SUMMARY
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 par uline warblers.
CONCLUSION
Many of the hypotheses for the use of different singing modes by hooded warblers and other parulines are complementary.   Repeat mode could serve for attraction and stimulation of females, both situations likely to be noisy from the signaler's point of vi ew and thus to favor stereotyped signals.   Mixed mode might serve primarily for interactions with established neighbors at relatively short range or in locations where short-range interaction might develop, situations in which greater variability of signa ls would allow assessment and negotiation. In this combined view, the receiver primarily addressed by a singer, whether potential mates or rival males, would determine the optimal form of signalling, either more stereotyped and recognizable or more variable and informative.   To obtain responses f rom females, the former might serve best; to interact with established neighbors, the latter might serve best. In natural circumstances, attraction and stimulation of unfamiliar females is often likely to require long-range or, from the signaler's point of view, noisy communication but little communication of behavioral tendencies;   stereotyped signals, like the hooded warblers's repeat mode, would then offer advantages. In many parulids, song patterns used in repeat mode have wide distributions across populations (Kroodsma 1981), a feature that would also have advantages for communication with unfamiliar individuals in noisy circumstances.   Negotiation and reaffirmation of relationships with familiar territorial neighbors is often likely to have just the opposite requirements;   variable signals, like singing in mixed mode, would then have advantages.		In the interests of fostering further investigation of adaptations for signalling in high- and low-noise situations, we close with some specific predictions.   The complexity of natural situations will often frustrate simple tests, but these predictions should at least serve to clarify the issues. Males distant from neighbors should use less variable singing than males close to neighbors. Unfamiliar rivals should evoke less variable singing than familiar neighbors. When females choose mates quickly after their arrival, they should use less variable signals; when they have more time to assess potential mates, they should use more variable signals. Males should use less variable singing in interactions with unfamiliar females, for instance when seeking mates or extra-pair copulations, than in interactions with familiar ones.

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