A 30-year study of song birds in the San Francisco area has shown that minimum
frequency of their songs has risen over time, an adaptation to increased levels
of urban noise, say researchers Dr David Luther of the University of Maryland
and Dr Luis Baptista of the California Academy of Sciences.
The studies were conducted on adjacent dialects of the native white-crowned
sparrow over a 30 year period, from the late 1960s to 1998. The researchers
hypothesised that the growth of urban sprawl in the San Francisco area would
have become a selection pressure on the birds. 'Urban noise, which is louder
at lower frequencies increased during our study period, and therefore it should
have created a selection pressure for songs with higher frequency,' say the
researchers.
Researchers made recordings of the birds in 1969, 1970, 1990 and 1998. Three
'dialects' were recorded in 1969, but by 1998 the one with the lowest minimum
frequency had disappeared and the minimum frequencies of the other two dialects
had increased. The dialect with the highest frequency had become dominant.
'In response to high levels of low-frequency ambient noise, urban birds have
songs with higher frequencies,' says the study.
Bird song is normally stable over protracted periods of time, and the results
suggest that, as with human languages, the actual songs of birds are cultural
constructs, even if the facility for song, like the facility for language, is
a genetic adaptation. Although these sparrows have a generation of only two
years, there hasn't been enough time for there to be a genetic explanation for
the frequency changes.
Something comparable was reported among two groups of macaques in Japan. One
group was formed by 23 monkeys living on the southern Japanese island of Yakushima,
and the other group comprised 30 descendants from the same tribe moved from
the island to Mount Ohira, central Japan, in 1956. Results showed that the island
group had a tone about 110 hertz higher on average than the one taken to central
Japan.
Nobuo Masataka, professor of ethology at Kyoto University's Primate Research
Institute, said: "Differences between chattering by monkeys are like dialects
of human beings".
Monkeys on Yakushima Island have an accent with a higher tone because tall
trees on the island tend to block their voice, Masataka said. "On the other
hand, monkeys on Mount Ohira do not have to gibber with a high tone as trees
there are low," he said. "Each group adopted their own accent depending
upon their environment."
This suggests differences in voice tones are not caused by genes, Masataka
said, adding the results "may lead to a clue to the origin of human language."
