The Importance of Directional Sound Radiation in Avian Vocalization

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Abstract
Certain high-frequency calls of the black-capped chickadee [Parus atricapillus] are directional in radiation pattern. Directional beaming of avian communication signals may be favored by selection for the following reasons: beaming may help to achieve an effective link between sender and receiver, especially in noisy conditions; beaming may prevent receivers other than the adreessee(s) from receiving the signal; and beaming may increase the transmission distance of the signal. The 2 factors influencing beaming (the sound-shadow effect of the head and body, and the concentration of sound waves by an acoustical horn) are most effective for high-frequency sounds. The widespread occurrence of high-pitched avian alarm calls may be partially explained by selection for directional radiation; and selection for strong attenuation of the signal by the habitat. Both factors may act to reduce the probability of the alarm call reaching a predator''s ears.