The importance of high streamflow and bank erosion for the Bank Swallow, a sentinel species of alluvial river systems

Lowland alluvial rivers are rich in biodiversity, yet many are highly degraded, constrained by dams and riprap, and no longer support robust natural communities or critical ecosystem services. Restorative actions are needed, as are ecological indicators for evaluating their effectiveness. We evaluated factors that regulate the population growth rate of Bank Swallows to inform strategies for its recovery and to assess its value as an indicator of riparian ecosystem health. In North America, over the past 50 years, this species, which depends upon erosion for its cutbank breeding habitat, has declined at a rate considered extreme. Understanding the causes of this decline and identifying feasible recovery actions is an urgent conservation priority. In our 25-year study of the Bank Swallow population on the Sacramento River, the breeding population fluctuated widely but declined at an average overall rate of 1.5 % per year. Periodic high streamflow played an important role in maintaining the population, and its effect on the population growth rate was time-lagged by 1 year. This suggests that high flow conditions may have improved rates of Bank Swallow reproductive success and/or survival, leading to increased recruitment into the breeding population in the subsequent year. We also found evidence for density-dependent population growth, suggesting that when the breeding population was large, high-quality nesting habitat was limiting. Our study establishes the critical importance of high streamflow and bank erosion to maintaining Bank Swallow breeding populations. In so doing, it informs recovery strategies for the species and the rivers it inhabits and identifies the Bank Swallow as an appropriate indicator of alluvial river ecosystem processes.