A quieter forest: UH Hilo biologists document loss of bird song in Hawaiian honeycreepers on Kaua‘i

The researchers did the study on Kaua‘i because it is in crisis mode: bird populations are crashing due to disease and habitat loss, and with that, the species are losing their songs. 

By Leah Sherwood.

A study led by biologists at the University of Hawai‘i at Hilo documents the loss of bird song complexity and the convergence of the songs of three species of Hawaiian honeycreepers on the island of Kaua‘i.

The three species of Hawaiian honeycreepers, ‘akeke‘e (Loxops cauruleirostris), ‘anianiau (Magumma parvus), and Kaua‘i ‘amakihi (Chlorodrepanis stejnegeri), have seen rapid declines in their population numbers in the wild due most likely to avian malaria and habitat loss. The honeycreepers forage on insects and help to pollinate plants and disperse seeds in the forests of Kaua‘i, their natural habitat.

“We did this study specifically in Kaua‘i because it is in a real crisis mode,” says Kristina Paxton, an ecologist and post-doctoral researcher at UH Hilo, who was the 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 Kaua‘i 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.”

Paxton is affiliated with the LOHE lab, a bioacoustics laboratory at UH Hilo led by Patrick Hart, professor of biology, and Adam Pack, professor of psychology. The lab goes by the Hawaiian name LOHE, which means “to perceive with the ear” and is an acronym for Listening Observatory for Hawaiian Ecosystems.

• Related: UH Hilo’s new bioacoustics lab is helping revolutionize the field of ecology (UH Hilo Stories, Nov. 12, 2015)

Honeycreepers are songbirds (also called “passerines”) 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. The scientists 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 Kaua‘i 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, and that is what motivated us to look at how the complexity of honeycreeper songs may be changing which could potentially lead to honeycreeper songs sounding more similar.”

The scientists analyzed four decades of bird song recordings from Kaua‘i and compared recordings from the 1970s to the present day.

“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.”

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. “The spectrogram helps document and describe the types of elements or syllables that make up the song and how the birds put them together,” says Paxton.

Paxton describes the loss of song complexity in the present-day version of the song. “All three species sing a trill, a repetition of the same syllable over and over again. However, the last part of an ‘akeke‘e song often would drop in frequency, and the syllable would change. We have noticed that in a lot of instances they no longer change frequencies within their song or change the last note,” she says. “Or for Kaua‘i ‘amakihi, they often have an introductory note that starts the trill which is in a higher frequency than the main trill itself. In present day recordings, we are noticing fewer introductory notes. And as you start dropping those parts off, the songs of each species start sounding more similar because then their songs become just a trill at a similar frequency without a lot of variability in what they are putting in the song.”

Listen

Trill of the 1970s:

Trill of the present:

The study, “Loss of cultural song diversity and the convergence of songs in a declining Hawaiian forest bird community,” was published last month in the journal Royal Society Open Science. The authors are Kristina L. Paxton, Esther Sebastián-González, Justin M. Hite, Lisa H. Crampton, David Kuhn, and Patrick J. Hart.

 

Story by Leah Sherwood, a graduate student in the tropical conservation biology and environmental science program at UH Hilo. She received her bachelor of science in biology and bachelor of arts in English from Boise State University.