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: