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. The birds have been surveyed for more than 30 years, with some data going back to 1890 when nesting American white pelicans (Pelecanus erythrorhynchos) and California gulls (Larus californicus) were first noted in this area. Currently, pelicans, California gulls, and double- crested cormorants (Phalacrocorax auritus) nest with varying rates of success. Very wet and cold late spring weather and the declining cutthroat trout population could be factors. In 2015, 291 pelican chicks, and 9 cormorant chicks fledged. No California gull chicks were observed.
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 reason(s) for the declines is unknown, but is likely related to cutthroat trout population diminishing in Yellowstone Lake.
Caspian terns 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 nests were confirmed by 1928. Prior to the late 1970s, the Molly Islands were surveyed only intermittently, but have been surveyed annually since that time.
American white pelicans spend the summer mainly on Yellowstone Lake and the Yellowstone River. These large white birds are often mistaken for trumpeter swans until their huge yellow beak and throat pouch are seen. Their black wing tips also distinguish them from swans, which have pure white wings.
Songbirds and Woodpeckers
Passerine and near passerine species comprise the majority of bird species in Yellowstone.
Annual Bird Program Reports. National Park Service, Yellowstone National Park. https://www.nps.gov/yell/learn/nature/birdreports.htm
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
Last updated: July 17, 2018