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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 1. March 2012 
FIG. 1. Spectrogram above) indicating change in frequency (in kHz.) over time (sec) and waveform (below) indicating 
? "!L 'ff ( MPa.) over time (sec) of a typical yodel of the Common Loon. Yodels consist of two fundamental 
varv'thp “ t "“f ,ntroduc ' or y P hrase ’ follow ed *>.v a scries of two-syllable repeat phrases in which each individual can 
vary the number ol repeat phrases for each of the yodels they produce (this example has 5 repeat phrases). 
critical predictions: (I) males should give longer 
yodels during social situations in which (he 
probability of attack is greatest, and (2) territorial 
pairs should respond more aggressively to yodels 
that vary only in number of repeat phrases. 
METHODS 
Stmly Site and Resident Population.— W( 
conducted this study from 2002 to 2007 or 
~100 study lakes in Oneida County. Wisconsin 
USA, northwest of Rhinelander (45° 42' N 
89- 37' W). Loons are genetically monogamous 
and exhibit high territory and male fidelity (indi¬ 
viduals return to the same territories for 3-7 years 
on average; Piper et al. 1997a). Territorial intrusions 
occur frequently between April and July (Mageranc 
Walcott 2007) and occur most often between 043C 
and 1100 hrs each day (Mager 1995 ). 
Assessing the Effect of Social Context on 
Yodel Duration .—We assessed the impact of 
varying number of repeat phrases on behavior ol 
signalers and receivers by considering whether 
longer yodels were given during five stages of the 
intrusion process in which the probability of 
resulted* f" ** pr ° porlion °‘‘ situations that 
Pre.l ^ 
conspecifics flew over the' \eZory, % when 
conspeafics landed on the territory bu, remained 
fr ° m the res,dem m ale, (3) when intruders 
approached to within 20 m of the resident male 
bin did not engage in ‘social gatherings' that 
consist of stereotyped circling on the water's 
surlace in head-to-tail orientation that may 
include ‘splash dives’ (Sjdlander and Argen 
1972) with pair members, (4) when participants 
engaged in social gatherings, and (5) when 
intruders attacked or fought a resident. Daw 
collected between 2002 and 2004 verified that 
residents were more likely to attack an intruder 
(repeated-measures ANOVA F 3 . 246 = 1.654. P < 
0.0001; Fig. 2 ). when intrusions transitioned from 
flyovers to actual landings (paired / = 8.093. df - 
1 ^?- P < 0.0001). from landings to approaches 
(paired t = 2.178. df = 155. P = 0.0073), and 
from approaches to social gatherings (paired t = 
6.969, df = 155, P < 0.0001; Fig. 2). 
Observations of Natural Yodels.—We observed 
58 pairs of individually-banded loons between I- 
April and 31 July 2002-2007 between 0430 and 
1430 hrs (CDT). We conducted daily M 11 
behavioral-time samples (Altmann 1974) of 4-6 
territorial pairs, using ‘all-occurrences’ recording 
methods (Martin and Bateson 1993) to count 
flyovers, the extent of intrusions (e.g.. did pairs 
approach within 20 m of the intruder, did die 
residents and intruders engage in 3 social 
gathering), aggressive behavior (e.g.. surlace 
chases involving the swimming and flapping of 
wings of one individual toward another, or actual 
