Birds bring us much joy. Unfortunately, birds are in trouble. Bird populations, measured in numbers of adult birds, have declined by almost thirty percent since 1970, a loss of approximately thirty billion individuals. Bird decline has paralleled insect decline. A recently released comprehensive review of insect population trends in Biological Conservation reached the alarming conclusion that forty percent of all insect species are unsustainably declining and that a third are endangered. With a loss of 2.5% per year over the last quarter-century, insects could be functionally extinct within a century. Ninety-six percent of North American land birds depend on insects to feed their young. Caterpillars alone help sustain the young of over three hundred North American bird species. Fewer insects mean lower productivity for birds and other animals that depend upon insects. Even where adult birds are surviving, they are too often not finding enough food to feed their young.
Insects in turn rely on native plants. Non-native plants attract significantly fewer insects than native plants because insects are attracted to the plants with which they co-evolved. Oaks alone support over five hundred species of butterflies and moths. Willows, cherries, and birches support over four hundred species each. These tree species are among the best places to look for migrating warblers hungry for nutritious caterpillars. Spring migration coincides with the hatching of caterpillars which coincides with the nascent leafing out of trees. The birds in turn prevent the hungry caterpillars from completely denuding the trees. Everything is connected in nature.
Even fruit-eating birds depend on insects. Insects pollinate the shrubs and trees that produce the fruit needed by many fall migrants. Fruit-eating species like thrushes, tanagers, and orioles time their fall migration to the ripeness of the berries of the plant species with which the birds co-evolved. Look for the thrushes in the Spicebush thickets at Baldpate when the berries turn red. I once counted five thrush species, Swainson’s, Gray-cheeked, Hermit, Wood Thrushes and Veery in one Spicebush plant.
Native plants including Spicebush, Flowering Dogwood, Gray Dogwood, and Arrowwood Viburnum produce berries with a higher lipid content than most non-native plants. This means birds can put on the fat they need for migration more quickly, reducing the time they spend foraging during migration and the number of stops they make in migration. This in turn reduces their exposure to predators. Migration stops are more dangerous for birds because, unlike the familiar breeding and wintering grounds, the territory is unfamiliar and the movements of the local predators unknown. The fewer stops the bird makes, the better its chance of survival.
Unfortunately, most of central New Jersey is a food desert for birds, with acres paved over or devoted to lawn and non-native plants that are useless for most species. A recent study by Narango, Tallamy, and Marra showed that Carolina Chickadees, a common species that has adapted to urban landscapes, are struggling to raise young in a landscape of exotic plants. The study showed that the chickadees could not sustain population growth where the native plant biomass was less than seventy percent.
The situation is worse for species that are less adapted to human landscapes and have an even higher requirement for native plants. FoHVOS Stewardship Director, Michael Van Clef, and I studied the plant associations used by breeding Hooded and Kentucky Warblers (pictured above) at Baldpate Mountain and the Sourland Ecosystem Preserve. We found that the warblers were breeding in areas with an average of eighty-three percent native plant cover, mainly Spicebush thickets. The rarity of native understory is why Hooded, Kentucky, and Worm-eating Warblers are among central New Jersey’s fastest declining breeding birds. Primarily insectivores, warblers are the fastest declining family of Neotropical migrants.
Fewer native plants means fewer insects, which means fewer birds. A major step in reversing bird decline would be reversing insect decline by planting native plants. Reversing insect decline is a big undertaking. Douglas Tallamy points out in his book Nature’s Best Hope that two insect types are the most significant for healthy bird populations: butterflies/moths and pollinating insects. Concentrating on plants that support these two groups of insects would have the maximum impact on increasing bird populations.
Planting native plants is a step everyone can take, starting with their own property or volunteering to plant native plants in restoration projects on preserved land. Not using insecticides and supporting local organic farms that forego insecticides would help boost local insect populations and the population of the insectivorous birds that depend upon them. Insectivorous birds such as swallows, swifts, and nightjars are our fastest declining bird species.
Joining FoHVOS’s Residential Community Conservation would be an excellent start in restoring bird habitats in the Hopewell Valley. The Hopewell Valley is part of the Greater Sourland Important Bird Area. The ecosystem is highly stressed but intact. Unlike most of central New Jersey, we still have the pieces to restore the ecosystem to health. The time to act is now. The reward is outside your window.
This article was written by Sharyn Magee, past President and current Holden Grants & Science Chair of the Washington Crossing Audubon Society, and submitted to MercerMe by the Friends of Hopewell Valley Open Space (FoHVOS).
FoHVOS is an accredited nonprofit land trust dedicated to conserving the Valley’s character by collaborating with the community to preserve land, protect natural resources, and inspire a new generation of conservation. Since its inception, they have preserved over 7,500 acres of land and inspired thousands of partners and volunteers. To learn more about FoHVOS, call 609-730-1560 or visit www.fohvos.org.