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.