Natal dispersal, defined as the permanent movement of pre-reproductive individuals from their birth place to their first breeding site, is an important life-history trait that can profoundly influence geographic distribution as well as dynamics, persistence, and genetic structure of natural populations. One factor that is often invoked as an explanation for variation in dispersal distance is population density at natal site, with the expectation that limiting resources and suitable breeding sites, coupled with increased intra-specific aggression occurring at higher densities, should drive dispersal. We present the findings of a long-term study examining factors influencing natal dispersal distance for the non-migratory population of Peregrine falcons (Falco peregrinus) in South Scotland and North England during two distinct time periods: (1) 1974-1982, when the population was recovering from near extinction due to the effects of organo-chlorine pesticides, and (2) 2002-2010, when the population is considered to have stabilized around a presumed equilibrium density.
Results/Conclusions
The best supported model explaining natal dispersal distance included an additive effect of sex and study period, providing evidence that dispersal distance for both males and females differed between the two periods. A comparable model replaced the period effect with a year-specific estimate of density. In contrast to expectations based on the density-dependent dispersal hypothesis, we found that dispersal distance was greater during the recovery period than the stable period, and that distances and the difference between periods were both greater for females than males (females: 81.09 ± 15.95 vs. 37.60 ± 12.06 km; and males: 56.02 ± 14.55 vs. 22.46 ± 8.09 km, for the recovery and stable periods, respectively). Potential causes and population-dynamic consequences of the observed patterns are discussed.