Anthropogenic food availability and body mass maintenance in urban Red-winged Starlings Onychognathus morio

Abstract

Anthropogenic food availability is influenced by short-term fluctuations in human presence, especially in core urban areas. Few studies have explored responses to such day-to-week fluctuations, specifically on body mass and body condition of urban birds. Here, we investigated non-breeding body mass maintenance and body condition in Red-winged Starlings Onychognathus morio resident at the University of Cape Town, Western Cape, South Africa, in relation to the proportion of impervious surfaces within their home ranges ('built-up index'). We hypothesised that the built-up index correlates with the starlings' ability to maintain mass, which is mediated by anthropogenic food availability. In a community science project focused on the university campus, we estimated home range sizes and combined these data with systematic behavioural observations (focals), transects to estimate anthropogenic food availability and repeated body mass measurements. Individual starlings with more built-up home ranges were heavier; they were also heavier and in better body condition on high human presence days (weekdays in term time). We found no relationship between the amount of anthropogenic food available or the proportion of anthropogenic food consumed and built-up index, nor between the times spent foraging or foraging efficiency and built-up index. However, anthropogenic food availability was higher on high human presence days when starlings also consumed more anthropogenic food and tended to spend less time foraging, compared with low human presence days (weekends and vacation). There is growing concern that anthropogenic food could have negative consequences during the breeding season, when parents might be provisioning 'junk food' to their offspring. This study provides additional understanding of urban impacts on avian species and the role of anthropogenic food. Disponibilité de la nourriture d'origine anthropique et maintien de la masse corporelle chez les Rufipennes morio en milieu urbainLa disponibilité de la nourriture d'origine anthropique est liée à la fluctuation à court terme de la présence humaine, et ce plus particulièrement dans les centres urbains. Peu d'études ont exploré les réponses à de telles fluctuations quotidiennes à hebdomadaires, et plus spécifiquement sur la masse corporelle et la condition physique des oiseaux urbains. Nous avons étudié le maintien de la masse corporelle et la condition physique des rufipennes morio (Onycognathus morio) résidant à l'université du Cap, dans la province du Cap occidental, en Afrique du Sud, en dehors de la période de reproduction, en fonction des surfaces imperméabilisées comprises dans leur domaine vital (index du bâti). Nous avons émis l'hypothèse que l'index du bâti est corrélé avec la capacité des rufipennes à maintenir leur masse, elle-même liée à la disponibilité de la nourriture anthropique. Dans le cadre d'un projet de science communautaire focalisé sur le campus universitaire, nous avons estimé les tailles de domaines vitaux et avons combiné ces données avec des obs...

Pulses of anthropogenic food availability appear to benefit parents, but compromise nestling growth in urban red-winged starlings

“…Over 25,000 students are enrolled at UCT annually and, on weekdays during the academic term, most of these students attend classes on upper campus. This abundance of people corresponds with an abundance of anthropogenic food provided by cafeteria stalls and brought by students from off campus (Stofberg et al 2019 ; Risi et al 2021 ). Cafeteria stalls are closed during weekends and the number of students visiting campus is dramatically reduced—e.g.…”

“…Here, we took advantage of a ‘natural experiment’ occurring on the University of Cape Town campus in South Africa, where a breeding population of urban-exploiting red-winged starlings (hereafter ‘starlings’) are exposed to fluctuating amounts of anthropogenic food over short temporal scales, to address the question of whether adult and nestling starlings are differentially affected by diets high in anthropogenic food. During the week, on high human presence (hereafter ‘HHP’) days, thousands of students and an associated high abundance of anthropogenic food provide heavily pulsed foraging opportunities for the resident starling population, with this food supply withdrawn over weekends and vacations (‘low human presence’ or ‘LHP’ days), forcing starlings to switch to a more natural diet including more insects and berries (Stofberg et al 2019 ; Risi et al 2021 ). We hypothesised that adult starlings would benefit from readily available high-calorie food on HHP days; whereas nestlings would be negatively impacted as a result of being fed a high proportion of low-quality anthropogenic food.…”

Interactive effects of rising temperatures and urbanisation on birds across different climate zones: A mechanistic perspective

“…In urban areas, anthropogenic food sources are often abundant (Risi et al, 2021) and may (partly or wholly) compensate for an overall scarcity of natural foods (Plummer et al, 2019;Seress et al, 2020).…”

A southern perspective on urban birds

“…In urban centres (where large numbers of people work or receive education, but most live elsewhere), short-term fluctuations in human presence on weekly work schedules provide fluctuating anthropogenic food resource pulses that create a less stable food environment over faster time scales than seen in natural systems. In this special issue, we highlight a study conducted on a university campus (Risi et al 2021) which examines how pulsed foot traffic and associated anthropogenic food availability influence the feeding efficiency and body mass of city-dwelling Red-winged Starlings Onychognathus morio. Such short-term fluctuations might also vary tremendously between cities in temperate and tropical regions, being not only influenced by climate, but also by the historical evolution and socio-demographic structure of a city (Ramalho and Hobbs 2012).…”