The distinctive vocalizations of three Kaua‘i honeycreepers are disappearing.
Once, and not that long ago, an ‘akeke‘e, or a Kaua‘i ‘amakihi, or an ‘anianiau could be identified by its song without setting eyes on the bird. Today, the calls of the three are so similar, you need visual confirmation to know which species you are hearing.
And the birds’ calls are not only growing increasingly similar. They’re becoming simpler.
The trends were reported in a paper published last year in the journal Royal Society Open Science. The lead author is Kristina Paxton, a post-doc in the Listening Observatory for Hawaiian Ecosystems lab at the University of Hawai‘i-Hilo, under the direction of Patrick J. Hart, who is also a co-author of the paper, “Loss of cultural song diversity and the convergence of songs in a declining Hawaiian forest bird community.”
As the authors note, “the reduction in song complexity and diversity and the convergence of songs not only signals a loss of culturally transmitted behaviors in these endemic Hawaiian honeycreepers, but also potential challenges to the recovery of these rapidly declining species.” In addition, they write, their study of bird songs “highlights the hidden cost to declining populations beyond just the loss of individuals that is not often considered, the loss of culturally transmitted social behaviors.”
Lost Cultural Diversity
In the late 1970s, Douglas H. Pratt, widely respected for his knowledge of Hawaiian birds, recorded the songs of Kaua‘i honeycreepers. The recordings were deposited with the Macaulay Library at Cornell University’s Ornithology Lab, which provided the tapes of the Kaua‘i ‘amakihi, the ‘anianiau, and ‘akeke‘e to Paxton and her colleagues to compare with recordings of the same species made in the early 2000s and in more recent years.
When the frequencies and syllables of songs were plotted, the results were clear: For all three species, songs recorded in the 1970s were more intricate and distinct for each species than songs recorded in the present day. The songs recorded in the early 2000s were intermediate between the earlier and later periods.
The authors describe how these trends are tied to the rapid decline in the birds’ populations in their core ranges.
At present, their range is limited to between roughly 5,000 and 10,000 hectares on Kaua‘i’s Alaka‘i Plateau, less than a quarter of their maximum range in 1968. Their populations have fallen dramatically as well, due in large part to avian malaria. Between 1981 and 2012, the authors write, within the species’ core ranges, the ‘amakihi population fell 16 percent, that of the ‘anianiau fell 17 percent, while that for the ‘akeke‘e dropped by nearly half – 48 percent. Fewer than 1,000 ‘akeke‘e individuals are now thought to exist.
“Song diversity and complexity arises through the creation of new song elements during song learning via cultural mutations … and the cultural transmission of new songs among dispersing individuals,” they write. “However, based on changes in honeycreeper densities and range contractions during the course of this study, there was a two- to sevenfold decrease in the density of available tutors for Kaua‘i honeycreepers to learn from, along with a 60-77 percent reduction in the area from which young birds could sample songs.”
With regard to the birds’ songs losing their complexity, one reason for it may be “random drift,” with the songs of all three species “consisting of one to four unique syllables repeated on average over nine times … The loss of song complexity has led to present-day honeycreeper songs containing fewer unique syllables and fewer frequency changes within and among syllables,” the authors say.
Another reason could be that the birds of one species incorporate elements from the songs of the other honeycreepers as the young birds have fewer and fewer older birds of their own species to learn from. This could help explain why, even though densities of all three species have declined, density estimates for the ‘akeke‘e and Kaua‘i ‘amakihi are the lowest – and their songs were also the most similar, the authors write.
Impacts on Reproduction
The cultural losses could well impact the survival of all three species. “While the consequences of population declines are typically thought of in terms of the loss of genetic diversity,” the authors note, “the disruption or loss of learned traditions can also affect species persistence, particularly when social learning is an important driver of behaviors that influence survival and reproduction.”
“The complexity of vocal signals such as song can serve as an honest signal of an individual’s quality as well as the viability of a population,” the authors write.
Hart elaborated on that point. In the field of behavioral ecology, he said, “an honest signal is one that takes energy to perform. The ability to sing well may be an honest signal of a bird’s fitness – it’s doing well, eating well, had a good upbringing, grew up in a large population. It’s a signal that they can’t fake and can be used by other individuals to judge their fitness.
“Up to 30 percent of a bird’s brain capacity has been shown to be related to song production and interpretation. It’s reflective of their early years; if they grew up in a good environment and are fit, they’ll have a more complex, desirable song.”
But why should the diminished richness of songs possibly lead to lower population growth?
“It’s hard to say for sure,” Hart said. “It just may be that it doesn’t entice mating as much. Just like with humans, the song is supposed to entice the female to want to come and mate.”
What’s more, Hart and his co-authors suggest the impoverishment of the honeycreepers’ songs and their convergence “could lead to a breakdown in species barriers.”
Does that mean that hybridization is possible among these species?
“Well, yes, I think it is definitely a possibility,” he said. “We know that ‘i‘iwi and ‘apapane can hybridize. Kaua‘i birds are closely related. If a young ‘akeke‘e mostly learns songs of ‘amakihis, then it might be attracted to an ‘amakihi song when it’s an adult, since that’s what it knows more.
“They’re learning each other’s songs, which leads to a higher potential for hybridization,” he said. Or, he said, alternatively, a male and female from different species could pair up but fail to have any offspring at all.
“Neither possibility is good.”
Songs of the ‘Alala
The loss of song complexity has been observed in other species, including the ‘alala, the Hawaiian crow. Ann Tanimoto, a graduate student working closely with Hart, examined differences in the vocal repertoire of ‘alala in the wild, recorded in the early 1990s, from those held in captive breeding aviaries.
“They lost a lot of elements,” Hart said. “Whole things like territorial songs, things like that, had just disappeared in the aviaries. Now we’re tracking the individuals released into the wild and how their songs are becoming much more rich and complex again.”
Hart said one of his grad students continues to go twice a week to the area where the ‘alala were released, “videoing the ‘alala, cataloguing all their vocalizations, looking at dominant and non-dominant birds, and comparing it to the aviary birds.”
In another study, students from Hart’s lab compared the songs of ‘amakihi in a low-elevation population on the Big Island with those of populations in mid- to high-elevation sites. “The reduced complexity of ‘amakihi songs at low-elevation sites is most likely shaped by the effects of habitat fragmentation and a disease-driven population bottleneck associated with avian malaria and maintained through isolation, localized song learning and sharing, and cultural drift,” wrote authors Joshua Pang-Ching, Kristina Paxton, Eben Paxton, Adam Pack, and Hart (“The effect of isolation, fragmentation, and population bottlenecks on song structure of a Hawaiian honeycreeper,” Ecology and Evolution, 2018). (The Hawai‘i ‘amakihi is a different species from the Kaua‘i ‘amakihi.)
For more information on the work Hart and his colleagues are doing, visit the LOHE website: www.lohelab.org.