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, this study looks at 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.
First, this study revealed that urbanisation significantly shapes arthropod prey availability within urban mosaics, with likely important implications for bird foraging behaviour, nutritional status, reproduction, survival, and distribution. Specifically, urbanised areas support fewer web spiders and springtails but more aphids, crab spiders, barklice, and dipterans (Chatelain et al., 2023). These urbanisation-driven changes in arthropod communities suggest that urban landscapes act as environmental filters, reducing arthropod richness and diversity, particularly in the canopy and bush layers. Such modifications in arthropod communities are expected to disrupt ecological interactions, notably predation. Amongst others, this study underscores the likelihood of bottom-up effects on insectivorous and omnivorous birds. By altering prey availability, urbanisation may shift bird diets or increase foraging effort, potentially compromising bird nutritional status and reproductive success (Dresner & Moldenke, 2017) and influencing species occurrence (Planillo et al., 2020). This research thus provides valuable insights into potential cascading impacts of urbanisation on urban food webs and bird populations.urbanisation significantly shapes arthropod prey availability within urban mosaics, with likely important implications for bird foraging behaviour, nutritional status, reproduction, survival, and distribution.
Second, our study on the diets of great tits and blue tits revealed that both species rely heavily on arthropods during the breeding season (April to June) (Chatelain et al., 2026). However, urbanisation reduces the availability of key arthropod groups, such as lepidopterans and spiders. In blue tits, the reduced consumption of caterpillars was associated with an increased intake of other arthropod groups, particularly spiders and aphids, suggesting that blue tits maintain a broad and flexible diet in both urban and rural habitats. In great tits, we observed an increase in diet diversity (the number of taxa consumed) and individual dietary variation along the rural-urban gradient. This indicates that urban environments promote opportunistic feeding behaviour in great tits, likely reflecting the fragmented and diverse nature of urban habitats. Our study also found that bird feeding plays a significant role in the diets of great tits and blue tits, with both species consuming high-fat foods, such as sunflower seeds and peanuts, from October to April. Notably, urban great tits consumed more sunflower seeds during the breeding season compared to their rural counterparts. Therefore, our study highlights that urbanisation-linked dietary shifts may come with nutritional trade-offs. While urban-adapted species like great tits and blue tits can exploit new food sources, these alternative diets may lack the nutritional balance of natural food sources. For instance, a decrease in moth consumption likely results in reduced omega-3 intake, while a lower intake of protein-rich arthropods, such as caterpillars and spiders, may limit the birds’ ability to produce eggs and provide essential amino acids for embryo development. Sunflower seeds are rich in omega-6 fatty acids but lack the proteins and omega-3 fatty acids found in arthropods (Andersson et al., 2015). An imbalance favouring omega-6 over omega-3 can lead to chronic inflammation, reduced immunity, and compromised egg quality, potentially resulting in lower hatching success and chick survival (Attia et al., 2022; Gutiérrez et al., 2019). Therefore, our results highlight the role that play urbanisation-linked diet shift in the lower productivity of urban populations compared to their rural counterparts.
Updates on the project can be found at: bit.ly/kohl-und-blaumeisen
