Birding at the Right Time? Listening for Climate Signals in Olympic’s Soundscapes

AUGUST 2026

From a tangle of moss-covered branches emerges a quick, whistling song. You pull out your phone and press record. Within seconds, Merlin Bird ID identifies a Pacific wren.

Sound identification tools like Merlin are taking off in the birding world—and in research, too. In a study published earlier this year, ecologists used a similar algorithm to classify bird sounds recorded over a six-month period in Olympic National Park.

Merlin runs on a machine learning algorithm developed by the Cornell Lab of Ornithology. Birders can add their observations to eBird, another Cornell Lab platform used to crowd-source bird sightings for scientific research.

NPS / Sophia Hammerle

Bird vocalizations change throughout the year following seasonal patterns of breeding and molting. Some birds are altering the timing of key life events, known as phenology, in response to warmer spring temperatures. With data demonstrating a shifting climate in Olympic, this study is the first in the park to examine links between warming temperatures and bird behavior.

“There’s evidence that birds’ breeding phenology is shifting due to climate change,” said Dr. Mary Clapp, acoustic ecologist at the Institute for Bird Populations and lead author on the study. For over 20 years, IBP has collaborated with the NPS to monitor bird populations in parks. Expert field technicians conduct point count surveys during peak breeding season each year, working their way along transects through different habitats beginning at dawn.

There’s evidence that birds’ breeding phenology is shifting due to climate change.

—Dr. Mary Clapp, acoustic ecologist at the Institute for Bird Populations

This annual data is key to the long-term study of bird populations, an ecological vital sign monitored by the NPS North Coast Cascades Network (NCCN). Results from NCCN landbirds monitoring show that bird populations have been steady or increasing in Olympic and nearby parks—at a time when North America’s breeding bird populations have decreased by 30% since the 1970s.

Changes in the breeding season could impact this crucial field work. To address this risk, IBP and NPS ecologists sought to answer a simple question. “Do we need to shift the timing of our in-person surveys to accommodate such shifts in phenology?” Clapp asked.

Across the US and Canada, survey data shows that evening grosbeak populations have declined over 90% since 1970. NCCN monitoring shows that they continue to use habitats in Olympic, North Cascades, and Mount Rainier in large numbers; the species was the third most frequently detected in 2025 counts.

Institute for Bird Populations / Eric Guzman

Listening In

To find out required only a little eavesdropping. Researchers used autonomous recording units (ARUs) to record birds across different habitats and elevations in Olympic.

Acoustic data must be sorted by species before it can offer insight into population dynamics. BirdNET, an algorithm similar to Merlin and also developed by Cornell University scientists, classified each sound by species. Researchers then modeled each population’s vocal activity from late spring through summer.

The results were clear: IBP’s monitoring technicians are in the field at the right time at Olympic National Park. The point count surveys coincided with a strong peak in vocal activity as male birds sing and defend territories during the breeding season.

Repurposing a science tool already in the field, the study used data from ARUs deployed annually in 185 sites across Olympic for northern spotted owl monitoring. Each year, NPS field technicians trek to remote areas in the park to place and retrieve the low-profile recording units—making studies like this one possible.

NPS / Sophia Hammerle

Sometimes eavesdropping means hearing things you don’t expect. In addition to the predicted peak in vocal activity during the early breeding season, researchers also found late-season peaks for several species.

“I didn’t really expect to find a secondary peak, and I certainly didn’t expect for it to be as repeatable across species as we saw,” Clapp said. “Those relationships are less well understood."

Landbirds monitoring technicians go through an intensive training period during which they learn to recognize the calls of up to 150 bird species.

Institute for Bird Populations

More fieldwork is needed to investigate the behaviors that drive these secondary sonic peaks. Are they the product of male songs as birds initiate second broods? Or do they reflect calls related to activities like foraging or migration? The answer could be different for each of the 29 species studied.

“What that really calls for is ... to actually go out into the field and observe the behavior during that time,” Clapp said.

Merlin Bird ID is Not Coming for Your Job

Moving forward, this study establishes a method for using passive acoustic monitoring to track trends in phenology. A longer-term data set will offer insight into how songbirds are responding to environmental changes, enabling comparisons among species and habitats. But first, the technology needs some work.

“These tools are really exciting because they allow us to collect data at very fine temporal scales. And they allow us to be in 180 places at once from April through September, which no army of seasonal field techs could do,” Clapp said. “At the same time, to make sense of their outputs still requires this intense rooting in natural history.”

BirdNET’s results needed significant verification from reviewers with expertise in bird sounds. Variation in species’ vocalizations toward the latter half of the season posed a challenge for the algorithm. Birds may shift from singing to calling, two distinct sounds that BirdNET recognizes with different success rates. The quality of an individual’s song can also shift with hormone fluctuations over the season—think of a teenager with a cracking voice.

Clapp said she hopes to get the process to a point where it requires less human verification. But the technology itself remains a tool in a field where on-the-ground research is indispensable.

Beyond Birdsong

When using Merlin, it’s easy to see birdsong as a binary—you either hear it, or you don't. But birdsong is a far more dynamic product of birds’ life histories and environment. Sound ID algorithms are a useful tool that comes with limitations. People remain an integral part of the science: both in the refinement of machine learning and in the field, asking and answering questions prompted by studies like this one.

NCCN long-term ecological monitoring has revealed a pattern of increased area burned by wildfire in Olympic, and larger fires burning across the NCCN. “National parks ... are such an interesting laboratory because they lack a lot of the human disturbances that are impacting wildlife populations,” said Clapp. “But there are some disturbances like climate change that national parks aren’t necessarily immune to.”

NPS

Park visitors and birders can get involved, too.

“National parks are some of the most important refuges for birds, and yet it’s where very few people actually eBird,” Clapp said. “Tools like eBird allow people to collect really valuable data. ... I would encourage people to not only fill out an eBird checklist but add behavioral observations to them as well.”

Birdsong is just the tip of the iceberg. Clapp encouraged birders to let sound IDs be the beginning of a deeper attentiveness to bird behavior—the same kind of observation that remains so crucial to fieldwork.

“Anyone who visits a park could contribute to us better understanding these very patterns of phenology,” Clapp said.

Further Information

• Read the full study: "Passive Acoustic Data as Phenological Distributions: Uncovering Signals of Temporal Ecology"
• Learn more about now the North Coast & Cascades Network conducts long-term ecological monitoring of landbirds in Olympic, North Cascades, Mount Rainier, Lewis and Clark, and San Juan Island.

Tags: olympic national park, monitoring, birds, passive acoustic monitoring, acoustic recording units, science, research, climate change, phenology, birding, nccn science