Chapter 1: The Decline of Hawaiian Songbird Populations

Not long ago, I discussed a concerning report revealing that North America's migratory bird population has dwindled by around 3 billion since 1970. This staggering decline raises questions about how such a significant reduction across various species may impact the social behaviors that are culturally passed down.

While many may not consider avian culture, birds indeed possess their own unique cultural expressions. For instance, the songs of songbirds are a reflection of their culture; these birds learn their melodies from family and neighbors. This vocalization is vital for social interactions and maintaining group dynamics. Moreover, female songbirds often evaluate the health and genetic fitness of potential mates based on the complexity and variety of their songs.

Surprisingly, comprehensive studies examining the acoustic intricacies and diversity of songbirds are quite rare. However, a recent study stands out as one of the first to evaluate the diversity, complexity, and similarity of songs from three endangered species of Hawaiian honeycreepers located on Kaua‘i, recorded over three different time periods since 1970.

Historically, Hawaiian forest birds thrived across the islands, yet they have suffered significant population declines and even extinctions due to habitat destruction, invasive species, and disease threats.

According to lead author Kristina Paxton, a conservation biologist at the University of Hawai‘i Hilo, “We focused our research on Kaua‘i because it is in a state of crisis. The populations are collapsing, with avian malaria being a major contributor to these declines.”

The decline of native bird populations on Kaua‘i accelerated in the early 2000s, primarily due to climate change, which facilitates the spread of avian malaria—a disease that is deadly to most Hawaiian honeycreepers. A 2016 study highlighted that population decreases among these birds were 1.4 to 5.7 times greater between 2000 and 2012 than in the preceding two decades. Concurrently, these honeycreepers have retreated to a limited, isolated area of forest, which spans approximately 40–60 km² on the Alaka’i Plateau. Conservationists warn that if current decline trends continue, we may face multiple extinctions in the coming years.

Chapter 2: Homogenization of Songs in Kaua‘i's Honeycreepers

As the population of honeycreepers dwindles, their songs are becoming increasingly similar, leading to a less vibrant acoustic environment. Dr. Paxton observed, “Walking through the forests of Kaua‘i, it’s noticeably quieter, which indicates that the ecosystem is under stress.”

Birding field guide David Kuhn, who also contributed to this study, noted the difficulty he faced in distinguishing between species solely based on their songs. Dr. Paxton explained that identifying songbirds by their vocalizations—known as “birding by ear”—is an advanced skill. This skill is crucial for recognizing species based on their songs, especially when visual identification is challenging.

“It’s becoming progressively more difficult to differentiate between species by their songs,” Dr. Paxton stated. “We are not just losing individuals; we are losing their songs as well.”

Intrigued by Kuhn’s observations, Dr. Paxton and her team hypothesized that the swift decline in honeycreeper populations has led to a decrease in the complexity and variety of songs that younger birds learn, as the number of available "singing teachers" has diminished.

To investigate this hypothesis, the researchers analyzed song recordings from three species of Kaua‘i honeycreepers over a span of 40 years: the Kaua‘i ‘amakihi, the ‘anianiau, and the ?akeke?e.

Kaua‘i ‘amakihi (Chlorodrepanis stejnegeri):

‘anianiau (Magumma parva):

?akeke?e (Loxops caeruleirostris):

Through this extensive analysis, the team sought to answer two primary questions: (a) Has the complexity and variability of the songs of Kaua‘i honeycreepers changed over time? (b) Have the current songs of these honeycreeper species become more similar over the years?

The findings were alarming: there was a notable reduction in song complexity and diversity over the 40-year study period. Additionally, the songs of the three honeycreeper species showed increasing similarities. This loss of unique song characteristics mirrored the dramatic population declines observed in these birds.

Dr. Paxton and her collaborators propose that the diminishing complexity and diversity of songs, along with the increasing similarity among species' songs, signifies a loss of culturally transmitted behaviors in these native Hawaiian honeycreepers.

“Songs are integral to a bird's culture, playing a vital role in mate attraction,” Dr. Paxton elaborated. “For honeycreepers to learn their songs, they must hear them from their parents and neighbors. With fewer birds available, the learning opportunities diminish, leading to simpler songs with less variety and fewer unique melodies.”

This research marks the first rigorous examination of song complexity loss in songbirds facing population declines. It highlights an often-overlooked cost associated with rapid population reductions: the erosion of culturally significant social behaviors like bird song. Furthermore, it prompts crucial conservation considerations regarding how these changes may impact the ability of species to find suitable mates and reproduce effectively.

Source:

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.