Common Loon

The common loon is a species of concern in Yellowstone.

 

The majority of the Greater Yellowstone Ecosystem’s (GYE) breeding common loon (Gavia immer) population occurs in Yellowstone and is one of the most southerly breeding populations in North America. The common loon is listed as a Species of Special Concern in Wyoming because of its limited range, small population, sensitivity to human disturbance, and loss of breeding habitat outside of Yellowstone. The GYE’s breeding loon population is isolated from populations to the north by more than 200 miles, limiting immigration from other populations. The Yellowstone Loon population appears stable; however, detailed data from a study initiated in 2012 indicate that the number of loons present in the park can vary from year to year. Continuing research will try to analyze any trends in productivity, nesting success, and number of breeding pairs to attempt to determine why some years are more productive than others.

 

Number in Yellowstone

• In 2023, 38 loons in total. 12 territorial pairs. Eight successful nests produced 10 young.

Identification

• Breeding adults (March–October) have black and white checkering on back, a black bill, red eyes, and iridescent green head and neck. The neck has a black and white chinstrap and distinctive collar.
• Loon chicks hatch with a blackish-brown down and white belly and retain this plumage for two weeks. Body feathers emerge at 4½ weeks on the chick’s upper back. By six weeks, brown down only remains on the neck and flanks.
• Gray juvenile plumage is present at seven weeks.
• Juveniles and winter adults have dark upperparts and white underparts.

Habitat

• Summer on ponds or lakes: large lakes, such as Yellowstone, Lewis, and Heart Lakes; and smaller ones such as Grebe and Riddle lakes.
• Winter on open water.
• May be found foraging or resting on larger, slow moving rivers.
• Nest sites are usually on islands, hummocks in wetlands, or floating bog mats.
• Pairs nesting on lakes smaller than 60 acres usually require more than one lake in their territory. Lakes smaller than 15 acres are rarely used.

Behavior

• Primarily eat fish (4–8 inches).
• Unable to walk on land.
• Migrate in loose groups or on own, not in organized flocks. Arrive at summer lakes and ponds at or soon after ice-off.
• Four common calls: wail—for long distance communication, yodel—used as a territorial signal by males only, tremolo—a staccato call, usually by an agitated adult, and hoot—a contact call, often between adults or adults and their young.
• Females generally lay two eggs, typically in June.
• Males and females share incubation duties equally. Chicks hatch after 27–30 days. Both adults also care for their young.
• Chicks are able to fend for themselves and attain flight at 11–12 weeks.
• In late summer, adults form social groups, especially on larger lakes, before leaving in October.

Management Concerns

• The breeding population in Wyoming is isolated; populations to the north are more than 200 miles away.
• Loons can be bioindicators of the aquatic integrity of lakes, responding to lead and mercury levels.
• All factors affecting loon reproduction in Yellowstone are unknown, but human disturbance of shoreline nests has a negative impact.

 

Population

In 2023, cooperating biologists and park staff identified 23 occupied territories throughout the GYE, 18 of which were located within YNP. In total, the park housed 38 adult loons. Twelve pairs attempted to nest, and four of those failed. The eight successful pairs produced 10 loonlets during 2023.

 

Common loon spotted in Yellowstone.

NPS/Neal Herbert

Distribution

In the western United States, common loons breed in Idaho, Montana, Washington, and Wyoming. The total western US breeding population is estimated at 90 territorial pairs. In 2023, Yellowstone National Park hosted 60% of the GYE’s total loon population and 57% of the breeding pairs. Furthermore, YNP loons produced 63% of the ecosystem’s fledged chicks, highlighting the park’s important role in regional loon population stability and persistence. Western populations of breeding common loons are known to overwinter from Washington south to California. Spring and fall migrants in Wyoming represent breeding populations from Saskatchewan that overwinter around Mexico’s Baja California peninsula.

 

Outlook

There are several threats to Wyoming’s loon populations. Direct human disturbance to shoreline nests and chicks lowers survival rates and adversely affects numerous loon territories in YNP each year. Increased outreach to the public to minimize shoreline disturbance could help improve the long-term outlook for loons within the park. Throughout YNP and Wyoming, the loss of breeding habitats and water level fluctuations (e.g., erratic spring flooding) also impact loon nest success and productivity.

Contaminants like lead (from sinkers) and mercury, in combination with hazards on wintering grounds (e.g., marine oil spills and fishing nets) challenge loon reproduction and survival even further. Visitors to Yellowstone’s lakes can help minimize disturbance of loon nests by staying on trails during the breeding season, avoiding shorelines, and, as with all wildlife species, giving adult and fledgling loons plenty of space.

On average, one loon is killed per year on Yellowstone Lake by gill nets as bycatch in the park’s effort to remove invasive lake trout. Ongoing research will better assess patterns in gillnetting mortalities to improve coordination with fisheries crews, thereby reducing the threat to local loons while allowing for continued lake trout removal.

Fish are the primary prey of loons. As part of a multi-park study on mercury concentration in fish, fish from various lakes where loons nest were screened for mercury. Fish were sampled from Beula, Grebe, Yellowstone, and Lewis Lakes. Fish from Beula, Grebe, and Yellowstone lakes exceeded the threshold at which fish-eating birds may be affected by mercury toxicity. Fish from Lewis did not exceed that threshold although they still contained mercury.

Loons can live up to 30 years, have relatively low chick production, and are poor colonizers to new breeding areas. Given the very small size and isolation of Wyoming’s breeding loon population, it is at a particularly high risk of local extinction.

 

Resources

Mcintyre, Judith W., Jack F. Barr, David C. Evers, and James D. Paruk. Common loon. The birds of North America Online. https://birdsna.org/Species-Account/bna/species/comloo/introduction