Recovering Green Roofs to Feed Native Birds
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For decades, green roof advocate Dusty Gedge has kept watch over birds on London’s brownfields and green roofs, including Black Redstarts and Linnet’s, like this one feeding a Viper’s bugloss (Echium vulgare) plant on a large green roof in London. England has passed a policy that requires a biodiversity metric for new projects. Biodiverse green roofs (40+ plant species) are included, because mobile wildlife is looking for habitat with various sources of food, water, and protection. Dusty said that in his lifetime, the Linnet population has collapsed in the wider country and Europe because of Intensive farming. So, the positive story here is that Linnets and some other birds in peril that need native plants to feed on are finding relief with biodiverse green roofs across greater London and Europe. The white flower is Chicory (Chichorium intybus) , another native food source for Linnets in London. Photo: dustygedgephotography. See Sustainable Futures Podcast with Dusty Gedge.
Recovering Habitat and Food Sources for Birds in Peril
Bird populations are in steep decline worldwide. Grassland bird populations have suffered the most, with a 64 percent loss in North America since the 1960s. Agriculture and urbanization are the primary causes of the bird population decline (Rosenberg, Dokter et al. 2019). Industrial agriculture has largely replaced prairie ecosystems that birds co-evolved with. These endangered ecosystems were once some of the most biologically rich biomes on the planet. Today, pesticide use on agricultural lands has continued to impair bird populations by indiscriminately killing insects, (a major food source for birds) and directly poisoning them. In addition, urbanization has displaced bird habitats. Some bird species have been able to adapt well to urban settings; however, many species of birds require particular species of plants for food and shelter that cannot be found in cities.
A female Northern Parula (Setophaga americana) visits a green roof at the Javits Center in New York City. Their winter range extends from southern Florida to Central America and the Caribbean Islands. Northern Parula’s migrate to North America each spring to breed. These adaptable warblers can make use of green roofs with plants that attract their favorite foods including spiders, damselflies, locusts, bugs, grasshoppers, aphids, beetles, caterpillars, flies, wasps, bees, and ants, but also berries, seeds, and nectar when available. Photo: Gail Karlsson, NYC Audubon
We can help to diversify urban bird species by implementing new ways to recover and reclaim habitat for birds. Fortuitously, green roofs can be designed and maintained to become habitats for birds to find nourishment, rest, or places to roost (sleep). When designed and maintained appropriately, vegetation that is native to prairies, savannas, and meadows performs well on green roofs (Dvorak and Bousselot 2021). In urban areas, green roofs can help supplement the needs of local and migrating birds, and even some rare species of birds (Grant 2006). Here are some ways these have been accomplished.
Feeding Birds through Direct and Indirect Food Sources
There are a number of studies that have investigated how green roofs can support birds. Some researchers have studied sedum roofs exclusively, while others have looked at a variety of biodiverse planting approaches. Generally, green roofs (including sedum roofs) make a positive difference for birds that tolerate urbanization, but green roofs alone can’t make up for lost habitat (Partridge and Clark 2018). While ground-based habitats are usually more useful for all birds, even an isolated green roof in a city can provide some important resources for urban birds such as food and shelter (Eakin, Campa III et al. 2015).
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Researchers are confirming that plant choices make a big difference for attracting birds to green roofs (Fernández Cañero and González Redondo 2010). Generally speaking, the more kinds of native plants that are present and that produce seeds, nectar, berries, nuts, fruits, and other plant parts the more beneficial the green roof will be to support local and migrating birds directly and indirectly (Fernández Cañero and González Redondo 2010). Non-native sedum monocultures on green roofs typically offer more for birds compared to conventional rooftops, because they can support insects, for example. The design and installation of diverse patches of the appropriate native plants can be added to simple green roofs to help improve habitat structure with food sources for birds during their different stages of growth (Fernández Cañero and González Redondo 2010).
‘Direct support’ means that a bird might use a plant part for food, such as nectar, seeds, berries, fruits, or nuts. ‘Indirect support’ means that a plant attracts insects which in turn attracts birds that feed on those insects. A good way to understand which kinds of plants that birds may use on a green roof is to investigate the conservation needs of the ecoregion. Once several species of birds are targeted, it is important to then study their habitat needs (food, cover, water, shelter). It is also good practice to specify plants to serve a variety of common bird species as well. Many plants can serve multiple species of birds, bees, butterflies, moths, and other beneficial insects that use green roofs.
Urban prairie plant-butterfly bipartite network showing nectaring, ovipositing, mating, and basking interactions between plants and butterflies during 2017 and 2018. Thicker lines indicate a greater proportion of interactions. Original Figure 4.17 by Pamela Blackmore, Master’s Thesis 2019 (K-State University, thesis advisor Lee R. Skabelund). Addition of photos and text by B. Dvorak 2024. (Diagram with permission. Butterfly photos from Wiki Commons)
Plant it and They Will Come
Some of the birds that have been observed on green roofs in North America include various species of gulls, doves, ducks, falcons, geese, lapwings, larks, oystercatchers, pigeons, plovers, skylarks, terns, and passerines (a large order of grassland and woodland birds). In New York City, a mixture of native and non-native birds use green roofs including finches, bluejays, cardinals, cedar waxwings, cowbirds, goldfinches, gulls, mockingbirds, robins, sparrows, starlings, swifts, as well as ruby-throated hummingbirds and warblers (Partridge and Clark 2018). Kildeer are one species that make good use of green roofs in dense urban areas, including their use for breeding (Washburn, Swearingin et al. 2016). Even some flying mammals such as bats have been observed frequenting green roofs in NYC.
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Chicago City Hall
On the Chicago City Hall green roof, pink blooms of Joe-pye weed (Eutrochium sp., center) attract native bees and its seeds feed swamp sparrows in the fall. The lavender blooms (lower right) of blazing star (Liatris spicata) feed nectar to bees, butterflies, and hummingbirds. Big bluestem (Andropogon gerardii) (behind blazing star) provides cover for 24 species of songbirds and nesting sites or seeds for Grasshopper Sparrow, Henslow’s Sparrow, and others. Big bluestem provides seeds during the fall and nesting sites for Sedge Wrens and Western Meadowlarks and many species of insects. Photo: Bruce Dvorak
During the installation of the prairie crabapple tree (Malus ioensis) on the Chicago City Hall green roof, one of the landscape contractors shared with us (a small group from Conservation Design Forum) that a Peregrine Falcon scouted out the new habitat and perched on the tree on the same day it was planted. This bird obviously understood the feeding opportunities that a new habitat with thousands of new native plants would offer twelve stories above LaSalle Street. It was the first raptor to arrive and call “dibs” on the new habitat. Since the early years, the Chicago City Hall green roof has been a favorite habitat for peregrine falcons along with many other native birds (Millett 2004). There are at least one hundred species of local and migrating birds that use the Chicago green roof each year, and sometimes the migrating birds are so exhausted from their travels, that they don’t move or fly away when visitors walk nearby on the roof.
The diverse planting approach, much of which has been installed during the maintenance, includes over 200 species of native plants with some introduced species. Despite being located right downtown and surrounded by buildings, these plants provide birds with sources of food and cover and places to rest or nest. Direct food sources include nectar, seeds, fruits, and berries. Indirect sources include soil-based and mobile insects, as well as some small birds that birds of prey might pursue for a meal. The tallgrass prairie vegetation grows in an FLL compliant substrate that is 15.24 cm (6 inches) deep. Drip irrigation is located on the surface of the growing media and is used during the summer and anytime drought persists. Two wooden bee hives are maintained on the roof near the prairie habitats, to supplement pollination.
Over the years, biologists have observed that a number of taxa of local and migrating birds make regular use of the Chicago City Hall green roof including field sparrows, cape may warblers, chickadees, wrens, thrushes, woodpeckers, peregrine falcons, and olive-sided flycatchers (Millett 2004).
Native plants on the Arizona State University Transportation Center green roof include red aloe, yellow yucca (front left), giant hesperaloe, and lady’s slipper (back row) which is native to the Sonoran Desert (northwestern Mexico) and central Baja California. Photo: Bruce Dvorak
Tempe Transportation Center
Located at the north edge of the campus of Arizona State University, the LEED Platinum Transportation Center building makes use of a green roof and seven other energy conserving design features, used to reduce energy consumption by up to 52 percent. The building and site were designed to demonstrate how to reduce urban heat islands. In addition to conference rooms and office space, the center connects METRO light rail, bus and bike transportation needs for the city of Tempe and the university.
The green roof includes five native species planted in monocultures including red aloe (Hesperaloe parvoflora), yellow yucca (Hesperaloe parviflora ‘Yellow’), giant hesperaloe (Hesperaloe funifera), and lady’s slipper (Pedilanthus macrocarpus). These plants provide birds with nectar and seeds and attract insects to feed on. The 31 cm (12 inch) deep substrate is drip irrigated several times a week during the summer (Dvorak and Coseo 2021). The rooftops and paved areas collect and harvest rainwater which is used to irrigate plants on the ground. The nearby Hayden Butte Preserve and the Salt River greenway offer birds a corridor to connect to the soft and cool landing pad of the green roof compared to the hot and lifeless roofs of surrounding buildings. The green roof vegetation offers nectar, a source of energy for nectar-feeding birds.
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A hummingbird feeds on nectar in a drift of lady’s slipper (Pedilanthus macrocarpus) growing on the Tempe Transportation Center green roof in July. Fifteen species of hummingbirds frequent Arizona typically from April to October. Photo: Bruce Dvorak
NOAA Southwest Fisheries Science Center
The green roofs at the NOAA Southwest Fisheries Science Center in La Jolla, California (LEED Gold certified building) were designed by Jeffrey Bruce & Company to recover some of the plant communities displaced by the building and address local conservation needs of the Scripps Coastal Reserve (Dvorak and Drennan 2021). This award-winning green roof (GRHC Award of Excellence) provides support for the scientists at the center conducting research on the conservation of regional marine life.
Thirty-two native plant species were included on the center’s nine green roofs. They are planted in a locally mixed substrate that varies from 10 cm (4 inches) up to 23 cm (9 inches) in depth and the green roofs are irrigated with harvested rainwater. The vegetation includes native herbaceous perennials, annuals, woody shrubs, and succulents. Food sources for birds provided by plants include nectar, seeds from wildflowers and grasses, and berries. Popular herbaceous perennials planted on the green roof include hummingbird sage, (Salivia spathacea) and white sage (Salivia apiana). During the spring blooms, hummingbirds are one of the several bird species observed using the green roofs. Sarah Mesnick, a research liaison at the research center has an office that faces out to the green roof. Sarah has observed that when the sages are in bloom, hummingbirds occupy the habitat as they fight and compete for nectar.
It is interesting to note that leaves of the hummingbird sage are also grown for tea, and shortbread. It spreads by rhizomes and grows like a groundcover. It is a host plant for hummingbirds, songbirds, bees, and at least seven species of butterflies and moths. It tolerates full shade, part shade and full sun. With its low water use, it needs little irrigation.
The coastal scrub habitat at the NOAA Southwest Fisheries Science Center. Varied plant heights, varied forms of plants and varied sized stones all support habitat for insects and birds. White sage (Salivia apiana) and hummingbird sage, (Salivia spathacea) grow as part of the coastal scrub habitat seen and are a direct food source for hummingbirds (shown in the foreground). Grasses (background) provide habitat for birds to seek insects, or as a shelter, rest, roost, or nest. Photo: Bruce Dvorak
Attractive Nuisance?
Birds and buildings do not always have a mutually beneficial relationship. Building facades need to be designed so that birds don’t see the reflection of blue sky in a glass façade and believe that it is real. This deception causes dire consequences for birds in major cities each year. In Chicago, for example, one large building with glass facades along the shores of Lake Michigan took out nearly 1000 birds in one day during a mass migration. Audubon estimates that nearly one billion birds die from bird collisions with glass facades of buildings each year in North America. There are guidelines that have been developed to make buildings bird-safe at very low cost and a number of jurisdictions, including NYC and Toronto require bird safe glass on new construction projects.
Other issues with birds on green roofs include cases where geese or ducks nest on a roof and the hatchlings have no experience with navigating rooftops. Hatchlings have been known to sometimes fall to their death on green roofs without parapets. Attempts to relocate baby birds to the ground by humans have been largely unsuccessful. This means that if a green roof is being designed for birds, especially birds that may nest on the roof, the edges of buildings (parapets) should be designed to retain and keep birds in place on a building until they mature. Other instances include the presence of some birds that peck at and pull up seedling plants, disturb substrate to dig for insects, or cause disruption to plants (Fernández Cañero and González Redondo 2010).
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Summary
In life, we generally strive to protect things that we understand and care for. Given the current state of decline of biodiversity worldwide, it appears that we need to learn and care a lot more about the ecological consequences of our land use policies, practices, and regulations. Green roofs can be used to support declining bird populations in urbanized regions, if they are planted with sources of food that birds are looking for. Although there are some pros and cons to attracting birds to green roofs, if green roofs and building façades are designed to be bird-safe, they can make positive contributions to urban areas for local and migrating birds. In part, this includes the opportunity for education, human well-being, as well as direct and indirect support for the birds themselves.
Bird safe glass adjacent to an intensive, accessible green roof at Humboldt University in Berlin. Photo: Steven Peck
With the right plants, a green roof can become a long-term (multiple decade) investment for bird conservation and other forms of wildlife. Although there has been some research on how birds use green roofs, most of the research to date has taken place in Europe. There are only a few long-term studies of how birds and wildlife use biodiverse green roofs in North America. New studies are needed so we can better understand how green roofs can be designed to support birds and wildlife.
In this article, several key takeaways are:
Bird populations are an indicator of overall ecosystem health.
Bird populations are in decline worldwide.
Biodiverse green roofs support various needs of birds.
More research and policies are needed to support how green roofs can better serve birds.
Planting a diverse mix (40+ species) of native plants that support direct and indirect needs of birds is necessary.
Direct support includes specifying plants on green roofs that develop seeds, nectar, berries, nuts, and can attract local or migrating birds.
Indirect support means that the green roof vegetation attracts airborne or soil-based insects that birds feed on.
Selecting vegetation local to the region can offer native birds something that they need to survive.
Including a source of water on the roof will attract insects and birds as will different compositions and depth of growing media, rather than a uniform application.
Buildings that support biodiverse green roofs need to be designed to prevent bird collisions with glass facades. Proven methods should be implemented for buildings with green roofs.
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New Green Roof Biodiversity Courses
To learn about how green roofs can support heat-tolerant plants and biodiversity, see the new Living Architecture Academy Course: Case Studies of Biodiverse Green Roofs, by Bruce Dvorak.
Bruce Dvorak, FASLA is a Professor at Texas A&M University in the Department of Landscape Architecture and Urban Planning, where he has been conducting green roof and living wall research since 2009. Bruce is a member of the GRHC Research Committee and founded a new Regional Academic Center of Excellence in 2022, the Southern Plains Living Architecture Center. Bruce received the GRHC Research Award of Excellence in 2017 and teaches green roofs and living walls in his courses in landscape architecture programs at Texas A&M University. His edited book, Ecoregional Green Roofs: Theory and Application in the Western USA and Canada (2021) provided inspiration and content for this article.
Acknowledgments
Thank you to Dusty Gedge for sharing his wonderful photography and for sharing his knowledge and stories of the success of biodiverse green roofs in London and Europe. Thank you also to Dustin Partridge and Gail Karlsson with New York City Audubon for sharing their photography.
References
Dvorak, B. and Bousselot, J. (2021) "Theoretical development of ecoregional green roofs." Ecoregional Green Roofs: Theory and Application in the Western USA and Canada Cham, Springer International Publishing: 41-79.
Dvorak, B. and P. Coseo (2021). Green Roofs in Desert Southwest Ecoregions. Ecoregional Green Roofs: Theory and Application in the Western USA and Canada. B. Dvorak. Cham, Springer International Publishing: 201-256.
Dvorak, B. and P. Drennan (2021). Green Roofs in California Coastal Ecoregions. Ecoregional Green Roofs: Theory and Application in the Western USA and Canada. B. Dvorak. Cham, Springer International Publishing: 315-389.
Eakin, C. J., H. Campa III, D. W. Linden, G. J. Roloff, D. B. Rowe and J. Westphal (2015). "Avian response to green roofs in urban landscapes in the Midwestern USA." Wildlife Society Bulletin 39(3): 574-582.
Fernández Cañero, R. and P. González Redondo (2010). "Green roofs as a habitat for birds: a review." Journal of Animal and Veterinary Advances, 9 (15), 2041-2052.
Grant, G. (2006). "Extensive green roofs in London." Urban habitats 4(1): 51-65.
Millett, K. (2004). "Birds on a cool green roof." Chicago Wilderness 7: 6-9.
Partridge, D. R. and J. A. Clark (2018). "Urban green roofs provide habitat for migrating and breeding birds and their arthropod prey." PLoS One 13(8): e0202298.
Rosenberg, K. V., A. M. Dokter, P. J. Blancher, J. R. Sauer, A. C. Smith, P. A. Smith, J. C. Stanton, A. Panjabi, L. Helft and M. Parr (2019). "Decline of the North American avifauna." Science 366(6461): 120-124.
Washburn, B. E., R. M. Swearingin, C. K. Pullins and M. E. Rice (2016). "Composition and diversity of avian communities using a new urban habitat: Green roofs." Environmental management 57: 1230-1239.