Population decline has all Hawaii’s songbirds singing the same tune
Rapid population declines of songbirds may reduce overall song diversity and complexity, and increase similarity between learned songs, so all songbirds end up singing the same song
NOTE: This piece was a Forbes Editor’s pick.
Not long ago, I shared a report that the total population of migratory birds in North America has declined by roughly 3 billion individuals since 1970 (here). Three billion is a lot of birds. But how has that massive population decline across so many species affected social behaviors that are culturally transmitted?
The truth is that people don’t often think about avian culture, but birds, like people, do have their own cultures. For example, when we listen to songbirds, we are hearing their culture in their song. Songbirds learn their songs and calls from their family and neighbors. These sounds are critically important for social behaviors associated with same-sex interactions and for group cohesion. Further, female songbirds rely on song complexity and diversity as honest indications of the health and genetic quality of a prospective mate.
Considering how crucial song is to songbird culture, it may surprise you to learn that detailed studies of the acoustic complexity and diversity of songbird songs are very rare. But a recently published study is one of the first to analyze the diversity, complexity and similarity of songs produced by three declining species of Hawaiian honeycreepers living on the island of Kaua‘i that were recorded during three time periods since 1970.
Hawaiian honeycreeper populations are in rapid decline
Hawaiian forest birds were once widespread and common, but they all have experienced population declines or extinctions, thanks to habitat destruction and degradation, and threats posed by introduced mammals, birds, plants and diseases.
“We did this study specifically in Kauaʻi because it is in a real crisis mode”, said lead author, conservation biologist Kristina Paxton, a postdoctoral researcher at the University of Hawai‘i Hilo. “Their populations are crashing and malaria is probably the largest driving factor of the declines.”
Although already declining, native bird populations on Kaua‘i began to plummet starting in the early 2000s due to the effects of climate change, which aids the spread of avian malaria, a parasitic disease that is fatal to most Hawaiian honeycreepers (ref). For example, a 2016 study found that the population declines in Hawaiian honeycreepers were 1.4–5.7 times greater between 2000 and 2012 than was documented during the previous 20 years (ref). At the same time on the island of Kaua‘i, most honeycreepers are undergoing a rapid range contraction to a small, remote forested area comprising just 40–60km2 on the Alaka’i Plateau. Conservation biologists predict that if these rates of decline continue unabated, multiple extinctions will occur in the coming decades.
Rapid population declines mean Kaua‘i honeycreeper songs are starting to sound alike
“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,” Dr. Paxton said. “The quietness of the forest is a sign that the forest is facing challenges.”
Not only is the forest quieter, but the songs these birds sing sound more and more alike, according to David Kuhn, a birding field guide on Kauaʻi and a co-author on the paper.
“Kuhn was having a hard time telling one honeycreeper species from another only by listening,” Dr. Paxton explained. Identifying songbirds only from hearing their songs or call notes is an advanced bird watching skill that is known as “birding by ear”. Because song is critically important for attracting mates of the correct species and to warn away competitors from breeding territories, it also is an important tool for people to eavesdrop on birds to identify species when they cannot be easily seen.
“It became harder to distinguish the birds by their songs in the field,” Dr. Paxton said. “[W]e are not only losing the individuals, we are losing their songs.”
Dr. Paxton and her colleagues were intrigued by Mr. Kuhn’s observations. The team proposed that rapid declines in the density and distribution of honeycreeper populations on Kaua‘i has reduced the complexity and diversity of songs that young birds learn (ref) by reducing the number of singing teachers available.
To identify whether this is the case for Kaua‘i honeycreepers, Dr. Paxton and her colleagues analyzed songs recorded during three time periods over a 40-year time frame for three species of Kaua‘i honeycreepers: the Kaua‘i ‘amakihi, Chlorodrepanis stejnegeri; the ‘anianiau, Magumma parva; and the ʻakekeʻe, Loxops caeruleirostris.
Kaua’i ‘amakihi, Chlorodrepanis stejnegeri:
‘anianiau, Magumma parva:
ʻakekeʻe, Loxops caeruleirostris:
Based on their amassed song recordings, Dr. Paxton and her colleagues specifically tested the following questions:
- Over time has the (a) complexity and (b) variability of acoustic characteristics of Kaua‘i honeycreepers’ song changed within each species?
- Have the present-day songs of the three Kaua‘i honeycreeper species included in this study become more similar to one another over time?
Dr. Paxton and her colleagues’ acoustic analysis not only revealed a loss of song complexity and diversity over the 40-year time period, but they also found that, over time, the acoustic characteristics of the three honeycreepers’ songs became more similar to one another (Figure 1). Further, they found that this observed loss of song complexity, diversity and distinctiveness paralleled the dramatic population declines in these species.
Dr. Paxton and her collaborators propose that the reduction in song complexity and diversity and the convergence of songs between species represents a loss of culturally transmitted behaviors in these endemic Hawaiian honeycreepers.
“For birds, their song is their culture, and a very important part of finding and attracting a mate,” Dr. Paxton explained. “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 is the first time that the loss of song complexity and diversity has been rigorously examined in songbirds that are experiencing population declines. Further, this study indicates that loss of a culturally transmitted social behavior, like bird song, is a generally unrecognized cost that can accompany a rapid population decline. It also raises important conservation questions, such as how the loss of a distinct species song influences whether species-appropriate mates can find each other and reproduce at all.
Kristina L. Paxton, Esther Sebastián-González, Justin M. Hite, Lisa H. Crampton, David Kuhn and Patrick J. Hart (2019). Loss of cultural song diversity and the convergence of songs in a declining Hawaiian forest bird community, Royal Society Open Science 6(8):190719 | doi: 10.1098/rsos.190719
Originally published at Forbes on 29 November 2019.