Food-driven movement of birds in urban landscapes

The global land surface covered by cities is expected to triple between 2000 and 2030. Urbanisation is a major driver of environmental changes, including landscape fragmentation and food availability modifications. Nonetheless, cities are also inhabited by plants and animals, which have great ecological importance and social value. How does urbanisation affect wildlife in cities? Although timely, this question remains largely unanswered. In particular, our knowledge on how food availability and patchiness in the cities affect movements over time is at its infancy. Providing wild urban animals with food (e.g. seeds) during winter became a very common practice. This extra food particularly benefits birds that consequently cope better with harsh conditions in winter. For this reason, we hypothesize that cities are optimal wintering grounds and would attract individuals from outside the city or from areas with few human settlements within the city. On the contrary, birds produce less offspring and those offspring are of lower quality in cities compared to less disturbed environments, likely because of differences in the availability of essential nutrients during growth. Therefore, we assume that cities are sub-optimal breeding grounds. Altogether, the changes in cities ability to sustain bird populations over the year is likely to generate movements within the city and the surrounding environment. The great tit Parus major and the blue tit Cyanistes caeruleus are flagship animals that are now becoming reference species to study the ecological and evolutionary effects of urbanisation. In this project we will investigate how great and blue tit populations respond to urbanisation employing a gradient of urbanisation in Innsbruck and its surroundings. Specifically, we will study the effects of urbanisation on: 1) food availability for omnivorous bird species (i.e. arthropod availability in trees and bushes and human-provided food), 2) great and blue tit diet by identifying the prey DNA in their faeces, and 3) great and blue tit movements within the city and between the city and its surroundings. For the latter a novel combination of re-catching data and the metal pollution signature in bird feathers, which correlated with urbanisation level, will be used. Long-distance migration will also be investigated using feather isotopic signature. This project will improve our understanding of how animals move within cities in response to food availability and therefore answer key questions in the new field of urban ecology. Compared to previous studies, it will take into account the fine-scale structure of urban settlements to better catch the variability in urban habitat quality. Moreover, it will rely on innovative and non-invasive methods in molecular and chemical ecology to efficiently measure diet and movements of birds in the urban environment.

In Europe, urban areas represent 17% of the land and it is home to many wild animals and plants. Urban ecologists such as myself aim at investigating how organisms interact with each other and with their environment in the urban space. However, all too often the urban space is considered as a homogeneous environment. Yet, the urban space is a mosaic of different habitats such as forest remnants, residential areas, etc. Therefore, my first aspiration was to study songbird populations in the urban space using a study design accounting for such habitat diversity. For this purpose, this study included 180 study sites (in contrast to a couple of sites in most studies in urban bird ecology) within a 56.6 km area including the populated area of Innsbruck and its surrounding. Using remote sensing data on land use/land cover, the environment at each site was characterised in terms of percentage of built-up and paved-over area, of trees, of grasslands, etc. Thanks to this study design, I was able to study 456 birds (281 great tits and 175 blue tits) from highly urbanised areas to green areas. My intent was to measure (1) spatial and temporal variation in food availability for birds and (2) its effects on bird diet, health and seasonal movements within the urban mosaic. I decided to focus on food availability because (i) it is one of the main limitation for individual and population growth, and (ii) urban areas, with their exotic plants and numerous bird feeders, are expected to offer different type of food and at a different timing, compared to forest areas for instance. By sampling arthropods at the 180 sites where I also studied birds, I showed that, indeed, arthropod prey in trees and bushes are available in different amount within the urban mosaic: while web spiders and springtails occur less and are less abundant in more urbanised areas, such areas offer more aphids, crab spiders, barklice and dipterans. Interestingly, preliminary results on great tit and blue tit diet, measured using the prey DNA present in the bird droppings, showed that the two species adapt their diet to what is available: in more urbanised environments, birds were eating more crab spiders, aphids and dipterans, especially so during the reproductive season. Interestingly, in highly urbanised areas but also in rural areas, the consumption of cereals and sunflowers, undoubtedly from bird feeders, was very frequent outside of the reproductive season. This last result highlights the significant impact that may have bird feeding on birds close to human settlements. Ongoing analyses will soon provide results on bird movements and nutritional status within the urban mosaic.