The Effects of Environmental Noise on Communication in Birds

When noise masks the biologically important signals of birds in the wild, and interferes with their ability to communicate effectively, it surely has a detrimental effect on their normal behavior and breeding biology. We know much about the hearing sensitivity of birds from laboratory tests with simple stimuli, but almost nothing about how they perceive the vocalizations of their own species in noise. Laboratory data on the effect of noise on the masking of tones can be combined with the scattered information available on other factors influencing the distance over which biologically meaningful signals can be used under natural conditions, such as the location and source intensity of a singing bird, the sound-attenuating and masking characteristics of the environment, and the location and hearing sensitivity of the bird receiving the signal. From such data we can obtain a rough estimate of possible communication distance, but without more information we have only a very crude idea of the effect of environmental noise on the perception of vocal signals in nature. There are virtually no data published on the effects of different types of noises (e.g. traffic or aircraft noises) on the perception of species-specific vocalizations. Such laboratory data are critical for understanding the effect of noise on acoustic communication and for developing reasonable guidelines for noise abatement.

The figure at right illustrates the theoretical maximum distance that one song sparrow can detect the song of another bird in this species. The dependent variables represented here are different source intensities, assuming an excess attenuation of 5 dB/100 m. Background noise level is given as overall SPL (C-weighted), spectrum level (per cycle energy distribution over the entire band of noise), and the amount of masking assuming a critical ratio of 26 dB (as measured in the laboratory). We know from laboratory studies that the threshold in the quiet for a 2 kHz pure tone is about 0 dB SPL for song sparrows. Critical ratio data for this species shows that background noise levels must be at least 26 dB below the power in a 2 kHz pure tone in order for the tone to be detected by song sparrows (Okanoya & Dooling 1988, 1990). Thus, for a noise spectrum level (energy per Hertz) of -26 dB, masked threshold is the same as absolute threshold in the quiet, and no masking occurs. If the spectrum level of the background noise is 0 dB SPL, however, the threshold of the signal is raised to 26 dB SPL, causing 26 dB of masking. The spectrum level of the background noise can be calculated from the overall sound pressure level, and depends on the bandwidth and distribution of the noise. For a typical sound level meter reading of noise in the frequency band 0 - 10 kHz, an overall SPL of 40 dB is equivalent to a spectrum level of about 0 dB if the noise is flat.