Use of Two Singing Modes By Hooded Warblers as Adaptations for Signalling

Abstract

Abstract Hooded warblers Wilsonia citrina use two modes of singing, repeat mode (one pattern sung repeatedly) and mixed mode (2-4 other patterns sung in irregular sequence). Intensive focal-individual sampling of 14 males documented the use of these modes of singing throughout the nesting cycle. Males of different ages (first breeding season or later) did not differ in use of the two modes. Time spent singing in repeat mode decreased markedly after acquiring a mate, but time spent singing mixed mode did not change significantly across stages of the nesting cycle. Males sang more when their neighbors sang at a distance of 25 m or more. Repeat mode increased in this situation before a male acquired a mate, while mixed mode increased afterwards. Near a neighbor (within 25 m), males avoided use of repeat mode. Nearby females before the onset of incubation evoked increased use of repeat mode. More distant, calling females elicited little response before incubation, but thereafter calling females tended to suppress all singing. Males used mixed mode proportionately more in locations nearer neighbors. There were no indications that variation in singing influenced the dates on which males acquired mates. Unmated males late in the breeding season sang persistently in repeat mode, even more than eventually mated males had early in the season before they acquired mates. These results provide support, with some reservations, for three hypotheses for the evolution of distinct modes of singing: (1) specializations for male and female listeners; (2) specializations for indicating conditional behavioral tendencies; and (3) specializations for communication in low- and high-noise situations. These hypotheses are not mutually exclusive, and all three in combination might offer the strongest explanation for the evolution of distinct singing modes in this species and other paruline warblers.