Predation on artificial bird nests along an urban gradient: predatory risk or relaxation in urban environments?

Abstract

1999. Predation on artificial bird nests along an urban gradient: predatory risk or relaxation in urban environments? -Ecography 22: 532-541.Urbanizationthe anthropogenic conversion of natural ecosystems into humandominated ecosystemshas occurred on global scales. The human-dominated landscape presents particular challenges to researchers because the effects of urbanization on ecological processes are not well understood. We investigated the influence of urbanization on predation by conducting an artificial nest experiment along an urban gradient of six sites ranging from natural to urbanized ecosystems. Previous hypotheses suggest that predation pressures in urban environments will either 1) increase because of the high abundance of exotic species which act as predators or 2) decrease due to the lack of natural predators. To determine relative predation pressures among sites along the urban gradient, we monitored the fates of 16 artificial avian nests at each of the six sites for a total of 96 nests in each year (1996, 1997). We analyzed the dependency of nest fate (depredated or undisturbed) on intensity of urbanization (sites along the urban gradient), nest height (ground, above-ground), and year using loglinear models. The frequency of nest predation was strongly dependent on site along the urban gradient, indicating that urbanization intensity was an important determinant of nest fate. Predation pressure exhibited an overall decline from natural to urban sites in both years, suggesting that urban environments have low predation pressures relative to natural areas. The predatory relaxation in urban environments may partially explain the greater abundance of some species in urban environments, particularly urban exploiters such as european starlings Sturnis vulgaris, house sparrows Passer domesticus, and rock doves Columba livia.

Habitat urbanization and its effects on birds

“…The general assumption is that predation pressure is reduced in urban areas (also known as predation-relaxation or safe-zone hypothesis) which is mainly based on urban birds' reduced fearfulness to humans and the sometimes low abundance of their native predators (Gering & Blair 1999, Shochat et al 2006. The predation relaxation is suggested to partially explain the great biomass of avian prey species in cities.…”

Garden Bird Feeding: Insights and Prospects from a North-South Comparison of This Global Urban Phenomenon

“…Many provide invaluable accounts of how urbanization (and sub-urbanization) impacts birds in terms of their behavior, ecology, physiology, abundance, and distribution. The next major challenge, however, is to determine how food availability, especially through the provision of food supplements, influences the biology of birds in our urban centers while contemporaneously being able to control extrinsic factors that influence the biology of urban birds equally strongly (e.g., predation -Gering and Blair, 1999;temperature-Stager et al, 2016;light pollution-Kempenaers et al, 2010;noise pollutionArroyo-Solís et al, 2013).…”

Conserving Biodiversity in Urbanizing Areas: Nontraditional Views from a Bird's Perspective

“…Such changes in predator communities are frequently cited as causes of the commonly reported negative association between some sensitive Neotropical migratory species and urbanization, yet the link between urbanization and increased nest predation has surprisingly mixed empirical support from studies of natural nests (Phillips et al 2005; Bakermans and Rodewald 2006; Burhans and Thompson 2006;Rodewald and Shustack 2008b). Most studies reporting higher rates of predation in urban environments have used artificial nests (Gering and Blair 1999;Jokimaki and Huhta 2000;Thorington and Bowman 2003;Jokimaki et al 2005), which can result in biased estimates of nest predation due to species-specific responses of nest predators (Thompson and Burhans 2004). Our long-term studies of nest predation in central Ohio and western Washington suggest that nest predation in urbanizing landscapes show high temporal and spatial variability, and urbanization seems to be inconsistently related to daily mortality rates (Marzluff et al 2007; A.D. Rodewald, unpublished data).…”