Density dependence in a recovering osprey population: demographic and behavioural processes

Abstract

Summary 1.Understanding how density-dependent and independent processes influence demographic parameters, and hence regulate population size, is fundamental within population ecology. We investigated density dependence in growth rate and fecundity in a recovering population of a semicolonial raptor, the osprey Pandion haliaetus [Linnaeus, 1758], using 31 years of count and demographic data in Corsica. 2. The study population increased from three pairs in 1974 to an average of 22 pairs in the late 1990s, with two distinct phases during the recovery (increase followed by stability) and contrasted trends in breeding parameters in each phase. 3. We show density dependence in population growth rate in the second phase, indicating that the stabilized population was regulated. We also show density dependence in productivity (fledging success between years and hatching success within years). 4. Using long-term data on behavioural interactions at nest sites, and on diet and fish provisioning rate, we evaluated two possible mechanisms of density dependence in productivity, food depletion and behavioural interference. 5. As density increased, both provisioning rate and the size of prey increased, contrary to predictions of a food-depletion mechanism. In the time series, a reduction in fledging success coincided with an increase in the number of non-breeders. Hatching success decreased with increasing local density and frequency of interactions with conspecifics, suggesting that behavioural interference was influencing hatching success. 6. Our study shows that, taking into account the role of non-breeders, in particular in species or populations where there are many floaters and where competition for nest sites is intense, can improve our understanding of density-dependent processes and help conservation actions.

The Osprey reintroduction in Central Italy: dispersal, survival and first breeding data

“…Although strong direct persecution reduced this population at only three breeding pairs in 1974, local management actions since then allowed a rapid demographic recovery. The population was considered stable with around 30 breeding pairs , Bretagnolle et al 2008. A total of 6-8 chicks was collected per year from the donor population without compromising its survival, for a minimum duration of five years of releases (Dominici et al 2007).…”

Behavioral responses of territorial red squirrels to natural and experimental variation in population density

“…In one exception, Bretagnolle et al (2008) documented a positive correlation between population density and antagonistic interaction rates between neighboring ospreys (Pandion haliaetus; see also Watson et al 1994;Amsalem and Hefetz, 2011;Innocent et al, 2011). Several other studies have experimentally manipulated population densities and monitored subsequent changes in feeding and foraging (e.g., Dobbs et al 2007;Rutten et al 2010) or offspring provisioning rates (e.g., Sillett et al 2004;Bretagnolle et al 2008). Although the results of these studies suggest that behavioral responses of individuals to variation in population density could yield insights into density-dependent population regulation as well as how individual behavioral variation affects population-level processes (Kokko and López-Supulcre 2007), this intersection between behavioral and population ecology remains understudied given the centrality of density-dependent behavior to the concepts of density-dependent population growth.…”

“…In one exception, Bretagnolle et al (2008) documented a positive correlation between population density and antagonistic interaction rates between neighboring ospreys (Pandion haliaetus; see also Watson et al 1994;Amsalem and Hefetz, 2011;Innocent et al, 2011). Several other studies have experimentally manipulated population densities and monitored subsequent changes in feeding and foraging (e.g., Dobbs et al 2007;Rutten et al 2010) or offspring provisioning rates (e.g., Sillett et al 2004;Bretagnolle et al 2008).…”

Floater strategies and dynamics in birds, and their importance in conservation biology: towards an understanding of nonbreeders in avian populations

“…In such cases floaters can become a drain on breeders, mainly because: (1) owners may spend a great deal of time and energy chasing floaters from their territories (Sunde & Bølstad, 2004;Carrete, Don´azar & Margalida, 2006a), with floater intrusions representing an additional cost during reproduction (Birkhead, 1982;Sandell & Diemer, 1999;Pilz & Smith, 2004;Bretagnolle, Mougeot & Thibault, 2008); (2) high levels of floater pressure may oblige owners to reduce the sizes of their territories (Norton, Arcese & Ewald, 1982); (3) floaters may compete for food with territory owners; (4) in some species (notably raptors) the intruders can kill owners in territorial fights (Newton, 1979).…”