l-reater aggressive motivation (e.g., Capp and 
Searcy 1991 Langetnann et al. 2000). 
Apartment of Biological and Allied Health Sciences. 
Ohio Northern University. Ada. OH 45810. USA. 
Department of Neurobiology and Behavior, Cornell 
University, Ithaca. NY 14853. USA. 
Department of Biological Sciences. Chapman Umver- 
s "> One University Drive. Orange, CA 92866. USA, 
Corresponding author: e-mail: j-mager@onu.edu 
Male Common Loons (Gavia irnmer) defend all¬ 
purpose territories on freshwater lakes by aggres¬ 
sive threat vocalizations called yodels (Sjolander 
and Argen 1972: Rummel and Goetzinger 1975. 
1978). Yodels are given only by male Common 
Loons, and are considered to be territorial threat 
signals because males aggressively approach and 
yodel at eonspecific territorial intruders (Vogel 
1995; J. N. Mager. unpubl. data). Structurally, 
yodels consist of a 3-4 note introductory phrase 
followed by a strophe of two-syllable repeat 
phrases (Fig. I). Most frequency elements of a 
yodel exhibit low intra-individual variability 
(Barklow 1979; Vogel 1995; Walcott et al. 
1999; Mager et al. 2007a, b) and each male can 
lengthen yodels by increasing the number of 
repeat phrases. Barklow (1979) observed males 
added more repeat phrases to their yodels when 
intruders wandered deeply into breeding territo¬ 
ries, and suggested that because intrusions pose a 
greater threat to resident males, longer yodels 
might communicate a greater willingness to 
attack. This hypothesis has been generally accept¬ 
ed. but has been supported by few. and only 
anecdotal, data. For example, males add repeat 
phrases to yodels when territory quality (and 
resource value) is enhanced (Mager et al. 2007b), 
which may indicate they defend higher quality 
territories more aggressively. There has been no 
published study to date that has examined critical 
predictions of this long-standing hypothesis. 
We specifically tested Bark low's hypothesis 
that yodels containing more repeat phrases reflect 
a greater motivational state by examining two 
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