The Condor, Volume 109, issue 1 (February 2007)
SOUND TRANSMISSION AND SONG DIVERGENCE: A COMPARISON OF URBAN AND
FOREST ACOUSTICS
SLABBEKOORN, HANS; YEH, PAMELA; HUNT, KIMBERLY
Degradation of acoustic signals during transmission presents a
challenging selection pressure for animals dependent on vocal
communication. Sound transmission properties differ among habitats and
may drive the evolution of vocal signals in different
directions. Urban habitat is expanding worldwide and an increasing
number of species, including many birds, must now communicate around
buildings and over concrete. Urban habitats are evolutionarily new,
although to some extent they may acoustically resemble rocky habitat
such as cliffs and canyons. Neither urban nor these natural habitats
have been studied in any detail for the selection pressure they may
exert on animal communication. Dark-eyed Juncos (Junco hyemalis)
commonly inhabit montane pine forests across North America, but for
about 25 years an isolated population has been successfully breeding
in an urban environment in southern California. We investigated
potentially divergent selection pressures on junco songs, using sound
transmission experiments with artificial sound stimuli, in natural
forest habitat and in this urban habitat. Transmission properties
differed significantly, resulting in tails of reflected sound with
gradually declining amplitude in the forest and in multiple discrete
echoes in the urban environment. We expected environmental selection
in urban habitat to favor shorter songs with higher frequencies and
slower trill rates. Despite the presence of relatively short urban
songs, there was no significant shortening overall. There were also no
differences in trill rates, but we did find a significantly higher
minimum frequency in the urban junco population compared to three of
four forest populations. Although the pattern of song divergence was
not consistent and it is difficult to draw firm conclusions from this
single urban population, our transmission results suggest that echoes
could be important in shaping urban birdsong.
OFFSPRING DISCRIMINATION WITHOUT RECOGNITION: CALIFORNIA TOWHEE
RESPONSES TO CHICK DISTRESS CALLS
BENEDICT, LAURY
Accurate offspring discrimination improves parental fitness by
ensuring appropriate parental investment. In colonial avian species,
offspring discrimination is often mediated by recognition of
individual offspring vocalizations, but spatially segregated species
do not necessarily need sophisticated recognition abilities if parents
can use alternative information to distinguish offspring from
nonoffspring. I experimentally tested the hypothesis that territorial
California Towhee (Pipilo crissalis) parents use a location-based
decision rule, instead of true vocal recognition of offspring, when
deciding whether to respond to chick distress calls. Accurate
responses to offspring distress calls should be favored by natural
selection because they can have large fitness benefits if parents
succeed in chasing away potential nest predators. Responses to
nonoffspring, in contrast, may be costly and should not be favored by
natural selection. Towhee parents were presented with a series of
three playback experiments in which I manipulated the identity of the
vocalizing chick, the age of resident chicks, and the location of the
distress call broadcast. Parents showed no evidence of individual
vocal recognition and no pattern of differential response to distress
calls when offspring age differed from that of the calling
chick. Parents did, however, exhibit a significant tendency to
approach distress calls originating near their offspring more often
than distress calls originating elsewhere on their territory. These
results provide support for the evolution of an offspring
discrimination strategy based on a simple location-based decision rule
instead of true vocal recognition.
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