Territory occupancy and breeding success of Peregrine FalconsFalco peregrinusat various stages of population recovery

Abstract

Organochlorine pesticides disrupted reproduction and killed many raptorial birds, and contributed to population declines during the 1940s to 1970s. We sought to discern whether and to what extent territory occupancy and breeding success changed from the pesticide era to recent years in a resident population of Peregrine Falcons Falco peregrinus in southern Scotland using long‐term (1964–2015) field data and multi‐state, multi‐season occupancy models. Peregrine territories that were occupied with successful reproduction in one year were much more likely to be occupied and experience reproductive success in the following year, compared with those that were unoccupied or occupied by unsuccessful breeders in the previous year. Probability of territory occupancy differed between territories in the eastern and western parts of the study area, and varied over time. The probability of occupancy of territories that were unoccupied and those that were occupied with successful reproduction during the previous breeding season generally increased over time, whereas the probability of occupancy of territories that were occupied after failed reproduction decreased. The probability of reproductive success (conditional on occupancy) in territories that were occupied during the previous breeding season increased over time. Specifically, for territories that had been successful in the previous year, the probability of occupancy as well as reproductive success increased steadily over time; these probabilities were substantially higher in recent years than earlier, when the population was still exposed to direct or residual effects of organochlorine pesticides. These results are consistent with the hypothesis that progressive reduction, followed by a complete ban, in the use of organochlorine pesticides improved reproductive success of Peregrines in southern Scotland. Differences in the temporal pattern of probability of reproductive success between south‐eastern and south‐western Scotland suggest that the effect of organochlorine pesticides on Peregrine reproductive success and/or the recovery from pesticide effects varied geographically and was possibly affected by other factors such as persecution.

Dispersal: a matter of scale

“…, McGrady et al. ). We interpret the significant effect of year at the larger spatial and temporal scales to primarily reflect the increase in Peregrine population density observed during the study period (Fig.…”

“…, McGrady et al. ). We therefore tested for the effect of year and year 2 in breeding numbers as representative indices of density across spatial scales, partially reflecting the increase in population size over the course of the study.…”

“…, McGrady et al. ). Thus, we expected a monotonically increasing linear or quadratic temporal trend in Peregrine dispersal distances over time, corresponding to Peregrine population recovery.…”

“…Through the 1980s and 1990s, following restrictions on the use of these compounds, the population recovered as the direct and residual effects of organochlorines subsided. More recently, the Peregrine population of the British Isles is thought to have fully recovered and stabilized around equilibrium densities, except in grouse-moor areas where the species is illegally persecuted (Banks et al 2010, Amar et al 2012, Smith et al 2015, McGrady et al 2017.…”

“…We could directly test this hypothesis with estimates of population density for the intensively surveyed limited study area. Although rigorous estimates of absolute or relative abundance of Peregrines for the extended study area are not available, periodic surveys have shown that the British Peregrine population went through a period of recovery as described above (Ratcliffe 1990) and is now relatively stable (Banks et al 2010, Smith et al 2015, McGrady et al 2017). Thus, we expected a monotonically increasing linear or quadratic temporal trend in Peregrine dispersal distances over time, corresponding to Peregrine population recovery.…”