Listening to Alaska’s Parks: Bird Detectors, Bat Calls and Whale Noise

National Park Service (NPS) scientists in Alaska are using sound recorders to learn about wildlife that is hard to see: songbirds across a season, bats that call above human hearing, and humpback whales under the water. A 2021 Alaska Park Science article by Davyd Betchkal, Paul Burger and Chris Gabriele describes three examples and what park managers do with the results.

Why record sound

Animals are surrounded by sounds from the Earth's surface, from other animals and from people's vehicles and machines. The NPS protects acoustic environments, meaning all the sounds at a place over time plus the landscape's capacity to carry them. Audio recorders, invented in 1877, let scientists listen in remote places for months at a time, hear above and below the range of human hearing (ultrasonic and infrasonic frequencies), and listen underwater.

Bioacoustic technique depends on the frequency of an animal's sound and the medium it travels through; the article covers three acoustic domains. Image from NPS's page.

On land: letting software find the birds

Analysis can be careful listening with headphones, which is slow but detailed, or numeric approaches such as acoustic indices, which are faster and flexible and show promise for estimating biodiversity. For particular species, detection by computer is now close to human accuracy. Programs such as Raven, Kaleidoscope, Ishmael and warbleR let users build their own detectors, and machine learning can build high-quality ones from large libraries of example sounds.

One tool made for parks is Avian Acoustic Discovery: Alaska, an open-source detector trained on Alaskan bird dialects. Its current release covers 20 species. Some, like the white-crowned sparrow, are abundant; others, like the orange-crowned warbler, are declining rapidly. Most are migratory, though a few, like the willow ptarmigan, stay all year. Automatic detection lets staff track biological events across a season or several years. Glacier Bay National Park and Preserve has used recordings to monitor the timing of bird migration since 2012, and a Central Alaska monitoring program has used recorders to document when wood frogs breed since 2011.

The article gives a sense of the gain. In 1974, researchers told apart groups of Gambel's white-crowned sparrow by their songs, using a spectrograph that would cost over $13,000 in 2019 dollars. Now any computer can extend that work, and a recording with a GPS coordinate and a sparrow's song becomes a population-biology study site. The authors add that publicly funded recordings should be accessible to researchers, through archives such as NOAA's National Centers for Environmental Information, Cornell's Macaulay Library and Xeno-canto.

Ultrasound: bats

Bat calls are ultrasonic, so people rarely hear them. Alaska has six bat species, with most of the diversity in the southeast, and in the south-central and interior regions only the little brown bat has been found. Bats are small, fast and out only a few hours a night, so direct observation is hard. Their echolocation calls can be picked up by ultrasonic microphones and matched to libraries of calls, at least to major species groups, which also shows how active they are through the night and the year.

A little brown bat captured in Klondike Gold Rush National Historical Park in 2014. Image from NPS's page.

Little is known about Alaska bats' habitat, prey, life cycle and migration. Parks have found little brown bats in structures, sometimes overwintering outside the parks, but not much is known about natural habitats. That matters for decisions such as leaving old-growth stands and snags in place because bats roost in them. Some acoustic and recapture data suggest that bats in interior Alaska move to the coast in fall, which would give them more chances to meet southern populations and carry White Nose Syndrome north. That fungal disease has spread west and north since 2002 and in some cases has wiped out more than 90% of a bat colony. The article notes that bats provide nearly 4 billion dollars a year in natural pest control.

Bat monitoring locations in Alaska, 2015 to 2019, and the road-based route in Wrangell-St. Elias National Park and Preserve. Image from NPS's page.

Since 2015 the team has placed 82 detectors in 10 parks, for over 4,500 nights of monitoring across coastal, forest and tundra habitats. It also ran 4 driving transects in Wrangell-St. Elias and several hiking transects along the Chilkoot Trail in Klondike Gold Rush National Historical Park. Three findings stand out:

• Bats are most active in the warmer months, but they feed intensely for much shorter periods than predicted, frequently only about two hours a day. The authors suggest the short or absent darkness in northern Alaska makes it hard to avoid predation by owls and others.
• Bats were active over saltwater in Glacier Bay and Kenai Fjords National Parks. It is unclear whether they were foraging or crossing open space between feeding and roosting.
• A late-fall spike in calls on the coast at Klondike Gold Rush roughly matches the drop in calls inland, hinting at migration to the coast. Netting and banding with Alaska Department of Fish and Game and Canadian researchers is meant to check this, but there are no results yet.

Under water: Glacier Bay

Bioacoustic studies have helped show how humpback whales communicate and rely on Glacier Bay's underwater soundscape. Image from NPS's page.

Humans seldom experience Alaska's underwater soundscape directly, yet many ecologically important animals depend on it. In Glacier Bay the concern has long been that vessel noise could disturb whales and other wildlife, so the studies describe natural and human sounds and how vessel rules, such as quotas and speed limits, change them.

The monitoring has grown step by step. Researchers first measured the bay's underwater sound in the early 1980s with portable hydrophones lowered from a boat. In 2000 park scientists and U.S. Navy acousticians installed a calibrated hydrophone near the bay's mouth, joined by a 5-mile (8-km) cable to a computer at park headquarters. It gave the first full description of sound sources (wind, rain, animals and vessel engines), how often they occur and their pitch and duration. In a 2015-2016 study with Oregon State University, an array of hydrophones located each calling animal, measuring how loud harbor seal roars and humpback whale calls are and showing how both change in the presence of motorized noise. In summer 2019 and 2020 a small autonomous recorder repeated the 1980s measurements to see how the sound has changed.

Autonomous recorders that work for weeks or months before retrieval are now the right tool for most applications in Alaska parks. Combining the sound data with a computer model, the team found that cruise ship speed was the dominant factor in how much noise whales are exposed to in Glacier Bay, and that exposure is lower when ships are scheduled to arrive in sync. In quiet conditions two whales can hear each other at about a mile and a half (2.3 kilometers); with typical vessel noise during the tourist season, that distance shrinks to about 75 yards (70 meters) or even less. A related study found whales call more loudly as background noise grows, and are less likely to call at all in the presence of vessel noise. The authors conclude that synchronizing vessel entries is one way managers can improve the underwater sound environment, and that managers should start collecting vessel sound signatures and baseline noise measurements now.

Advice for others

Think carefully about the question first, and if you take quantitative measurements use standard metrics so results can be compared across places. Find the right partners at the start. The authors point to the NPS Natural Sounds Program as a resource for equipment and collaborators.

Sources

• Davyd Betchkal, Paul Burger and Chris Gabriele, National Park Service, Making Sound Decisions Using Bioacoustics in Alaska's National Parks, Alaska Park Science 20(1): 66-73 (2021); rewritten in hubnx's own words. The article's photograph of a named staff member and its hydrophone figure are left out.