Do Green Roofs Help Urban Birds?

The answer is YES, but with lots of caveats. It all depends on the bird species that you are trying to help; the structure, plant species composition and the area of the rooftop habitat; distances from source bird populations; and management of the surrounding urban landscape. Let's explore these issues in greater detail.

What is a Green Roof?

A green roof is a vegetated green space on top of a building, and created by adding layers of growing medium (root protection, drainage, and substrate or soil layers) and plants on a waterproof membrane. Green roofs are usually grouped into two categories: extensive and intensive green roofs. Extensive green roofs have a thin growing medium, are vegetated with slow-growing succulent shrubs (e.g. Crassulaceae species) and grasses, require little maintenance, and rely on rainfall and dew as their only sources of moisture. Intensive green roofs have much thicker layers of growing medium, usually contain more complex vegetation associations and layering, including small trees, require a lot of maintenance, and usually have an established irrigation system.

Extensive and semi-intensive green roofs are generally simple habitats in a harsh environment, similar to the hard ground surfaces, thin substrates and low-moisture conditions found in brownfields and grasslands. Therefore, fauna that use these areas are usually tolerant of moderate to high ambient temperatures, bright sunlight, dry conditions, acidic and low-nutrient soils, and originate from grasslands and pioneer habitats.?

Intensively-managed green roofs often have a more limited value as fauna habitat because they often contain areas of mowed lawn and pesticide and fertiliser application. They are also likely to be subject to more disturbances due to human and pet activity, which can also result in trampling of vegetation and compaction of the growing medium.

What Bird Species Use Green Roofs?

Globally, at least 50 bird species have been recorded using green roofs, 29 of these have nested on them.?Bird taxa recorded using green roofs include gulls and terns; oystercatchers, plovers and lapwings; ducks and geese; pigeons and doves; larks and skylarks; falcons; and grassland, scrubland and woodland passerines. While the number of bird species recorded using green roofs is not very high, some are species of conservation significance.?

How Do Green Roofs Help Birds?

The main values of green roof habitats for birds are provision of food; shelter from extreme weather and predators; breeding habitat; refuge habitat; drinking water (dew), or drinking and bathing water (if the green roof is irrigated or has a pond); resting and refuelling spots for migratory and dispersing birds; and additional habitat for individuals of resident bird populations to colonise or occupy. They may also act as “stepping stones” between larger green spaces in the urban environment in the absence of continuous corridor links.

Food may be in short supply for birds that live in city environments and this may be one of the main reasons that birds visit green roofs. Arthropods that are potential food for birds readily colonise green roofs. These include spiders, beetles, bugs, springtails, flies, butterflies, bees, wasps and ants. Seeds, fruit and nectar are also important food sources for some bird species that use green roofs. Some birds forage on green roofs while on migration and these sites are likely to be important refuelling spots on short stop-overs.

Birds nest on green roofs if there are nest sites. The vegetative cover can provide protection against extreme weather, including shade from the heat, relief from the wind and rain and radiant heat loss at night. However, there is little evidence of birds breeding successfully on green roofs.?

Green Roofs in Context of the Surrounding Landscape.

Green roofs are still really not that common in world cities. Abundances and designs of green roofs are restrained by building architecture, the practical difficulties of establishing them (especially on the roofs of tall buildings), exposure to extreme ambient conditions (e.g. climate, weather and sunlight), fire risk, the costs and time involved in maintaining them, and the inadequacy of many city councils in encouraging and regulating the establishment of green roof spaces.

At least 10-35 ha of continuous green space are required to support most urbanised bird populations. Green spaces greater than 50 ha in size are necessary for the conservation of most threatened and urban avoider species. However, most city parks fall below this size range, and even small urban green spaces can support considerable biodiversity provided that there is sufficient habitat quality. The minimum theoretical patch size threshold for sustaining populations of urban-adapted species is 4.4 ha. Existing extensive and semi-intensive green roofs range from less than 0.01 ha to over 1.3 ha in area, whereas intensively-managed green roofs are known to range from 0.1 to 10 ha. Therefore, on size alone, while green roofs may be important stopping-over points for migrating birds, important stepping-stones between larger green spaces, or refuge habitat for displaced birds or individuals seeking shelter from the weather and predators, they are most likely only of value in helping to sustain city populations of urban-adapted grassland, shrubland and open woodland species. This may change in the future if more green roofs with suitable habitat, and shorter distances between them, are established in cities, adding significantly to a city’s total green space area.