Abstract
Aggressive signaling is integral to territory defense in most animals that defend territories. Territorial signals may communicate a territory owner's level of aggressive intent, announce its fighting ability, or simply proclaim ownership. Song sparrows (Melospiza melodia) are songbirds whose territorial signals have been especially well studied. In this species, multiple territorial displays function as conventional signals of aggressive intent. The most reliable signals of aggressive intent are low-amplitude soft song, an auditory signal, and wing-waving, a visual signal. In a western population of song sparrows, these two displays are part of a hierarchy of signals that allow song sparrows to communicate a graded series of threats, starting at the low end with repertoire matching, escalating to song-type matching, and culminating in soft songs and wing waves. In this and another western population, levels of song sharing between males are high and sharing level is positively correlated with territory tenure. By contrast, in an eastern population, song-type matching apparently does not function as an intermediate threat, sharing levels do not correlate with territory tenure, and males rarely share whole songs. Although these two patterns are internally logical, why particular populations exhibit one pattern and not the other is unclear. In addition to communicating threat, song sparrow songs also allow individual recognition of the singer by nearby residents. The ability of neighboring males to recognize each other by song enables them to maintain dear-enemy relations, in which established neighbors show mutual forbearance. This limited level of cooperation is sustained by both direct and indirect reciprocity.
