Birds may pitch songs to win mates despite urban din.
City birds sing higher-pitched songs than their country cousins. The trick could make their mating calls audible over the low roar of traffic, researchers suggest1.
Hans Slabbekoorn and Margriet Peet of Leiden University in the Netherlands surveyed 32 male great tits (Parus major) around Leiden, recording songs and measuring the level of background noise. "Some were next to a really busy road - others were in quiet residential neighbourhoods," Slabbekoorn explains.
Town tits hit the high notes, the pair found, whereas rural ones favour their lower registers. Urban birds may stand a better chance of being heard over the loud, low-frequency rumbling of engines if they use mainly high notes. Especially since they don't seem to wait for a quiet moment before performing. "They continue to sing regardless of whether cars are going past," says Slabbekoorn.
Species that can't modify their songs to compete with man-made noise could suffer as a result, he adds. Great tits expand their repertoires throughout life.
It's a plausible idea, agrees behavioural ecologist Peter McGregor of the University of Copenhagen, Denmark. But it needs proof that high notes are indeed easier for metropolitan females to hear.
One way to test this would be to play different songs to females in urban areas and see which ones elicit the greatest response, McGregor suggests. "We need to know how the songs are perceived by female great tits," he says.
The tit study builds on Slabbekoorn's earlier finding that little greenbuls (Andropadus virens) in the rainforest of central Africa sing lower songs than those elsewhere2. In this setting, he explains, the birds compete against high-pitched racket from insects such as cicadas and grasshoppers.
“We need to know how female great tits perceive the songs Peter McGregor , University of Copenhagen”
Slabbekoorn hopes to establish that different habitats have predictable influences on birdsong. He has recorded songs in all of Europe's capital cities, and compared them with those recorded in nearby countryside spots.
He believes that a bird singing under the Eiffel Tower in Paris will have more in common with its metropolitan counterparts than with a bird from the nearby French countryside. "I want to see if there's an overall pattern of 'city' and 'country' birds," he says.
Slabbekoorn, H. & Peet, M. Birds sing at a higher pitch in urban noise. Nature, 424, 267, (2003).
Slabbekoorn, H. & Smith, T. B. Habitat-dependent song divergence in the little greenbul: an analysis of environmental selection pressures on acoustic signals. Evolution, 56, 1849 - 1858, (2002).
University of Copenhagen