Consistent apparent adult survival and nest-site fidelity of whimbrel Numenius phaeopus near Churchill, Manitoba, Canada over a 40 year period

Reliable estimates of adult survival for many shorebird species are lacking. We used Cormack–Jolly–Seber (CJS) methods to provide an estimate of apparent, or local, survival (φ) of a population of whimbrels, Numenius phaeopus, breeding in the subarctic Churchill, Manitoba region. We used data collected in two time-periods: 1973–1976 and 2010–2014. We also quantified nest-site fidelity in 2010–2014 to provide context to our apparent survival estimates because mark-recapture analyses cannot distinguish between mortality and permanent emigration. The most parsimonious CJS model did not include effects of sex or time on apparent adult survival in either period (φ = 0.76 ± 0.13 SE; φ = 0.75 ± 0.04 SE, 1973–1976 and 2010–2014, respectively). Additionally, observations of marked whimbrels between 2010 and 2019 (n = 124) showed that 61 of the 105 marked individuals (58.1%) were resighted. These estimates of return rates are, as expected, much lower than estimates of apparent survival. The median year-to-year distance between nests (n = 139) in 2010 to 2014 was 198 m ± 88 SE and did not differ significantly (p = 0.84) between females (x̄ = 721.9 m ± 119.8 SE) and males (x̄ = 720.3 m ± 83.1 SE). If our apparent survival estimate is indicative of true survivorship, then adult mortality during the non-breeding season has remained constant over the last five decades, implying that the recent decline in whimbrel populations may stem largely from reduced fecundity, including egg or juvenile survival, rather than primarily from reduced adult survival.