Estimating abundance in unmarked populations of Golden Eagle

1. Estimates of species abundance are of key importance in
population and ecosystem level research but can be hard to obtain. Study
designs using camera-traps are increasingly being used for large-scale
monitoring of species that are elusive and/or occur naturally at low
densities. 2. Golden eagle (Aquila chrysaetos) is one such species, and we
investigate whether existing large-scale monitoring programs using baited
camera-traps can be used to estimate the abundance of golden eagles, as an
alternative to traditional labour-intensive searches for active
territories and nest sites during the breeding period. 3. The camera-trap
data allowed two measures of abundance to be estimated within each of four
main study areas in mid and northern Norway; occupancy was measured as the
probability of camera site use, and population size was measured as the
number of eagle individuals using the camera sites within a study area.
Spatial and temporal patterns in occupancy and population size were
explored and evaluated against independent estimates of the breeding pair
density in the study areas. 4. Annual estimates of golden eagle occupancy
showed low precision, while estimates of population size were more precise
in relation to both estimated and anticipated abundance fluctuations.
Estimates of population size may therefore be suitable for monitoring
within study area temporal abundance trends, while estimates of occupancy
seem unsuitable for such in golden eagles. Across study areas, patterns in
both average occupancy and average population density estimated from
population size, were consistent with the spatial pattern in average
breeding pair densities (r = 0.99, and r = 0.89 respectively). This
suggests that camera-trap based estimates of occupancy and population
density reflect territory density at large spatial scales. In conclusion,
our results suggest that baited camera-traps can be a cost-effective
strategy for monitoring the abundance of golden eagles.