Nestboxes and immigration drive the growth of an urban Peregrine FalconFalco peregrinuspopulation

Abstract

Drivers of wildlife population dynamics are generally numerous and interacting. Some of these drivers may impact demographic processes that are difficult to estimate, such as immigration into the focal population. Populations may furthermore be small and subject to demographic stochasticity. All of these factors contribute to blur the causal relationship between past management action and current population trends. The urban Peregrine Falcon Falco peregrinus population in Cape Town, South Africa, increased from three pairs in 1997 to 18 pairs in 2010. Nestboxes were installed over this period to manage the interface between new urban pairs of Falcons and the human users of colonized buildings, and incidentally to improve breeding success. We used integrated population models (IPMs) formally to combine information from a capture-mark-recapture study, monitoring of reproductive success and counts of population size. As all local demographic processes were directly observed, the IPM approach also allowed us to estimate immigration by difference. The provision of nestboxes, as a possible stimulant of population growth, improved breeding success and accounted for an estimated 3-26% of the population increase. The most important driver of growth, however, was immigration. Despite low sample sizes, the IPM approach allowed us to obtain relatively precise estimates of the population-level impact of nestbox deployment. The goal of conservation interventions is often to increase population size, so the effectiveness of such interventions should ideally be assessed at the population level. IPMs are powerful tools in this context for combining demographic information that may be limited due to small population size or practical constraints on monitoring. Our study quantitatively documented both the immigration process that led to growth of a small population and the effect of a management action that helped the process.

The breeding performance of raptors in urban landscapes: a review and meta-analysis

“…Barn Owls, Burrowing Owls, Mississippi Kites (Ictinia mississippiensis), Red-shouldered Hawks and Swainson's Hawks (B. swainsoni) all produced more young to fledge at urban sites, but none of these differences were significant in the studies, despite large sample sizes ( Fig. 4; Bloom and McCrary 1996;Parker 1996;Salvati et al 2002;Dykstra et al 2009;Berardelli et al 2010;Alsup 2012). In an urban-gradient study, Burrowing Owls responded positively to urban cover up until their numbers reached high levels and stabilised (Millsap and Bear 2000).…”

“…For example, in Charter et al's (2007) study on Eurasian Kestrels (F. tinnunculus), rural habitat was defined as small villages with few buildings and a human population size smaller than 700. Yet in other studies, rural habitat was defined simply as agricultural land (Tella et al 1996;Alsup 2012;Chiang et al 2012;Lin et al 2015) or woodland (Solonen and Ursin 2008), or measured by the intensity of human use (Stout et al 1998;Millsap et al 2004). For our own data, urban Peregrine Falcons were defined as those nesting in a city or town, whereas rural ones were defined as those nesting outside of the town or cityscape.…”

Studying the demographic drivers of an increasing Imperial Eagle population to inform conservation management

“…Demographic models have been used to improve the understanding of population dynamics and the development of conservation strategies for several raptor species (Katzner et al 2006;Soutullo et al 2008;Ortega et al 2009), but estimating the abundance and survival of cryptic immature age classes remains a formidable challenge (Katzner et al 2011;Margalida et al 2011). Movement of individuals between populations is known for several raptor species (Ortega et al 2009), and this immigration may mask conditions inherent in the target population and potentially lead to erroneous conclusions about the effectiveness of management (Katzner et al 2006;Schaub et al 2010;Altwegg et al 2014). Assessing the effectiveness of raptor conservation management therefore requires approaches that can explicitly account for immigration.…”

Breeding performance of an apex predator, the peregrine falcon, across urban and rural landscapes

“…Peregrines are specialist bird-eating raptors found in cities globally (Altwegg et al 2014;Wilson et al 2018), an environment in which some potential prey species of peregrines are especially abundant. Despite being one of the best-known urban predators, it is unclear whether the peregrine is benefitting from nesting in urban environments.…”