Perilous choices: landscapes of fear for adult birds reduces nestling condition across an urban gradient

Abstract

Predator fear effects influence reproductive outcomes in many species. In non-urban systems, passerines often respond to predator cues by reducing parental investment, resulting in smaller and lighter nestlings. Since trophic interactions in urban areas are highly altered, it is unclear how passerines respond to fear effects in human-altered landscapes. Nestlings of passerines in urban areas also tend to be smaller and lighter than their rural counterparts and are often exposed to high densities of potential predators yet experience lower per capita predation-the predation paradox. We suggest fear effects in urban habitats could be a significant mechanism influencing nestling condition in birds, despite lowered predation rates. We manipulated exposure of nesting birds to adult-consuming predator risk in residential yards across a gradient of urbanization to determine the relative influence of urbanization and fear on nestling condition. We found nestlings had reduced mass in nests exposed to predator playbacks as well as in more urban areas. Despite lower per capita predation rates in urban areas, fear effects from increased predator densities may influence passerine fitness through reduced nestling condition. As urban development expands, biodiversity conservation hinges on a deeper mechanistic understanding of how urbanization affects reproductive outcomes.

Phenotypic plasticity and the anthropause: an urban bird becomes less aggressive

“…Having to actively avoid humans may strongly encourage vigilance behavior in adult birds. This in turn draws investment away from foraging and provisioning chicks and promotes avoidance of areas of the landscape with high predator activity (57)(58)(59). Juncos perceiving less predation risk from humans may be less vigilant toward competitors as well if they shift energetic investment to parental care.…”

“…Because vigilance and nest defense behaviors are associated with trade-offs in parental care (58,59), limiting human disturbance of urban bird populations long-term could potentially improve the overall health and survival of chicks. This trade-off was evident in a pandemic lockdown-era study which measured the body condition of nestling urban great tits (Parus major) in urban parks of two cities during the lockdowns: one in which community park attendance increased, and one in which it decreased.…”

Urban landscapes are richer in bird species when compared to farming lands: evidence from Morocco (Northwest Africa)

“…However, most previous research did not explore avian groups across a wide-range gradient of urbanization, from undisturbed landscapes to highly urbanized ecosystems (Crooks et al 2004;Minor and Urban 2010;Grade et al 2021;Roselli et al 2021). Current studies that did explore avian communities across urban gradients demonstrated that both species richness and total abundance peaked at non-urbanized or moderately urbanized levels (Sanz and Caula 2015;Ciach and Fröhlich 2017;Xu et al 2018).…”

Chronic disturbance induces attenuation of the acute glucocorticoid response in an urban adapter, the dark-eyed junco

“…Urban environments often have a higher density of vertebrate predators such as cats and raccoons compared to rural environments (Haskell et al, 2001), although it is not known whether predation rates differ for urban vs non-urban populations of our study species, the dark-eyed junco (Junco hyemalis). Humans, vehicles, anthropogenic noise, and domestic animals may also be interpreted as potential predators (Frid and Dill, 2002;Davies et al, 2017), and perception of predation risk may affect urban animals regardless of predation rates (Beckerman et al, 2007;Grade et al, 2021;Garitano-Zavala et al, 2022). Here we focus on glucocorticoid hormones because they are known to respond to predation risk (Scheuerlein et al, 2001;Hawlena and Schmitz, 2010;Fischer et al, 2014;Jones et al, 2016) and may mediate physiological and life-history responses to risk such as changes in reproductive rate (Clinchy et al, 2011) and changes in population dynamics (Sheriff et al, 2009;Finn et al, 2023).…”