The Ecology of A vi an Acoustical Signals
2.3 Refraction and Turbulence: Effects of Wind and
Temperature
81
Vertical gradients of wind velocity and temperature are a main source for changes
in sound levels. Only close to the ground can sound with reduced phase velocity
and strong distance-dependent attenuation (creeping wave) penetrate the shadow
region created by upward refraction. Downward refraction may cancel ground
attenuation and excess attenuation may become zero (Embleton 1996, Sutherland
and Daigle 1997).
Air turbulence, as it builds up over open areas during the day, scatters the sound
energy. Turbulence fluctuations modulate mainly highly directed sound beams at
medium distances and are insignificant for spherically expanding waves.
Backscattering also attenuates sound negligibly (Sutherland and Daigle 1997).
This contrasts with the deliberations of Wiley and Richards (1982), who
considered possible severe attenuation of higher frequencies due to turbulence.
Turbulence counteracts ground attenuation at high frequencies and long distances
even at the moderate sound amplitude and phase fluctuations associated with
temperature fluctuations of ± 1 °C (Chessell 1977, Embleton 1996). It also offsets
upward refraction by transferring sound energy into the shadow zone (Daigle et al.
1986) with scattering as the principal mechanism at frequencies above 500 Hz
(McBride et al. 1992).
2.4 Reverberation and the Effects of Vegetation and Habitat
Signals can also be degraded in their temporal structure. Particularly pulsed
signals and those with complex temporal structure are influenced by
reverberations that cause trailing echoes (Richards and Wiley 1980; Brenowitz
1986). Reverberation depends chiefly on frequency and range. Richards and Wiley
(1980) found that reverberations in forests are least in the 2-8 kHz range. Studies
that relate to habitat influences mainly concentrate on gross habitat differences,
such as those between open and closed (forest) habitats (Chappuis 1971; Hunter
and Krebs 1979; Ryan and Brenowitz 1985). Generally, birds in open habitats
produce higher-pitched signals. Particularly studies of neotropical passerines
suggest that forest-dwelling species use lower frequencies at smaller bandwidths,
and exhibit low repetition rates (Morton 1975; Richards and Wiley 1980). These
features are largely shared by species of temperate American or subtropical Old
World habitats (Wasserman 1979; Lemon et al. 1981; Wiley and Richards 1982;
Sorjonen 1986; Wiley I 991; Smith and Yu 1992). Wiley (1991) in his
comprehensive survey of eastern North American birds could demonstrate that
habitat had much stronger effects on temporal features than on spectral ones.
Suggestive evidence for habitat influences comes from variation within songs,
within species and between closely related species (e.g., Nottebohm 1975b;
Bowman 1979, Hunter and Krebs 1979; Handford 1981; Shy 1983, 1984;
2.3 Refraction and Turbulence: Effects of Wind and
Temperature
81
Vertical gradients of wind velocity and temperature are a main source for changes
in sound levels. Only close to the ground can sound with reduced phase velocity
and strong distance-dependent attenuation (creeping wave) penetrate the shadow
region created by upward refraction. Downward refraction may cancel ground
attenuation and excess attenuation may become zero (Embleton 1996, Sutherland
and Daigle 1997).
Air turbulence, as it builds up over open areas during the day, scatters the sound
energy. Turbulence fluctuations modulate mainly highly directed sound beams at
medium distances and are insignificant for spherically expanding waves.
Backscattering also attenuates sound negligibly (Sutherland and Daigle 1997).
This contrasts with the deliberations of Wiley and Richards (1982), who
considered possible severe attenuation of higher frequencies due to turbulence.
Turbulence counteracts ground attenuation at high frequencies and long distances
even at the moderate sound amplitude and phase fluctuations associated with
temperature fluctuations of ± 1 °C (Chessell 1977, Embleton 1996). It also offsets
upward refraction by transferring sound energy into the shadow zone (Daigle et al.
1986) with scattering as the principal mechanism at frequencies above 500 Hz
(McBride et al. 1992).
2.4 Reverberation and the Effects of Vegetation and Habitat
Signals can also be degraded in their temporal structure. Particularly pulsed
signals and those with complex temporal structure are influenced by
reverberations that cause trailing echoes (Richards and Wiley 1980; Brenowitz
1986). Reverberation depends chiefly on frequency and range. Richards and Wiley
(1980) found that reverberations in forests are least in the 2-8 kHz range. Studies
that relate to habitat influences mainly concentrate on gross habitat differences,
such as those between open and closed (forest) habitats (Chappuis 1971; Hunter
and Krebs 1979; Ryan and Brenowitz 1985). Generally, birds in open habitats
produce higher-pitched signals. Particularly studies of neotropical passerines
suggest that forest-dwelling species use lower frequencies at smaller bandwidths,
and exhibit low repetition rates (Morton 1975; Richards and Wiley 1980). These
features are largely shared by species of temperate American or subtropical Old
World habitats (Wasserman 1979; Lemon et al. 1981; Wiley and Richards 1982;
Sorjonen 1986; Wiley I 991; Smith and Yu 1992). Wiley (1991) in his
comprehensive survey of eastern North American birds could demonstrate that
habitat had much stronger effects on temporal features than on spectral ones.
Suggestive evidence for habitat influences comes from variation within songs,
within species and between closely related species (e.g., Nottebohm 1975b;
Bowman 1979, Hunter and Krebs 1979; Handford 1981; Shy 1983, 1984;