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Animal social behaviors are often mediated by signals that provide
information about signaler attributes. Although some signals are
structurally simple, others are temporally dynamic and multifaceted. In
such cases, exaggeration of some display components is likely to curtail
the expression of others. We quantified features of the acrobatic,
multimodal “leap display” of blue-black grassquits (Volatinia jacarina),
which appears to entail moderate-to-high performance levels in terms of
vigor and skill. We video recorded and quantified leap parameters (height,
duration, rotation angle, launch velocity, and number of wing beats) and
assessed how these parameters covaried with each other and with vocal
parameters, display rates, and body mass index. Our analyses revealed
correlations among multiple performance variables: leap height, duration,
launch velocity, and number of wing beats. Leap height also correlated
positively with song duration. By contrast, no leap parameters covaried
with rotation angle. Our analyses also revealed a trade-off in vigor and
skill-based leap attributes: birds with a lower body mass index showed a
negative relationship between leap heights and the proportion of displays
that included leaps (vs. perched vocalizations only). Our results identify
directions of display evolution subject to mechanical or timing
constraints and provide evidence that display attributes that emphasize
vigor and skill may limit one another. Our results also support a key
expectation of handicap models of display evolution, which is that costs
of display execution should be borne disproportionately by signalers of
lower quality.
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