The influence of anthropogenic food on bird behaviour and community structure in urban environments

Stofberg, Miqkayla

The influence of anthropogenic food on bird behaviour and community structure in urban environments

Thesis / Dissertation

2023

Abstract

Anthropogenic food sources have been recognized as an important trophic input in urban systems, and a major driver of the colonization of urban areas. Among various taxonomic groups, birds have exhibited a remarkable adaptation to incorporate anthropogenic food into their diets. This may have profound impacts on almost every aspect of their ecology, with the potential to trigger cascading effects on the functionality of urban ecosystems. Despite the clear significance of these dietary shifts, they have only recently begun to receive attention. To address this knowledge gap, this thesis aims to investigate the impacts of anthropogenic food on avian populations thriving in urban environments. By examining its influence on the selection of avian ecological traits, diversity, behaviour, physiology, and morphology of urban birds, I aim to gain a comprehensive understanding of their adaptation to this novel ecological niche. Species vary widely in their ability to exploit these novel foods; therefore, it sees evident that anthropogenic food is likely to favour some species over others. In Chapter 2, I employed a traitbased approach to investigate the utilization of anthropogenic food by avian species. I review the global literature of studies quantifying the diet of urban-inhabiting bird species. The results showed a diverse range of bird species exploiting anthropogenic food sources, with varying degrees of utilization, ranging from 0% to 93% of their diets across species. There was no evidence of a phylogenetic relationship between species and the level of exploitation. Instead, the percentage of anthropogenic food in the diet of birds differed with dietary guild. Scavengers, generalists, and species with aquatic diets exhibited a greater propensity to consume anthropogenic food in substantial quantities when compared to dietary specialists, such as insectivores and carnivores. I also observed a notable disparity in the utilization of anthropogenic food between hemispheres, with birds in the Northern Hemisphere demonstrating a considerably higher consumption of anthropogenic food compared to their Southern Hemisphere counterparts. This likely mirrors the prevalence of bird feeding practices that are more established and prominent in the Northern Hemisphere, particularly in European cities, where residents are deeply motivated by their desire to enhance bird survival during the harsh winter months. My research underscores the significance of preserving natural resources for specialist species while also managing and controlling the availability of anthropogenic resources to mitigate the dominance of particular species in urban ecosystems. The spatial distribution of food resources may vary substantially in urban environments and be directly responsible for the patterns of diversity observed in urban environments. For example, within urban areas there is considerable evidence that species diversity and abundance correlate positively with socioeconomic status, a pattern termed the ‘luxury effect'. The potential influence of anthropogenic food abundance on the luxury effect has not yet been investigated. In Chapter 3, again using a trait-based approach and citizen science data, I explored the reporting rates of species in relation to household income within urbanised areas across South Africa. Carnivores, generalists, granivores and aquatic species exhibited the most pronounced rise in their likelihood of occurrence with increasing income. I also found evidence that the luxury effect was steeper for species endemic to South Africa and for species which were locally threatened. In contrast, species with differing nesting location, clutch size, body mass, residency and territoriality responded similarly to income. My results support to the idea that avian functional diversity in urban environments may be influenced by differences in the availability of both natural and human-provided food in urban areas and may have far-reaching implications for the delivery of ecosystem services and disservices to urban residents. Under adverse natural food conditions, the abundance of anthropogenic food can compensate for natural food scarcity, thereby improving survival. The most notable evidence for this comes from the effects of supplementary feeding during winter in cities in the Northern Hemisphere. The potential for anthropogenic food in urban areas to buffer the potential costs associated with foraging under elevated temperatures however have been less considered. In Chapter 4, I explore the impacts of air temperature on the foraging behaviour and body mass of an urban-dwelling passerine, the Redwinged starling, Onychognathus morio at a university campus, where anthropogenic food abundance fluctuates over short timescales, with food being more abundant on weekdays and less abundant on weekends. I hypothesized that abundant anthropogenic food on weekdays could mitigate the negative effects of elevated temperatures on behaviour and mass change. More specifically, I predicted that the onset of evaporative heat dissipation (i.e., panting) would occur earlier with greater access to water resources from anthropogenic food sources (e.g., fruit). With regards to behaviour, I predicted that foraging effort would decline more rapidly with elevated temperature as birds can avoid prolonged activity and heat exposure under unfavourable temperature conditions. As a consequence of higher anthropogenic food availability on weekdays, I also predicted that starlings would be able to maintain food consumption rates with under elevated temperature conditions and that diurnal body mass changes would be less pronounced. I found that individuals increased panting with temperature while foraging effort declined. Despite reduced foraging, starlings were able to maintain food intake rate and body mass with rising temperatures irrespective of the short-term food fluctuations. This suggests that both natural and anthropogenic food sources in this urban environment were abundant enough to buffer individuals from the effects of elevated temperature. Anthropogenic food may lack the essential nutrients typically provided by more balanced natural diets and could have detrimental effects on health and fitness. While bird species extensively exploit anthropogenic food, there has been limited exploration of the physiological implications associated with such consumption. In Chapter 5, using a food supplementary food experiment, I investigated the effects of a “processed” and “unprocessed” diet on the (1) behaviour (foraging effort, food consumption, provisioning, perching and preening) and (2) body mass change of adult Red-winged starlings and on the (3) relative plasma fatty acid (FA) percentages and (4) morphology (body mass, head, wing, tarsus and tail length) of their nestlings. My results showed no difference in adult behaviour and body mass with treatment. Instead, the blood plasma of nestlings assigned to a processed diet were significantly higher in saturated FAs and lower in -6 polyunsaturated FAs, while treatment had no effect on nestling morphology. These findings provide limited support for the idea that anthropogenic food benefits adults or poses detriments to the growth and development of their nestlings. They may indicate that urban species display physiological flexibility, allowing them to withstand substantial changes in morphology despite distinct variations in the physiological effects of anthropogenic food. This thesis significantly advances our understanding of urban avian species' interactions with anthropogenic food resources. It highlights the central role of anthropogenic sustenance in shaping urban ecosystems, driven by species-specific tolerance levels. The research sheds light on why certain species, like generalists, scavengers, granivores, and aquatic species, thrive in urban environments while others face challenges. These insights have broad implications, emphasizing the need to regulate food access and cater to dietary specialists. The study also explores the positive and negative effects of processed anthropogenic food, with potential species-specific variations. These findings are vital for urban management, especially amid increasing urbanization and climate change challenges.

Keywords

Biological Sciences

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Collections

PhD / Doctoral

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