However, female and male songbirds don’t sing from the same hymn book. These differences can’t always be easily detected by the untrained ear—birds’ hearing is four times more sensitive than humans—but people such as Dianne Brunton of Massey University’s Institute of Natural Resources can, or if she can’t, she can with spectrographic analysis. Brunton has done a lot of research with New Zealand songbirds, particularly bellbirds. “It’s like the females have a sentence, but vary the order of the words,” she says, “while the males seem to have different sentences.”
As with many avian species, the bellbird’s song varies from region to region. This allowed Brunton to confirm the origins of the bellbird population at Tawharanui (a peninsula north of Auckland) which self-introduced soon after the building of a predator-proof fence. It was thought likely that the birds had come from Little Barrier Island, but they also could have come from Tiritiri Matangi Island, which has a thriving population of bellbirds a similar distance away. Through song, Brunton was able to confirm they came from the former.
Bellbirds also have different songs for different occasions. As Brunton has revealed, a female bellbird reserves her most aggressive tune for the female neighbour, which she is able to distinguish from a visiting stranger acoustically. This, Brunton speculates, is not only because the neighbour more often intrudes on her turf, but also because it’s the female next door rather than a passing stranger that is most likely to woo away her mate.