A study led by biologists at the University of Hawaiʻi at Hilo documents the loss of bird song complexity and the convergence of songs of three species of Hawaiian honeycreepers on the island of Kauaʻi.
The Hawaiian honeycreepers, ʻakekeʻe (Loxops cauruleirostris), ʻanianiau (Magumma parvus) and Kauaʻi ʻamakihi (Chlorodrepanis stejnegeri), have seen rapid declines in population numbers in the wild due most likely to avian malaria and habitat loss.
“We did this study specifically in Kauaʻi because it is in a real crisis mode,” says Kristina Paxton, a post-doctoral researcher at UHHilo and lead author of the study. “Their populations are crashing and malaria is probably the largest driving factor of the declines. But we are not only losing the individuals, we are losing their songs. When you go into the forest in Kauaʻi it is now quieter, and that’s losing a part of what makes the Hawaiian forest what it is. The quietness of the forest is a sign that the forest is facing challenges.”
Honeycreepers losing their teachers
Honeycreepers are songbirds who use their songs to attract mates and defend resources like food or a territory. They learn the songs in their repertoire through practice of the songs they hear from other birds of the same species.
Researchers wanted to know how the repertoire size and structural components of the songs can change in the face of a rapid decline in the size of the bird populations, especially in small, sparse populations.
The research was prompted when David Kuhn, a guide on Kauaʻi and a coauthor on the paper, made a puzzling observation. “Kuhn was having a hard time telling one honeycreeper species from one another only by listening,” explains Paxton. “It became harder to distinguish the birds by their songs in the field.”
The scientists analyzed four decades of bird song recordings from Kauaʻi and compared recordings from the 1970s to the present day. This study is the first in-depth analysis of the technical components of the honeycreeper’s song. Using a spectrogram, a visual display that shows changes in intensity at different frequencies over time, the researchers were able to show the structure and acoustic characteristics of the songs.
“For birds, their song is their culture, and a very important part of finding and attracting a mate,” explains Paxton. “In order for honeycreepers to learn their song, they have to hear it from parents and neighbors. As they hear different songs from their parents and neighbors, they are building their song repertoire. If there are too few birds, and they are too spread out in the forest, then there are fewer birds to learn from, fewer song types to learn, and also an increased chance of losing song types. This can lead to songs with fewer notes, less variety of notes, and fewer songs learned in the environment.”
The study, “Loss of cultural song diversity and the convergence of songs in a declining Hawaiian forest bird community,” was published in the journal Royal Society Open Science in August.
—By Leah Sherwood, a UH Hilo graduate student in the tropical conservation biology and environmental science program