Depleted suburban house sparrow Passer domesticus population not limited by food availability

Abstract

Little is known about the environmental factors that limit the demography and abundance of wild vertebrates in highly modified urban environments. The House Sparrow Passer domesticus is a globally widespread species whose urban populations have recently undergone substantial declines particularly in Europe. The environmental drivers of these declines remain unknown. In a previous study we showed that invertebrate availability during the breeding season limited reproductive success but not population size in a suburban sparrow population. In this study we test experimentally whether year-round food availability limits demography and population size. Supplementary feeding involved the provision of invertebrate prey (during the breeding season) plus unlimited high-energy seed (year-round) at 33 sparrow colonies spread across suburban London over two successive calendar years. Thirty-three unfed colonies served as controls. Supplementary feeding increased fledgling abundance, but had no impact on overwinter survival or population size. We conclude that this depleted suburban sparrow population is not limited by food availability, and conservation efforts based primarily on food provision are unlikely to succeed. We also tested whether cross-colony variation in sparrow abundance was correlated with a set of potential environmental stressors including measures of predator abundance and pollution. Sparrows were more abundant, or showed more positive temporal changes in abundance, at localities containing large areas of seed-rich habitat and low levels of nitrogen dioxide air pollution. Further research is merited into the potential impacts of air pollution on the fitness of urban birds.

Are house sparrow populations limited by the lack of cavities in urbanized landscapes? An experimental test

“…For example, it has recently been reported that great tits could exclude flycatchers from nest‐boxes, and could even kill them in order to take over a nest‐box (Samplonius and Both ). However, this inter‐specific competition is also quite unlikely to explain our results because previous studies have suggested that house sparrows are dominant over tits for nest‐box occupancy (Charter et al 2013, Goldshtein et al ). Instead, our results suggest that house sparrows do not have to compete for cavities in urban landscapes similar to Niort.…”

“…For example, in many cities, old buildings are progressively replaced by new buildings, which are very often built with modern materials. These new structures without any cracks or cavities may preclude cavity nesters from breeding in cities and may cause population declines, especially in cavity‐nesting birds (Moudra et al ).…”

“…Surprisingly, the causes of this decline remain poorly understood despite its importance and rapidity (De Laet and Summers‐Smith 2007). This sharp decline may be linked to multiple recent modifications of the urban environment, such as 1) an increased predation risk due to the recent rise in the numbers of European sparrowhawks and feral cats in cities, 2) an intensification of multiple sources of pollution (noise, light, electromagnetic, and air pollution), 3) a modification of food availability and food type, or 4) a change in the urban habitat with a reduction of habitat availability for house sparrows (Robinson et al , Liker et al , Shaw et al , Peach et al , 2018, Herrera‐Duenas et al , Meillère et al , Moudra et al ).…”

Avian malaria-mediated population decline of a widespread iconic bird species

“…By contrast, Peach et al . [15] found a positive relationship between adult sparrow abundance and Eurasian sparrowhawk activity but no relationship between temporal trends in sparrow abundance and hawk activity. Studies on the impact of predation by domestic cats ( Felis catus ) have reached no definitive conclusions.…”

Urban resources limit pair coordination over offspring provisioning

“…In turn, these environmental changes have profound effects at the ecosystem level, modifying resource availability and ultimately breeding behaviour and fitness of urban populations 29 . For examples, correlative and experimental studies on insectivorous songbird breeding have shown that limited and poor-quality resource availability limits provisioning behaviour and reduces reproductive success in urban habitats [29][30][31][32][33][34][35] . Therefore, it is likely that differences in resource distribution and abundance as a result of urbanization may lead to certain parental behaviours to be more prevalent in urban areas with fitness consequences 36,37 .…”