Mager el al. • LOONS SIGNAL HIGH AGGRESSION WITH LONGER YODELS 
77 
6.0 
Flyover Distant intruder Close intruder Social gathering C base Fight 
FIG. 4. Mean (± SE) number of repeat syllables per yodel given by resident male Common Loons under social contexts 
associated with greater levels of agression. 
of study), and tended lo give proportionately more 
vulture yodels (33/130 or 25.4% of yodels) than 
residents (84/1.495. or 5.6% of yodels). 
Males had 3.6 ± 0.1 repeat phrases/yodel 
(range = 0.0-20.0). although the number of 
repeat phrases each male produced was a function 
of the number of yodels heard (y = 3.320 log x + 
1.064; r = 0.613; P < 0.0001). Most yodels 
(S6.l%) contained between two and five repeat 
phrases (mode = 3). There was individual 
variation in the number of repeat phrases given 
hv males per crouch posture yodel (ANOVA 
^ 7.129 = 3.270. P < 0.0001). but not per vulture 
Posture (ANOVA F 2S , 12 = 1.431. P = 0.26) 
.'"del. not controlling for the different contexts of 
territorial intrusion. 
Males had 42.9% more repeat phrases per yodel as 
intrusions escalated from flyovers and landings to 
wia| gatherings and physical attacks (Fig. 4). Males 
^ lun ger yodels when flyovers escalated to landings 
'Nred 1 = 3.158, df = 39, P = 0.0031); did not give 
’significantly longer yodels when intruders came 
U|1 hin 20 in of the male (paired / = 0.025, dl = 
P ~ 0.98); and gave longer yodels when close 
"'tnisions transitioned into social gatherings (paired 
t = 3 .I 68 . df = 10, P = 0.01), but not when social 
gatherings transitioned into chases and/or fights 
(paired t — 0.09, df = 17, P — 0.93). 
There was a clear difference in how pairs 
responded vocally to broadcast yodels that differed 
in the number of repeat phrases. Pairs vocalized 
over twice as quickly, and gave almost tour times 
as many tremolos to longer yodels. Resident males 
gave four times as many yodels to seven-repeat 
yodels versus one-repeat yodels (Table I ). 
DISCUSSION 
Individuals that experience frequent territorial 
contests often produce graded threat displays to 
communicate a heightened state of aggressive 
motivation (van Rhijn 1980, Bradbury and 
Vehrencamp 1998). Populations experience fre¬ 
quent conspecific intrusions that escalate into 
potentially lethal confrontations (Piper et al. 
1997b. 2000), and it is not surprising that male 
Common Loons, as Barklow (1979) proposed, 
lengthen territorial yodels to communicate greater 
aggressive motivation. 
Territorial interactions between residents and 
intruders among loons proceed through a se- 
T«U I. Vocal responses of territorial parrs („ - 3S> of Common Loons to playback yodels having one. four, and 
ieven re Peai phrases. 
Response 
One repeal 
Four repeats 
Seven repeals 
Individual growth 
model Fi n j 
p 
bivariate response 
^ ^0Cali7fltinnc 
9.68 ± 2.96 
17.80 ± 5.06 
26.76 ± 4.85 
6.89 
0.0099 
* Tremolos 
5.92 ± 2.79 
14.72 ± 4.91 
20.08 ± 4.53 
5.26 
0.0237 
* Wails 
3.95 ±1.14 
2.44 ± 0.79 
4.54 ± 0.91 
0.12 
0.7288 
# Yodels 
0.18 ± 0.08 
0.36 ±0.14 
0.73 ± 0.20 
6.51 
0.0121 
patency before 1st vocalization (sec) 
128.20 ± 52.72 
131.72 ± 49.12 
62.87 ± 17.27 
12.37 
0.0006 
