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.
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
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
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.