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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 1. March 2012 
quence of actions that provide more reliable 
information about fighting ability but entail 
greater costs consistent with sequential-assess¬ 
ment models of contests (Enquist and Leimar 
1983. 1987). Flyovers provide intruders informa¬ 
tion about territory quality (Piper ct al. 2006). but 
if a resident yodels, it also provides the prospect¬ 
ing loons information about territory occupancy, 
the resident male's identity (Walcott et al. 1999, 
2006; Mager et al. 2010), and fighting ability 
(Mager et al. 2007a). Flyovers constitute a lesser 
threat to a resident male versus a close intruder. 
Males yodel less often (-1 of every 3 flyovers), 
and always assume the low-risk, low-cost 
crouch posture. Not only is the “crouch" posture 
less costly physiologically to assume versus the 
"vulture" posture, but it also places the signaler in 
a less vulnerable posture for receiver attack 
(although no attack could possibly occur during 
a flyover) and a more effective posture to transmit 
information over great distances (McIntyre 1988). 
It is likely that both residents and intruders 
obtain more reliable or perhaps additional infor¬ 
mation about each other’s fighting ability and 
motivation when intruders approach within 20 m 
of the resident pair and engage in social 
gatherings. Residents, in response, yodel more 
often (yodeling approximately once for each such 
intrusion), and give more repeat phrases in each 
yodel. Longer yodels likely provide intruders 
more reliable information about the identity, 
health, and vigor of the signaler. However, this 
information comes with considerable costs to the 
signaler. Physiologically, longer yodels necessar¬ 
ily are more energetically expensive to produce. A 
male that produces longer yodels must be able to 
endure these costs. Producing longer yodels could 
also place the signaler at a greater risk of 
retaliation from the receiver. Our results have 
shown that longer yodels evoke quicker responses 
by and more tremolos (signaling greater alarm) 
and yodels (signaling greater threat) from con- 
specific receivers. These receiver-independent 
and receiver-dependent costs of producing longer 
yodels ultimately could prevent males from 
luffing about their motivation, and act as 
seective processes to maintain signal honesty 
(Vehrencamp 2000. Hurd and Enquist 2001) We 
fee both types of easts are likely more than offset 
by the significant costs of lighting, as aSfire „ ivc 
pursutts end fig hls (Rumn t el Li S 
1975, McIntyre 1988, Pnruk 2006) ettn lead to 
resident s evict,on or death (McIntyre 1988. pJruk 
1999. Piper et al. 2008). However, more detailed 
analyses are needed to consider these factor, 
regarding fitness costs associated with pnxiucing 
longer yodels. 
We found loons yodel in the vulture posture 
most olten when contests escalate into social 
gatherings and actual chases/fights. The vulture 
posture may amplily/reinlorce the communication 
of a higher motivational state and/or it may sene 
as u separate signal of aggression (Rummel and 
Goetzinger 1978, Johnstone 1996). The posture is 
clearly more costly for males to assume, as they 
must vigorously paddle their feet to "stand" 
upright upon the Water surface and extend dieir 
wings outward while ‘pointing’ their bills toward 
the potential receiver (McIntyre 1988). 
We provide empirical support of Barklow's 
(1979) contention that male Common Loons 
signal greater aggressive motivation hy adding 
repeat phrases to territorial yodels. However, 
signaling motivation by lengthening territorial 
vocalizations is not the conventional way birds 
signal greater motivation (e.g., Morton 1977, 
1982: Ripmeester et al. 2007). Many species that 
sing longer songs tend to communicate greater 
lighting ability (e.g., Lambrechts and Dhondt 
1986, 1987; Appleby and Redpath 1997). In 
contrast, male Common Loons communicate 
fighting ability through the dominant acoustic 
frequencies ot yodels: larger males of better 
fighting ability produce lower-frequency yodels 
and smaller males of poorer fighting ability 
produce higher-frequency yodels (Mager et al. 
2007a). Past studies have shown that males that 
signal lower fighting ability produce yodels with 
more repeat phrases (Mager et al. 2007a). We 
believe that males of poorer fighting ability must 
be more aggressively motivated (and produce 
longer yodels) to successfully defend their 
territories based on our empirical support of 
Barklow's (1979) hypothesis that males commu¬ 
nicate greater aggressive motiv ation by lengthen¬ 
ing yodels. This strategy may not be limited to 
loons, as other species similarly lengthen threat 
signals when more motivated to attack. Many 
examples are non-oscines (e.g.. Martin-Vivaldi el 
al. 2004), including some waterbirds (e.g.. Nelson 
1984, Ewins and Weseloh 1999. Mow bray et al. 
2002) living in open water environments where 
acoustic scatter and absorption are minimal. This 
raises the question regarding those selective 
factors associated with signal production, propa¬ 
gation. and reception among birds that call over 
