Urbanization is one of the primary factors affecting the decline of global bird populations. Bird habitat is being degraded and destroyed at alarming rates, and what habitat remains is often unevenly distributed across the landscape. This urban ecosystem favors species, such as pigeons and swallows, that survive well in cities because of their ability to nest on concrete structures. Walk around any city, and you will see plenty of birds. But, you are unlikely to see much bird diversity. Typically a few, non-native invasive species predominate.
Similarly alarming, many of the world’s major metropolitan areas lie along migratory bird flyways. Flyways are the routes used by migratory birds as they traverse between their summer habitat and wintering grounds. Birds must stop regularly along these flyways to rest and eat. In many urban areas stopover sites are limited, because of the scarcity of green space. As a results birds must expend scarce energy reserves in search of suitable stopover sites.
Well-designed green roofs may help alleviate some of the strain on bird populations. Typical green roofs attract common bird species with ease. But, in order to attract rare species, or target a particular endangered species greater attention must be paid to habitat needs. In order to attract both resident and migratory species green roofs should provide the food, water, cover, and space necessary to support birdlife.
Birds have been found to favor green roofs that have diverse invertebrate (insect and spider) communities. This can be achieved through planting a variety of plants rather than covering roofs with monoculture crops and limiting the use of insecticides. Similarly, many invertebrates and birds are attracted to fruiting and flowering plants. So rooftop vegetable gardens and wildflower plots that flower on a rotating cycle may be used to attract additional bird species.
Birds typically obtain water from drinking at surface water pools. Green roofs that incorporate water features like ponds and fountains are particularly successful at attracting birdlife. When surface water is unavailable some birds may be able to extract water from succulents, which are often very successful green roof plants.
Birds need cover to protect against adverse weather and predation. Small trees, bushes, and leafy plants all provide ample coverage from the elements. Some bird species may nest directly on the ground, but many will require man-made nest boxes or birdhouses. Birds can be quite finicky about their preferred nesting conditions, so it is important to research species of interest and build boxes that suit their individual needs. A beginner’s guide to building specialty birdhouses can be found on the Cornell Lab of Ornithology website.
Space is certainly a limiting factor in any urban environment. Birds will need space over which to graze and travel on a regular basis. A single rooftop, in most cases, will be inadequate to support a thriving bird community. A network of green spaces, that may include neighboring roofs, parks, and ground-level greenways connecting these spaces is essential.
The City of Toronto provides an excellent guide to designing Biodiverse Green Roofs.
Feature Image: Bird nest on a green roof. Image via greenroofs.com