Retrospective analysis of a declining trumpeter swan (Cygnus buccinator) population in Yellowstone National Park

Abstract

By 1933, the number of trumpeter swans (Cygnus buccinator) in the continental United States was reduced to roughly 70 individuals that nested and wintered in Yellowstone National Park (YNP) and the surrounding Greater Yellowstone area. While conservation measures saved the trumpeter swan, and their numbers have increased greatly across North America, abundance and productivity of YNP's resident trumpeter swan population declined from the 1960's through about 2010. Many hypotheses for the initial decline in YNP trumpeter swans exist, including human disturbance at nesting areas, changes in habitat quality, predation, and management of trumpeter swans outside of YNP. To improve knowledge and take advantage of long-term monitoring of trumpeter swans, this retrospective study was designed to evaluate the various competing hypotheses about possible factors associated with temporal and spatial variation in swan abundance and reproductive success in YNP for 1931-2019. Two different types of analyses were used: (1) analysis of annual park-wide counts of trumpeter swan territories with swans Absent, Present but unsuccessful (Present), and Successful, and (2) Bayesian reversible jump Markov chain Monte Carlo analysis that evaluated the utility of covariates representing swan decline hypotheses for explaining variation in annual, territory-level patterns of where swans were Absent, Present, and Successful each year. My results provide novel information on temporal patterns in the annual number of Absent, Present, and Successful territories, and analysis of covariates that are useful to explain variation in territory statuses identified several interesting covariate relationships. Swan territories within YNP were more likely to have trumpeter swans Present as opposed to Absent during 1931-2011 in years when total abundance of trumpeter swans in the broader geographic area around YNP was greater. Because several covariates have values that trend through time, it is difficult to distinguish between several alternative interpretations for the underlying causes of temporal trends. Identification of swan territories most likely to have swans Present and Successful can be a useful tool to help YNP staff manage important swan habitat or justify targeted management actions. Future work that utilizes satellite imagery to reconstruct lake/wetland hydrology is likely to be useful to describe potential changes in habitat quality.