Behavioral thermoregulation in avian embryos: Spectrum analysis of calls in warm and cold conditions

During the last days of incubation vocalization is a form of communication between the avian embryo and the incubating parent. Commonly, embryonic calls increase when ambient temperature (Ta) deviates from the optimal range, but no information is available on whether the characteristics of the calls differ between warming and cooling. Rate of calls, power spectra (distribution of the call's energy among its frequency components) and spectrograms (time-frequency plots) were obtained in chicken embryos during the external pipping phase, in normothermia (38 °C), during progressive cooling to Ta = 27 °C (C) or progressive warming to Ta = 43 °C (W) over a short (30 min) or a long (150 min) period. Over the Ta range investigated, the embryo's oxygen consumption did not change significantly from normothermia. Number of calls, average and peak amplitudes of the power spectrum increased as cold or heat increased, according to power functions with exponents significantly higher during warming than during cooling. The spectrum frequency at peak amplitudes did not vary with Ta. Number of calls and characteristics of the power spectra (average amplitude, peak amplitude and frequency at peak amplitude) at five degrees above normothermia (43 °C) did not differ significantly from those at ten degrees below normothermia (27 °C), whether the changes in Ta occurred rapidly or slowly. The incidence of spectrograms with characteristics of 'distress' (progressive decrease in frequency with time) at 43 °C was similar to that at 27 °C. It is concluded that the vocalization of the chicken embryo has a stereotyped sonogram independent of the Ta directional changes, while the Ta-sensitivity of vocalization (changes in number of calls and amplitude per °C change in Ta) is much higher when Ta rises than when Ta drops.