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Blue-throated hummingbirds produce elaborate songs extending into the ultrasonic frequency range, up to 30 kHz. Ultrasonic song elements include harmonics and extensions of audible notes, non-harmonic components of audible syllables, and sounds produced at frequencies above 20 kHz without corresponding hearing range sound. To determine whether ultrasonic song elements function in intraspecific communication, we tested the hearing range of male and female blue-throated hummingbirds. We measured auditory thresholds for tone pips ranging from 1 kHz to 50 kHz using auditory brainstem responses. Neither male nor female blue-throated hummingbirds appear to be able to hear above 7 kHz. No auditory brainstem responses could be detected between 8 and 50 kHz at 90 dB. This high-frequency cutoff is well within the range reported for other species of birds. These results suggest that high-frequency song elements are not used in intraspecific communication. We propose that the restricted hummingbird hearing range may exemplify a phylogenetic constraint. [ABSTRACT FROM AUTHOR]

Animals that rely on vocal communication must broadcast sound so that a perceptible signal is transmitted over an appropriate distance. We found that male blue-throated hummingbirds modified the amplitude of their vocalizations in response to both naturally occurring and experimenter-controlled changes in ambient noise levels. This phenomenon is known as the Lombard effect and may increase the efficiency of acoustic signalling. This study demonstrates the effect under natural field conditions and documents the first hummingbird species (Apodiformes: Trochilidae) to show this behaviour. We measured sound pressure levels (SPLs) of Serial Chip territorial advertisement calls across a natural range of ambient noise, primarily due to creeks within male territories. We found a significant correlation between the amplitude of Serial Chips and the amplitude of background noise. To test this relationship, we broadcast recordings of creek noise at high and low amplitudes while target individuals were producing Serial Chip vocalizations. We measured vocal SPLs before and during the playback. Individuals responded to changes in playback creek noise by changing the amplitude of Serial Chip production. We also measured transmission properties of Serial Chip calls through natural habitat to calculate the approximate amplitude of vocalizations at the position of the calling bird. We suggest that amplitude regulation of vocalizations contibutes to signal transmission distance along with the established relationships between singing behaviour, acoustic structure and habitat. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. [Copyright &y& Elsevier]