Insects are in freefall and that goes a long way in explaining our declining bird populations. 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 bird species. Fewer insects mean lower productivity for birds and other animals that depend upon insects. Many fish, mammal, reptile, and especially amphibian species that heavily depend on insect mass are also unsustainably declining. Insects also pollinate plants that produce the seeds and berries that many bird species need to sustain life as adults. The decline in bird numbers cannot be reversed without increasing the insect biomass.
Insects in turn depend 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.
Urban and suburban yards filled with exotic plants are food deserts for our native birds. 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. Species that are less adapted to human landscapes have an even higher requirement for native plants. Michael van Clif and I studied the plant associations used by breeding Hooded and Kentucky Warblers at Baldpate Mountain and the Sourland Ecosystem Preserve. We found that the warblers were breeding in areas with an average of eighty-two percent native plant cover, mainly Spicebush thickets, which are the best place to look for migrating Black-throated Blue Warblers that relish Spicebush Swallowtail caterpillars. Also, migrating thrushes are attracted by the lipid rich berries of the Spicebush.
Obligate insectivores like swallows and swifts are among our fastest declining bird species. Species such as Whip-poor-wills that depend on night flying insects have also significantly decreased. Decreases in insect abundance, diversity, and size has resulted in the unsustainable decline of these bird species, indicating that depletion of the bird’s food source was the main cause of the rapid decline in insectivorous birds.
Agricultural pesticides, such as neonicotinoids, called neonics, and chlorpyrifos, have been implicated in the decline of insect populations and the vertebrate species that depend upon them. A study in Germany showed significant insect population declines in preserved natural areas surrounded by farms utilizing intensive agricultural practices. Farming practices have a huge impact on insect populations. A study in Finland showed that organic livestock farming was the only mode of farming that significantly increased the population of insectivorous birds because the frequently rotated pastures used in organic livestock farms most closely resemble native grasslands. The swallow population at the St. Michael’s Farm Preserve supports this study. St. Michael’s supports healthy numbers of Tree Swallows, Barn Swallows, and Purple Martins and has a pair of Rough-winged Swallows under the Aunt Molly Road bridge.
Insecticides also have a direct effect on birds. An American Bird Conservancy study showed that a single neonic-coated seed could kill a Blue Jay sized bird. A recent Canadian study shows that non-lethal doses of neonics are having a significant effect on migrating birds by suppressing appetite. Birds exposed to neonics lose a substantial amount of weight at a time they need to be gaining weight, forcing the birds to delay migration which harms their chances to breed. Birds poisoned by neonics may not be in condition to breed once they reach their breeding grounds.
Reversing insect decline is an essential step in reversing bird decline. Fortunately, it is a step everyone can take, starting with increasing the native plant content on 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 also help boost local insect populations and make migration safer for birds. The reward is outside your window.