I always recommend that we grow as many native plants as we can to sustain wildlife, but to avoid overwhelming apprehensive gardeners I also mention that our yards don’t have to be exclusively native to be beneficial. Well, now there’s a number to aspire to: 70 percent native, minimum. That’s what a group of researchers have found is necessary for insectivorous birds to raise healthy young and keep their populations steady in human-dominated landscapes, the most swiftly growing ecosystem on the planet.
Their study, the first to examine the effect of non-native plants on an insectivore, looked at the connection between plants, the arthropods (insects, spiders and others) that eat and hang out on those plants, and the breeding success of one insectivorous bird species that, along with most other terrestrial birds, cannot survive without consuming arthropods. Published in Proceedings of the National Academy of Sciences, it was conducted in the Washington D.C. area by the usual suspects, University of Delaware researchers Doug Tallamy and Desirée Narango, along with Peter Marra, director of the Smithsonian Migratory Bird Center. They sought to determine how exotic plants affect songbirds’ reproductive success in urban and suburban landscapes.
Data was collected from about 150 citizen-scientist homeowners whose properties were provided with artificial nest boxes to attract paired Carolina chickadees*. Once their nests were complete, the researchers recorded life on plants within a 50-meter radius where nesting chickadees search almost incessantly for the most nutritious food they can find. During breeding season, arthropods make up more than 90 percent of their diet, which is composed primarily of moth and butterfly larvae, spiders, and Hemipterans (such as aphids and leafhoppers). During non-breeding season, chickadees will consume some plant material, but more than half of their diet is still animal-based, which may have important implications for annual survival. Throughout the year, caterpillars—rich in fat, protein and carotenoids—are an extremely important food item and essential to nestlings’ fast growth.
Unsurprisingly, native plants were teeming with “bird food,” while non-natives were nearly devoid of life. The reason? Most native insects need native plants because they are specialists—they co-evolved with certain plants and can feed only on them due to their chemical compositions; they cannot survive where those native plants don’t exist.
Nest boxes were also monitored, as was the survival of parents and fledglings. Analysis of data revealed rapid declines in populations of Carolina chickadees when yards supported mostly non-native trees and shrubs. As soon as the percentage of natives falls below 70, the probability of sustaining the species drops to zero. In other words, when there is little native plant biomass, the parents either do not establish nests or they cannot locate enough food and their babies starve to death. But at 70 percent or higher, the birds can thrive and sustain their populations. The number is a baseline: The more insectivorous a bird, the higher percentage of native plants needed to support them.
Developers and property owners typically convert native plant communities into habitats composed of mostly non-native plant species. Usually chosen for some aesthetic effect or because they’re so commonly available, they are extremely poor at supporting native invertebrates at the base of the food chain and those—such as songbirds—who cannot survive without such highly nutritious prey. Non-native plants—invasive or not—appear harmless, but substantially influence ecosystems in dangerous ways. Effects that begin at the bottom of the food chain go straight up, creating so-called ‘food deserts’ for birds, which may lead to starvation and possibly local extinction. Sadly, that is the case with most yards. If we really want to help birds, we need to realize that their lives are in our hands. Small changes for us will be colossal for them.
Though the study focused on just one insectivorous bird species in the mid-Atlantic region, the results are applicable to migratory birds who need high quality food at stopover sites as they undertake their arduous, exhausting semiannual journeys, as well as 431 other insectivorous species (in the U.S.) that need similar support in habitats far away. Because I live in an urban area where natural cavities for cavity-nesting birds (such as black-capped chickadees and woodpeckers) are scarce, each spring our clean chickadee nest box is dutifully placed in our back yard. We have photographed mom and dad chickadees feeding their young both spiders and insects or their larvae, and for the past five years every chickadee nestling has fledged (and, as far as I know, lived to adulthood). Nonetheless, the study mentions that when spiders are a sizable part of insectivorous birds’ diets, it’s due to non-native vegetation. I can’t do much about the non-natives in my neighbors’ yards, but I can replace exotics in mine.
How we can help
Reading about shocking, dramatic declines in insects and insectivorous birds, as well as countless other creatures in trouble due to human actions can be disheartening, but this study proves that when we (and our neighbors) prioritize regional native plants at home that have great capacity for supporting biodiversity, we can make positive change for them and ourselves as well, since supporting wildlife can be very rewarding. Clearly, countless lives depend on how we garden and which plants we choose. And the little invertebrates themselves—part of the intricate web of life—have value in and of themselves.
Quercus (oak), Prunus (wild cherry), Salix (willow), Betula (birch), Populus (aspen & cottonwood), and Acer (maple) were among the top performers on Tallamy’s list pf plants found to host lepidoptera (moth and butterfly larvae) in the mid-Atlantic states. So instead of a ginkgo tree, opt for a native oak tree. Instead of a flowering cherry hybrid, choose a native cherry (in the Pacific Northwest: Prunus emarginata). Instead of Japanese maple, plant native maple (in the PNW: Acer macrophyllum, A. circinatum or A. glabrum). Some woody PNW trees and shrubs known to host lepidoptera include native dogwood (Cornus spp.), western red cedar (Thuja plicata), serviceberry (Amelanchier alnifolia), elderberry (Sambucus spp.), oceanspray (Holodiscus discolor), western mock orange (Philadelphus lewisii), honeysuckle (Lonicera spp.), and herbaceous plants like checker mallow (Sidalcea spp.), monkey flower (Mimulus spp.), and milkweed (Asclepias spp.). Choose species that would have historically grown in your locale, whenever possible, and add associated species—those that would grow with them naturally—as well. The 30 percent leeway allows us to grow some non-natives that we love and/or food for the kitchen table.
* Carolina chickadees, which are very similar in appearance to black-capped chickadees, are almost entirely insectivorous during breeding. Although they are fairly common across their range, their populations declined by 16% between 1966 and 2019, according to the Cornell Lab of Ornithology.