Date Published: February 8, 2018
Publisher: Public Library of Science
Author(s): Petra Sumasgutner, Marius Adrion, Anita Gamauf, Petra Quillfeldt.
As the world experiences rapid urban expansion, natural landscapes are being transformed into cities at an alarming rate. Consequently, urbanization is identified as one of the biggest environmental challenges of our time, yet we lack a clear understanding of how urbanization affects free-living organisms. Urbanization leads to habitat fragmentation and increased impervious surfaces affecting for example availability and quality of food. Urbanization is also associated with increased pollution levels that can affect organisms directly, via ecophysiological constraints and indirectly by disrupting trophic interactions in multi-species networks. Birds are highly mobile, while an individual is not necessarily exposed to urban stressors around the clock, but nestlings of altricial birds are. Such a city-dwelling species with a long nestling phase is the Eurasian kestrel (Falco tinnunculus) in Vienna, Austria, which forage on a diverse diet differing in composition from rural habitats. Furthermore, prey items vary in nutritional value and contents of micronutrients like carotenoids, which might impact the nestlings’ health. Carotenoids are pigments that are incorporated into integument tissues but also have antioxidant and immunostimulatory capacity, resulting in a trade-off between these functions. In nestlings these pigments function in parent-offspring communication or sibling competition by advertising an individual’s physical or physiological condition. Anthropogenic disturbance and pollutants could have disruptive effects on the coloration of these traits. In this study, we measured carotenoid based coloration and other indicators of individual health (body condition and susceptibility to the ectoparasite Carnus hemapterus) of 154 nestling kestrels (n = 91 nests) along an urban gradient from 2010 to 2015. We found skin yellowness of nestlings from nest-sites in the city-center to be least pronounced. This result might indicate that inner-city nestlings are strongly affected by urban stressors and depleted their stores of dietary carotenoids for health-related functions rather than coloration. In addition, skin yellowness intensified with age and was stronger pronounced in earlier nests. Since the immune system of nestlings is still developing, younger chicks might need more antioxidants to combat environmental stress. Additionally, parasite infection intensity was highest in nestlings with less intense skin yellowness (paler or less yellow pigmented integuments) and in earlier nests of the season. In combination with results from previous studies, our findings provide further support for the low quality of the inner-city habitat, both in terms of productivity and individual health.
Wildlife around the globe faces the dangers of a novel, quickly spreading habitat infiltrating the natural environment: urban areas, i.e. cities and other human settlements. Through the drastic changes imposed by cities, such as increased impervious surfaces, habitat loss and fragmentation, noise, light and chemical pollution, introduced alien species and predators, and diet alteration, urbanization acts as a filter for species communities [1–3]. This leads to biodiversity loss through both random processes and mal-adaption of some species . Thus, urbanization is a novel and strong evolutionary force [5, 6]. In avian research, species are categorized according to their adaptability to urban landscapes in urban avoiders, adapters or exploiters [1, 7], reflecting their ability to cope with this novel habitat. Some species seem to cope well with urban environments and react with rapid evolutionary adaptations to city life [8–10]. Other species, however, show negative responses to anthropogenic stressors in terms of productivity and individual health. Furthermore, urbanization influences host susceptibility to parasites and infectious diseases and can intensify the detrimental effects of pathogens by weakening the immune system .
Along the urban gradient, we found skin yellowness of nestlings from nest-sites in the city-center to be least pronounced, indicating that they are stronger affected by novel urban stressors than nestlings from more rural areas, and indeed allocate these carotenoids as important micronutrients to antioxidant defense instead of coloration. In addition, skin yellowness intensified with age and was stronger pronounced in male nestlings. The other health indicators used, i.e. body condition and parasite infection intensity were not directly linked to the urban gradient, but one could argue some indirect effects (see Fig 6).