For decades scientists have warned of the decline of Hawaii’s colorful forest birds, which have long been victims of avian malaria and other environmental threats.

Now it appears the drop in native forest birds is taking a toll on their songs.

A scientific study that examined the historical recordings of three native honeycreeper species in the forests of Kauai over the past 40 years found a dip in the complexity and variety of songs by two of the species.

The study, led by University of Hawaii-Hilo researcher Kristina Paxton, reveals a quieter forest with fewer distinct honeycreeper calls, more simplistic trills and increasingly similar sounds across the species.

Paxton, who works for UH’s Department of Tropical Conservation Biology and Environmental Science, said it’s a disheartening discovery.

“When we got the results, it was exciting to see the differences (over time),” Paxton said. “But when you look at it, it’s definitely something not to be excited about.”

Known for their bright feathers and canarylike songs, Hawaiian honeycreepers are famous for “adaptive radiation,” in which a wide variety of species evolved to fill niches throughout their island environments.

The population of six Kauai honeycreeper species — among them the more common apapane and iiwi — have dropped an average of 65 percent from the 1970s, and those that remain have been forced into higher elevations in the northwest part of the island, Paxton said.

As the climate has warmed, malaria-infected mosquitoes are also reaching higher elevations, and at least one recent study predicts multiple extinctions if global warming continues at its current pace.

In the meantime the Kauai Forest Bird Recovery Project is working to improve the forest habitat, and the San Diego Zoo Global Hawai‘i Endangered Bird Conservation Program is raising Kauai’s akikiki and akekee honeycreepers at its Big Island and Maui captive-rearing facilities.

“It’s a beautiful sound to hear in the forest,” Paxton said of the honeycreepers.

Over time, however, scientists and conservationists have reported a much quieter forest on Kauai.

“It can be depressing to go into the forest. It doesn’t have the rich soundscape it used to have,” Paxton said.

It pales in comparison with the Big Island’s Hakalau Forest National Wildlife Refuge, “where you can barely hear yourself with all the songs.”

Paxton, who is also conducting genetic research to discover how the birds might be able to resist malaria, said she wondered what impact the dwindling population of native honeycreepers on Kauai was having on the bird calls in the forest.

She and her colleagues hypothesized the rapid declines in honeycreeper density and distribution would result in changes to the acoustic structure of the songs.

Paxton said the songs are important for mate choice in the bird world and are associated with greater reproductive success. Females generally prefer males with complex songs.

“The male is saying, ‘This is who I am — a healthy mate to mate with,’” she said.

For the study, researchers examined 11 acoustic characteristics of songs recorded during three time periods — the 1970s, early 2000s and present day — for the akekee, anianiau and Kauai amakihi.

The earliest recordings used for the study came from the 1975 expedition to the Alakai wilderness led by former UH-Manoa zoology professor Sheila Conant. Also on the trip was Hawaii bird expert H. Douglas Pratt, who made the recordings.

It was one of the last trips to occur when native forest birds were relatively plentiful on Kauai. Only one extinction was known over the previous two centuries, but within the next decade, four species observed on that trip would vanish.

The results of the latest study indicate the acoustic characteristics of present-­day songs of Kauai amakihi and anianiau have changed with reduced complexity, a narrower range of frequencies across songs and reduced diversity compared with songs from the 1970s.

Paxton said the best explanation for the change over time is the severe reduction in population size.

Because birds learn their songs from their parents and neighboring birds, fewer birds in an area results in fewer songs for young birds to learn and a quieter soundscape from which to build their song repertoire.

“It’s becoming quite challenging to distinguish between species,” she said.

Paxton’s study collaborators were UH-Hilo biology professor Patrick Hart; Lisa Crampton and Justin Hite of the Kauai Forest Bird Recovery Project; David Kuhn of SoundsHawaiian; and Ester Sebastian-Gonzalez of Miguel Hernandez University in Spain.

The paper is expected to be submitted for publication within the month.