Solving the challenges of monitoring mobile populations: insights from studies of waterbirds in southern Africa

Abstract

Highly mobile species are difficult to monitor. It is often unclear from abundance estimates whether a population is genuinely declining or simply elsewhere. If the spatial and temporal scales of monitoring are inadequate, the data will only be useful if the signal is very strong (i.e. the change in population size is large). We explored the ability of three major databases – the Southern African Bird Atlas Project (SABAP), the Coordinated Waterbird Counts (CWAC) and the South African Bird Ringing Unit (SAFRING) – to reveal the movement patterns of a highly mobile African waterbird, the Red-billed Teal Anas erythrorhyncha. Only ring recovery data provided clear evidence of long-distance movement. All three data sets were compromised to varying degrees by methodological biases and spatial and/or temporal resolutions that prevented the detection of movement patterns. By incorporating abundance estimates, increasing and quantifying sampling effort, and extending coverage at the data collection stage, the potential for these data sets to contribute to our understanding of bird movements could be greatly improved. Monitoring programmes for mobile species must be tested at an early stage to assess their ability to explain spatial and temporal population variation at the scales at which such variation occurs.