Glacier Bay's Mystery Murrelets: Counting Two Elusive Seabirds, 2011–2022

Two elusive seabirds

Glacier Bay National Park and Preserve in Alaska is a summer haven for two elusive seabirds: Kittlitz's murrelet (Brachyramphus brevirostris) and marbled murrelet (B. marmoratus). Murrelets are small diving seabirds — about 10 inches and 8 ounces — that eat small schooling fish and belong to the alcid family, with auks, puffins and murres.

The Park Service has monitored both since 2011, because of conservation concerns and because they are indicator species linking marine and land ecosystems. The goal is to estimate their numbers and distribution in Glacier Bay and understand what drives their populations.

How they're counted

Each July, two experienced observers on the bow of a boat record every murrelet of each species and its distance and bearing along set routes. Effort is weighted toward where Kittlitz's murrelets are expected, and the estimates account for birds missed and for uncertain identifications.

What the surveys found

• Numbers swing widely from year to year for both species — probably from births and deaths, imprecise estimates, and birds moving in and out of the bay.
• Marbled murrelets in 2022 far exceeded any earlier estimate. The cause is unknown; one possibility is post-breeding birds migrating north, which reach Glacier Bay by early July from as far south as British Columbia.

Estimated numbers (with 95% confidence intervals) of Kittlitz's (A, red) and marbled (B, yellow) murrelets in Glacier Bay (1,092 km²) in early July, 2011–2022. NPS.

• Kittlitz's murrelets again gathered mainly around Hugh Miller Inlet and Reid Inlet in the upper West Arm, near glacier-fed waters and nesting habitat. Why their numbers were lower in 2019 and 2022 is unknown, but unusually warm ocean temperatures in Glacier Bay and the Gulf of Alaska hurt many marine species.
• Marbled murrelets were widespread, densest in the lower and middle bay, likely because of seasonal schools of small fish and nearby conifer forest for nesting.

Where Kittlitz's (left, red) and marbled (right, yellow) murrelets were seen on survey routes, July 2022; larger circles mean larger groups. NPS.

The two species' different trajectories suggest different forces are at work. Besides birds moving in and out of the bay, warming ocean temperatures and changing tidewater glaciers likely play an important part.

Reports and data: the Southeast Alaska Network's murrelet page.

Sources

Based on S.T. Hoekman, J.N. Womble, T.R. Ziomek and L.E. Pearson, "Mystery Murrelets: Monitoring Two Elusive Seabirds in Glacier Bay National Park & Preserve" (2024), Southeast Alaska Network, National Park Service; a work of the United States government in the public domain. It cites Hoekman and others (2013, 2023), Murdoch and Goodwin (2022), Danielson and others (2022) and Suryan and others (2021). A photograph credited to an individual photographer is not reproduced.