Heart rate response in great tit nestlings (Parus major) to an alarm call

Nine great tit nestlings were exposed to two synthesized auditory stimuli: a replica of a high-pitched alarm call, a part of the species' vocal repertoire, and a control stimulus, identical to the alarm call with respect to amplitude, duration, frequency span, and frequency modulation but having a lower frequency range (8.6--8.1 kHz vs. 4.5--4.0 kHz). The alarm call released a defensive reaction characterized by (a) a prolonged heart rate (HR) acceleration, (b) increased electromyographic activity, and (c) behavioral immobility. The control stimulus elicited a transient HR acceleration and a muscular reaction indicative of a startle reflex. It is concluded that high pitch is a critical feature of the alarm call. It is proposed that the impact of the alarm call is a consequence of the nestlings' primary experience with sounds in the low to middle frequencies, in particular, with the parental feeding call and their own begging call. Embryological maturation of auditory responsiveness, starting with a sensitivity to low-frequency sounds, may contribute to the establishment of a basic tendency to approach low-frequency stimulation and show aversive reactions to high-frequency stimulation. In contrast to earlier interpretations, it is suggested that a strong accelerative HR response is not associated with a reduced stimulus sensitivity per se but rather with an altered distribution of attentiveness.