88
Hans Winkler
modulation potential (within individual variation), emotional coding (Morton
1977, 1982), and signal honesty are paramount to social communication. Some
further constraints derive from the morphology and physiology of the sender. The
possibility of eavesdropping predators and their sensory performance certainly
influences signal design as well (Klump and Shalter 1984; Klump et a!. 1986).
Although body size should theoretically constrain the ability to produce sound of
low frequency, correlations between avian body size and pitch of the sounds
produced are rather weak (Greenewalt 1968, Nottebohm 1975a).
Analyses related to this topic (Ryan and Brenowitz 1985; Lemon eta!. 1981) did
not, however, completely control for phylogenetic effects (Garland and Adolph
1994), which in the face of the observed low correlations might be rather relevant
since body size in birds is strongly dependent on phylogeny (Owens et a!. 1999).
4 Signal Design and Habitat Selection in the Acoustical
Communication of Woodpeckers
The vocal repertoire of most species-rich group of woodpeckers, the genus
Picoides (Winkler and Christie 1995), consists of more than ten categories
(Winkler and Short 1978). Differences within repertoires are large among
categories and rather consistent across species with species-specific differences
mainly encoded in the two categories Call Note and Rattle Call (Winkler and
Short 1978) that I will discuss further.
Call notes are the most frequent and general call. They are short (23-128 ms) in
overall duration, with one exception (red-cockaded woodpecker, P. borealis, see
below) simple chevron-like structures on the sonagram. Since sound energy is
clearly concentrated in the raising and falling parts, effective signal duration is
about 16 ms. Such a signal, transmitted from about 20 m height to a receiver at
I 0 m and at a distance of I 00 m, is too short to be subject to attenuation through
destructi~e interference. Its frequency is in the range of 1.7-3.6 kHz with an
average of 2.7 ± 0.14 kHz (N = 19 species) and lies roughly in the frequency
window of 1-2.5 kHz for sound transmission in forests (Morton 1986; Marten and
Marler 1977; Marten et al. 1977; Price eta!. 1988). These features indicate that
this signal type is designed for good transmission.
The rattle is used over long distances and basically consists of strings of call
notes in many species. In some species rattles consist of elements that are very
different from ordinary call notes and they are typically uttered with leading call
notes, thus representing composite signals (Fig. 3A) with an alerting and a
message component. The only condition listed in Section 1.2.2 that is not met is
the species-specific character of the leading call note (Winkler 1971, Winkler and
Short 1978). Although pied woodpeckers cover a wide range of body sizes (13160 g), there seems to be no relationship between body size and call pitch.
Hans Winkler
modulation potential (within individual variation), emotional coding (Morton
1977, 1982), and signal honesty are paramount to social communication. Some
further constraints derive from the morphology and physiology of the sender. The
possibility of eavesdropping predators and their sensory performance certainly
influences signal design as well (Klump and Shalter 1984; Klump et a!. 1986).
Although body size should theoretically constrain the ability to produce sound of
low frequency, correlations between avian body size and pitch of the sounds
produced are rather weak (Greenewalt 1968, Nottebohm 1975a).
Analyses related to this topic (Ryan and Brenowitz 1985; Lemon eta!. 1981) did
not, however, completely control for phylogenetic effects (Garland and Adolph
1994), which in the face of the observed low correlations might be rather relevant
since body size in birds is strongly dependent on phylogeny (Owens et a!. 1999).
4 Signal Design and Habitat Selection in the Acoustical
Communication of Woodpeckers
The vocal repertoire of most species-rich group of woodpeckers, the genus
Picoides (Winkler and Christie 1995), consists of more than ten categories
(Winkler and Short 1978). Differences within repertoires are large among
categories and rather consistent across species with species-specific differences
mainly encoded in the two categories Call Note and Rattle Call (Winkler and
Short 1978) that I will discuss further.
Call notes are the most frequent and general call. They are short (23-128 ms) in
overall duration, with one exception (red-cockaded woodpecker, P. borealis, see
below) simple chevron-like structures on the sonagram. Since sound energy is
clearly concentrated in the raising and falling parts, effective signal duration is
about 16 ms. Such a signal, transmitted from about 20 m height to a receiver at
I 0 m and at a distance of I 00 m, is too short to be subject to attenuation through
destructi~e interference. Its frequency is in the range of 1.7-3.6 kHz with an
average of 2.7 ± 0.14 kHz (N = 19 species) and lies roughly in the frequency
window of 1-2.5 kHz for sound transmission in forests (Morton 1986; Marten and
Marler 1977; Marten et al. 1977; Price eta!. 1988). These features indicate that
this signal type is designed for good transmission.
The rattle is used over long distances and basically consists of strings of call
notes in many species. In some species rattles consist of elements that are very
different from ordinary call notes and they are typically uttered with leading call
notes, thus representing composite signals (Fig. 3A) with an alerting and a
message component. The only condition listed in Section 1.2.2 that is not met is
the species-specific character of the leading call note (Winkler 1971, Winkler and
Short 1978). Although pied woodpeckers cover a wide range of body sizes (13160 g), there seems to be no relationship between body size and call pitch.