Avian Urbanites: Which Birds Can Make it in the Urban Jungle?
Source Article: Neate-Clegg, Montague H.C., Tonelli, B.A., Youngflesh, C., Wu, J.X., Montgomery, G.A., Şekercğlu Ç.H. (2023). Traits shaping urban tolerance in birds differ around the world. Current Biology 33(9). https://doi.org/10.1016/j.cub.2023.03.024.
Featured Image Caption:. Two feral pigeons (Columba livia) taking a rest on the Empire State Building, New York City. Feral pigeons are the most ubiquitous, if not the most beloved, of all urban birds. (Image Source: Feral pigeon -Empire State Building, New York City, USA-31Aug2008d by ZeroOne, CC BY-SA 2.0, via Wikimedia Commons.)
As the world’s urban centers continue to grow, so too do their impacts on wildlife. Cities are often located in ecologically rich areas, and the conversion of natural habitats into islands of concrete has predictably disastrous impacts on biodiversity. Some species, however, show a remarkable capacity to survive and even thrive in these new habitats. Which traits separate these urban adapters – the grey squirrels, house sparrows and cockroaches of the world – from those that vanish in the face of urban sprawl? What does it take to make it in the urban jungle?
Birds in an Urbanizing World
Birds, as a well-described and species-rich group containing many urban adapters, are ideal for addressing this question. Previous studies have examined the relationship between avian ecological and morphological traits, on the one hand, and urban success, on the other. While these have shown some consistent trends – social-living, for example, gives species an edge in urban areas – other traits show conflicting results. However, these studies have been restricted either taxonomically (raptors, for example), geographically (skewed towards Europe, North America, or Australia), or simply covered a small number of species. There is thus a need for a truly global, comprehensive study of urban bird adaptation.
A new study fills this gap by quantifying the urban adaptiveness of 3,768 bird species – around 35% of all those described. Researchers used records from the citizen science platform eBird in combination with night-time light data to calculate an Urban Association Index (UAI) for each species, with a higher number indicating higher levels of urban abundance. Their dataset covered 137 cities representing 11 biomes and 6 continents. They then compared UAI with morphological and ecological data from the database AVONET. Researchers focused on 10 traits thought to impact urban adaptiveness, ranging from bill and wing shape to territoriality, diet, and longevity.
Be Flexible, Be Small, Live Long and Prosper
Urban adapters tended to have wide dietary and habitat breadth, meaning that they were flexible both in terms of what they ate and where they lived. Urban birds are dealing with entirely novel habitats and food sources, so it makes sense that the most flexible species have an advantage! Habitat breadth was especially important in tropical cities, where species are often highly specialized to the forests where they evolved. In temperate regions, on the other hand, dietary flexibility was more important, as species there need to manage the ebb and flow of seasonal resource changes.
Species with large clutch sizes – meaning that they lay many eggs at a time – did better than average, confirming a result found in smaller-scale studies. These species tend to live shorter lives and evolve quickly, aiding their transition to new environments. Longevity on its own, however, correlated positively with urban adaptiveness, especially in densely populated cities. This is perhaps because longer-lived species can develop larger brains and higher levels of intelligence.
The strongest predictor of urban success was lower elevational limit, which reflects the fact that most cities are found at low elevations. Territoriality, meanwhile, had a negative impact on urban success. This could indirectly reflect sociality – as mentioned before, social species tend to do better in cities, in part because they can learn survival tips from group members. Territorial species are intolerant of other members of their species, and therefore deprived of crucial learning opportunities. They are also less willing to search high and low for new food sources, which could be fatal if their territory is in a resource-poor patch of the city.
Larger species were at a disadvantage, likely because they spend more time on the ground and are thus vulnerable to direct predation (by cats, for instance) and egg predation (by rats). Ground nesting was likewise a poor strategy for urban life, a conclusion reached by previous studies. Birds were also assessed according to their hand-wing index (HWI), a measure of wing shape often used to estimate dispersal ability. Birds with a high HWI did better in cities, which could reflect the ability of migratory birds to adapt to anthropogenic change; it could also be because birds with HWI are more efficient fliers in general and better able to exploit many of the food sources that cities offer.
City Dwellers and Awkward Urbanites
The ideal urbanite, then, is a small, aerodynamic, long-lived, and non-territorial bird that lives at low altitudes, lays large clutches, nests above the ground, and isn’t too picky about its food or habitat requirements. According to this study, the birds found in the most cities worldwide were feral pigeons (Columba livia), house sparrows (Passer domesticus), barn swallows (Hirunda rustica), ospreys (Pandion haliaetus) and peregrine falcons (Falco peregrinus); with a few exceptions, these urban icons match the profile described above.
Conversely, this study allows us to predict which birds will be ill-suited for city life. As cities expand, we should offer protection to the large, ground nesting, short-lived, and ecologically specialized species, as these are least likely to feel at home in expanding urban ecosystems.
