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Territorial intrusions are related to the peak frequencies of yodels given by male common loons.

Mager, J., Walcott, C., Backus, J., Nebgen, E., and D. Norris
Ohio Northern University
Ecology, Animal Behavior, Ornithology
In addition to their identity, male Common Loons (Gavia immer) communicate condition-dependent fighting abilities through the dominant frequencies (better condition individuals produce lower-frequency signals), and a heightened aggressive motivational state by lengthening their territorial yodels by adding two-syllable repeat syllables. Through an acoustic playback experiment, we experimentally examined whether the dominant frequencies of either resident or non-resident yodels affected an intruder’s (i.e., conspecifics flying over a territory) propensity to land upon a breeding territory. Among 33 territories from which we broadcast yodels having three repeat phrases, we found that potential intruders were more likely to land on territories when we broadcast higher-frequency yodels to them. These results provide the first empirical support of the idea that conspecific prospectors consider the dominant frequencies of a yodel in deciding whether to land upon a potential breeding territory, and support observational studies that infer that males that communicate poorer fighting ability can hold onto breeding territories by communicating greater aggressive motivation.
Behavior 2011: The Joint Conference of the Animal Behavior Society and International Ethological Conference
Biology and Allied Health