The Ecology of Avian Acoustical Signals
83
3 Communication
3.1 Birds as Receivers
Bird ears are most sensitive in the 1-5 kHz range. They can discriminate
frequencies differing by only 1-2% (Konishi and Knudsen 1979; Dooling 1982;
Saunders and Salvi 1993; Fay 1992, 1997; Langemann eta!. 1998; Bradbury and
Vehrencamp 1998). There are small but biologically significant species-specific
differences with respect to frequencies of best hearing (Okanoya and Dooling
1988; Kreutzer et a!. 1991) and frequency dependent critical noise masked
auditory thresholds (Langemann et a!. 1998). Birds have generally a narrower
hearing range than mammals and are even less sensitive than humans at higher
frequencies although the latter rank low among mammals in this respect. As in
other vertebrates, the detectability of a signal increases with signal duration
(Klump and Maier 1990; Fay 1992, 1997).
Birds discriminate among different signals not only on the basis of frequencyrelated features, such as position of center frequency, lowest and highest
frequency present, signal bandwidth, and alternations in pitch between successive
elements, but also on the basis of temporal patterns (Weary eta!. 1986; Kreutzer et
a!. 1991; Loesche et a!. 1992; Catchpole and Slater 1995). They especially
discriminate between changes in signals of their own species (Dooling et a!. 1992;
Kreutzer et a!. 1991; Loesche et a!. 1992). Motivation affects discrimination as
well. Johnsrude eta!. (1994) showed, for example, that some signal aspects remain
invariant across motivational contexts while others are more specific.
Birds recognize acoustical signals rather well even in the presence of high levels
of noise (Schubert 1971; Brenowitz 1982; Klump 1996; Fay 1997, Langemann et
a!. 1998). Frequencies of best hearing in the presence of noise may be
substantially different from those measured without noise, and may be better
predictors of species-specific signal frequencies (Okanoya and Dooling 1988;
Klump 1996; Langemann et a!. 1998). Amplitude modulations reduce masking by
noise (Klump 1996).
Birds can determine the direction of a sound (accuracies achieved by small birds
in the range of 10-30°, Bradbury and Vehrencamp 1998; Nelson and Stoddard
1998) although their ears are very close together. The mechanism birds use as
described by Lewis and Coles (1980), Coles et a!. (I 980), and Hill et a!. (1980) is
different from mammals and is based on interaural comparisons of differential
sound pressures facilitated by a direct connection between the ears. Birds locate
broad-band signals more accurately than pure tones, and frequency-modulated
tones better than non-modulated ones (Brown 1982; Klump and Shalter 1984 ).
The ability of the peripheral auditory system to process directional information
also increases signal-to-noise ratios in perception (Klump 1996).
The ability of birds to determine not only the direction of a sound source, but
also its distance has received great interest (Richards 1981 b; Morton 1982;
Morton et a!. 1986, 1998; Naguib 1997a, b, 1998; Wiley and Godard 1996).
83
3 Communication
3.1 Birds as Receivers
Bird ears are most sensitive in the 1-5 kHz range. They can discriminate
frequencies differing by only 1-2% (Konishi and Knudsen 1979; Dooling 1982;
Saunders and Salvi 1993; Fay 1992, 1997; Langemann eta!. 1998; Bradbury and
Vehrencamp 1998). There are small but biologically significant species-specific
differences with respect to frequencies of best hearing (Okanoya and Dooling
1988; Kreutzer et a!. 1991) and frequency dependent critical noise masked
auditory thresholds (Langemann et a!. 1998). Birds have generally a narrower
hearing range than mammals and are even less sensitive than humans at higher
frequencies although the latter rank low among mammals in this respect. As in
other vertebrates, the detectability of a signal increases with signal duration
(Klump and Maier 1990; Fay 1992, 1997).
Birds discriminate among different signals not only on the basis of frequencyrelated features, such as position of center frequency, lowest and highest
frequency present, signal bandwidth, and alternations in pitch between successive
elements, but also on the basis of temporal patterns (Weary eta!. 1986; Kreutzer et
a!. 1991; Loesche et a!. 1992; Catchpole and Slater 1995). They especially
discriminate between changes in signals of their own species (Dooling et a!. 1992;
Kreutzer et a!. 1991; Loesche et a!. 1992). Motivation affects discrimination as
well. Johnsrude eta!. (1994) showed, for example, that some signal aspects remain
invariant across motivational contexts while others are more specific.
Birds recognize acoustical signals rather well even in the presence of high levels
of noise (Schubert 1971; Brenowitz 1982; Klump 1996; Fay 1997, Langemann et
a!. 1998). Frequencies of best hearing in the presence of noise may be
substantially different from those measured without noise, and may be better
predictors of species-specific signal frequencies (Okanoya and Dooling 1988;
Klump 1996; Langemann et a!. 1998). Amplitude modulations reduce masking by
noise (Klump 1996).
Birds can determine the direction of a sound (accuracies achieved by small birds
in the range of 10-30°, Bradbury and Vehrencamp 1998; Nelson and Stoddard
1998) although their ears are very close together. The mechanism birds use as
described by Lewis and Coles (1980), Coles et a!. (I 980), and Hill et a!. (1980) is
different from mammals and is based on interaural comparisons of differential
sound pressures facilitated by a direct connection between the ears. Birds locate
broad-band signals more accurately than pure tones, and frequency-modulated
tones better than non-modulated ones (Brown 1982; Klump and Shalter 1984 ).
The ability of the peripheral auditory system to process directional information
also increases signal-to-noise ratios in perception (Klump 1996).
The ability of birds to determine not only the direction of a sound source, but
also its distance has received great interest (Richards 1981 b; Morton 1982;
Morton et a!. 1986, 1998; Naguib 1997a, b, 1998; Wiley and Godard 1996).