This post was contributed by Luis Sandoval, lead author of a recent paper in The Wilson Journal of Ornithology on cavity-nesting birds in Neotropical urban environments.

When we as a society build cities, we destroy natural habitats. The result is a new habitat that has never existed before for any wild species. It’s not surprising that not all species that lived in an area before a city was built survive in this new urban habitat. However, some species appear to deal well with these changes and persist inside cities, especially in areas where parks preserve fragments of the original natural habitats. A few species highly associated with humans (such as mice, rats, cockroaches, flies, and doves) even take advantage of urban habitats to survive and thrive.

Where species occur depends on not only on habitat, but on the availability of all of the resources they need, such as food, breeding sites, or material for nest building. For example, species that eat a wide variety of foods but need a very particular habitat or substrate to build a nest will not occur in areas without those breeding requirements, even if food is abundant. On the other hand, species that can nest anywhere and do not need a particular substrate or nest material, but eat a particular type of fruit or nectar, may not occur in areas without their specific food. Between these differences in species requirements and the reduction of natural habitat due to urban development, it’s easy to understand why surviving in cities is a challenge for most bird species. With all this in mind, we asked how cavity-nesting birds such as parrots, toucans, woodpeckers, swallows, motmots, flycatchers, and wrens are faring inside cities in our recent published paper in The Wilson Journal of Ornithology titled “Artificial cavities and substrates for cavity-nesting birds in Neotropical urban environments: A novel resource or the last opportunity to breed?”.

The occurrence of the cavity-nesting bird species is highly limited by the presence of their nesting substrates (dead trees or dirt banks), and therefore they may not occur in cities if those substrates are absent. However, as humans we also indirectly create new substrates that these species may use to build their nests, such as wood poles for electrical lines and other utilities, walls with artificial cavities, metallic pipes to support roofs, or cavities inside building walls. These new substrates may compensate for the lack of natural nesting sites and allow the occurrence of the cavity-nesting bird species in urban areas.

Our study showed that in Costa Rica, 24 cavity-nesting bird species are using artificial substrates in urban areas to build their nests and reproduce. We even observed a Cocoa Woodcreeper, a species that is associated with humid forest and normally nests inside cavities in trees, building a nest inside a wood wall that was part of housing for researchers. The House Wren and the Red-fronted Parakeet were the species that used the most substrate types inside cities (6 and 5 respectively). House Wrens, which are very small (12 g), built nests inside metal boxes, wall cavities, concrete utility poles, metal pipes, public lights, and structural support beams. Meanwhile, Red-fronted Parakeets built nests inside building roofs, concrete wall cavities, plastic tins, support beams, and styrofoam eaves. These are good examples of how we provide substrates indirectly for bird species inside cities, because none of these substrates were intended to make up for the lack of nesting sites for cavity-nesting birds inside cities.

Additionally, our observations are evidence of how good cavity-nesting bird species are at taking advantage of those new substrate types that were not built with them in mind. In a world where cities are increasing constantly, especially in tropical countries, the lack of natural nesting substrates for birds will be a constant. Therefore, the majority of them will decline or disappear inside cities. But a small group of individuals and species that are smart or have good luck will use the new resources that humans build and may survive and even increase in abundance in cities, as other, competing species move away.

Cities are a challenging habitat to live in for the majority of wildlife species. Some birds, even cavity-nesting species, may survive in cities if they learn to use the new resources we provide for nesting or as food. However, as society conscious of the negative effects of urban development and natural habitat destruction, we need to produce proactive solutions to solve the lack of nesting sites for cavity-nesting bird species in cities, not allow the survival of those species to be a random process. If we want to contribute actively to the conservation of cavity-nesting bird species, we need to provide or maintain dead trees, dead branches in trees, earth banks, and natural cavities inside cities, as these are the main resources they need to reproduce.