The Vocal Repertoire of the Ancient Murrelet

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Abstract
We investigated the vocalizations of the Ancient Murrelet (Synthliboramphus antiquus), a seabird of the North Pacific, to determine how the size and structure of their vocal repertoire relates to their nocturnal activity at breeding colonies. Nine distinct vocal displays were found, a repertoire of similar size to that of two other alcid species and several passerines. Most vocalizations consisted of broad band sounds with rapid frequency and amplitude modulation. The chirrup call, a short (0.5 sec) vocalization performed in many situations, did not differ in use or structure between the sexes. Chirrups had great individual stereotypy in structure, important in individual recognition of parents by young and possibly between mates. Song, a complex vocal display performed by males, was usually given from perches high in trees in the colony. Song may function in courtship and mate attraction, but apparently not for defense of long-lasting territories. Several song variants were recorded, at least one of which appeared to function as an agonistic signal to other males. Ancient Murrelet vocalizations have simple ''combinatorial'' properties in that they consist of a few acoustic elements combined in various sequences to produce vocal displays with different functions. Although there was little evidence that the Ancient Murrelet repertoire was larger than those of other alcids, the vocalizations were structurally more complex and include the song-like male advertising display. This likely relates to the nocturnal timing of social behavior of this species, which must restrict the usefulness of visual displays. We conclude that Ancient Murrelet vocal signals exhibit a number of adaptations to the unusual habits of this nocturnal, forest-nesting seabird.

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