Avian Conservation and Ecology (Dec 2019)
Apparent survival of a range-restricted montane forest bird species is influenced by weather throughout the annual cycle
Abstract
To conserve small and fragmented populations, we need an understanding of their population dynamics. With a global population estimate of < 120,000, Bicknell's Thrush (Catharus bicknelli) is considered one of the Nearctic-Neotropical migrants at greatest risk of extinction. This range-restricted songbird breeds in high-elevation fir (Abies balsamea) forests of the northeastern United States and eastern Canada, and primarily overwinters in forests of the Dominican Republic. The Conservation Action Plan for Bicknell's Thrush identifies numerous actions that may help stem population declines and promote recovery of the species, yet the empirical data needed to prioritize among these actions are lacking. We fit Cormack-Jolly-Seber models with mark-recapture data to test a series of hypotheses about the factors that limit apparent survival in 178 adult Bicknell''s Thrush (50 females and 128 males) captured on the breeding grounds in Vermont (June-July, 2001-2015). We focused on putatively important factors from throughout their annual cycle: cyclical population dynamics of nest predators, and weather effects on food abundance on the breeding and wintering grounds. Apparent survival of Bicknell's Thrush was relatively stable (mean Φ = 0.61, 95% CI: 0.54, 0.68) over our 15-year study, and most strongly associated with fir mast production. Apparent survival was higher following years during which fir trees produced large mast crops (mean Φ = 0.67, 95% CI: 0.55, 0.79), compared to following nonmast years (Φ = 0.56 ± 0.06, 95% CI: 0.43, 0.68). These results are likely driven by the reduced red squirrel density and increased nesting success and site fidelity of adult thrushes following nonmast years. Apparent survival of Bicknell's Thrush was also associated with relatively wet conditions on the wintering grounds in Hispaniola as assessed via the El Niño-Southern Oscillation (ENSO) precipitation index (ESPI). These relatively wet December-March periods are likely linked to greater primary productivity and the local availability of fruits and arthropods consumed by Bicknell's Thrush. Our research provides the most comprehensive examination of potentially limiting factors on Bicknell's Thrush populations to date and suggests future avenues of research exploring the relationship between food availability, survival, and climate change induced reductions in rainfall for the Greater Antilles.
