The Importance of Productivity to the Dynamics of a Swainson's Thrush Population

We analyzed the population dynamics of Swainson's Thrushes (Catharus ustulatus) breeding at the Palomarin Field Station of the Point Reyes Bird Observatory using 15 years (1980-1994) of long-term, standardized mist-net data. The capture rates of adults and hatching-year birds provided indices of adult abundance and productivity respectively. Annual variation of these indices was high, and linear regression analysis revealed no long-term trends. However, numbers of new and total adults captured in a given year were significantly dependent on the number of hatching-year birds caught the previous year. In addition, per capita productivity was inversely density-dependent and may partially regulate adult abundance. These results suggest that Swainson's Thrushes at Palomarin are most limited by the production of young on the breeding grounds. Return rates of hatching-year birds were exceptionally high compared to other species (18.3%). Strong habitat specificity on the breeding grounds may elevate this return rate and strengthen the link between productivity and adult abundance in this population.