Monitoring demographic rates to assess the status of a reintroduced swan population

Information about demographic vital rates is useful for assessing the status of reintroduced species and potential barriers to growth. We used an integrated population model to estimate and assess demographic vital rates of a Trumpeter Swan (<em>Cygnus buccinator</em>) population that was reintroduced into its former breeding range in south-central Ontario, Canada. Apparent adult mean survival, 2006–2019, was slightly higher than juvenile survival (0.84 and 0.80, respectively, with 95% credible limits of 0.83–0.86 and 0.75–0.85). The transition probability for a 3-yo sub-adult female to a breeding adult was much lower than for a 4-yo+ non-breeding female (0.07 and 0.53, respectively, with 95% credible limits 0.01–0.15 and 0.26–0.92). We found no trend in population growth rate (mean = 1.16, 95% credible limits 1.13 to 1.20) and change in growth rate was most sensitive to proportional change in adult survival (elasticity = 0.84, 95% credible limits 0.82–0.86). However, life table response experiment contribution was greatest for adult abundance (0.857, 95% credible limits 0.411–0.866). We found no evidence of density dependence on juvenile apparent survival. We forecast an increase in relative abundance of female swans by 11 times from 2019 to 2034 assuming no density dependent restrictions. Evidence suggests this Trumpeter Swan population had not reached carrying capacity during the period of study and the available breeding habitat or management action during winter may have circumvented density dependent limitation. We recommend continued monitoring of vital rates and their drivers as they can provide information on when carrying capacity may be reached and where management action may be needed if barriers to population growth are encountered that inhibit Trumpeter Swans from repopulating their former breeding range.