We examined two critical predictions of the hypothesis that male Common Loons (Gavia immer) communicate greater aggressive motivation by increasing the number of 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 intruders to stereotyped ‘social gatherings’ to escalated fights between residents and intruders was positively correlated to the number of repeat syllables given by individually-banded males. Males yodeling 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 state. Our study demonstrates the ability of loons to communicate greater aggressive motivation by lengthening acoustic territorial threat signals, which not only may be important for conveying imminent attack, but may also reflect important tactics for individuals of poorer fighting ability to deter territorial evictions. Our results also raise questions regarding what receiver-dependent and receiver-independent selective factors are responsible for maintaining signal honesty in this non-oscine bird.