Colonial nesting birds nest primarily on the Molly Islands in the southeast arm of Yellowstone Lake. These two small islands are cumulatively just 0.7–1.2 acres in size, depending on lake water levels, yet hundreds of birds have nested there in a single year.
Prior to the late 1970s, the Molly Islands were surveyed only intermittently. Some data goes back to 1890 when nesting American white pelicans (Pelecanus erythrorhynchos) and California gulls (Larus californicus) were first noted in this area. Caspian terns (Hydroprogne caspia) are suspected of nesting on the Molly Islands as early as 1917, although information on breeding status was not collected until 1933. Double-crested cormorant (Phalacrocorax auritus) nests were confirmed by 1928.
Currently, pelicans, California gulls, and doublecrested cormorants nest with varying rates of success. Photographic interpretation from three aerial surveys conducted June through August 2019, showed approximately 613 pelican nests that fledged an estimated 175 young. Biologists were unable to confirm if 16 nesting double-crested cormorants were successful in fledging any young. Though California gulls were observed on the island, none attempted to nest. As in recent years, no Caspian terns were observed on the islands.
Birds nesting on the Molly Islands are subject to extreme environmental conditions ranging from flooding, frosts that can occur at any time of year, and high winds. As a result, birds nesting there experience large year-to-year fluctuations in the number of nests initiated and fledglings produced. Populations of American white pelicans, California gulls, and double-crested cormorants have declined over the last 20 years. Caspian terns (Hydroprogne caspia) have not nested on the islands since 2005.
The reasons for the decline in colonial nesting birds are not well understood, but a previous study indicates that high levels of water in Yellowstone Lake are associated with low reproduction for nesting pelicans. Notably, quick spring melt-off events can cause a significant rise in the water level on Yellowstone Lake and flood the Molly Islands.
The decline in cutthroat trout, a known food source for the Molly Island colonial nesting birds, may also influence nesting success. Furthermore, bald eagles on Yellowstone Lake that formerly depended on cutthroat trout may have switched prey to target the flightless and vulnerable young of these colonial nesting species.
Evans, R.M., and F.L. Knopf. American white pelican. The Birds of North America Online. https://birdsna.org/Species-Account/bna/species/amwpel/introduction
Varley, J. D., and P. Schullery. 1995. The Yellowstone Lake crisis: confronting a lake trout invasion. Yellowstone National Park, Wyo: Yellowstone Center for Resources, National Park Service.
Songbirds and Woodpeckers
Passerine and near passerine species comprise the majority of bird species in Yellowstone.
Last updated: July 13, 2020