As fall advances toward winter here in the northern hemisphere, we say a fond farewell to most of the birds we have enjoyed watching in our backyards, local parks, tree-lined streets and no-mow patches scattered across the landscape. We’ll have to wait, in some cases, eight or nine months for them to return. At the same time, many people might welcome a decline in the number of insects sharing their outdoor spaces. The timing of those changes is not a coincidence though. Insects are an essential energy source for migratory birds throughout the year, and the timing of migratory movements of many species have evolved to track seasonal changes in insect abundance across their ranges.
Insects and other invertebrates are a main source of protein for many migratory landbirds. During the breeding season, female birds require protein to produce eggs, and parents feed insects to their young to provide protein for growth. During migration, birds need protein that insects provide to rebuild muscle mass that is lost during demanding flights that force birds to breakdown flight muscles for energy once they have depleted their fat reserves. This can happen when birds encounter poor weather conditions while flying over open water with no place to seek refuge, like Yellow Warblers and Cerulean Warblers crossing the Atlantic, Caribbean or Gulf of Mexico. And many migratory species continue to eat insects during the non-breeding season, especially as they prepare for their return to breeding grounds in the north.
Recent science has shown that insectivorous species are among the groups of birds showing the steepest population declines. We have lost about one third of the insectivorous birds in the U.S. and Canada in the last fifty years. Declines in insect populations are driven by the application of pesticides and loss of natural habitat. Additionally, human-induced climate change has disrupted the synchrony between migration and insect population dynamics; as the climate has changed, many birds are migrating earlier while insects are emerging later in what scientists call a phenological mismatch.
As a consequence of these human activities and environmental changes, the balance between birds and insects in healthy ecosystems is impaired. Birds have less insect prey, making it harder for them to survive and produce offspring. In turn, with fewer insectivorous birds, insects, including pests that damage food crops, are not kept in check, and farmers may respond by applying more synthetic pesticides. The grim outcome? A positive feedback loop where bird populations could spiral toward extinction without conservation interventions.
As we celebrate another World Migratory Bird Day, let’s highlight a key conservation action that can help: growing native plants.
Did you know that:
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Native plant species support larger, more diverse insect communities. Many birds will visit even small patches of native plants scattered across urban, suburban, and rural landscapes.
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Audubon has a native plants database that users in the United States can access to learn which plant species to grow and which types of birds they will support. To learn more, please visit Audubon’s Plants for Birds database.
While insectivorous bird populations are declining, there is still time to halt and reverse these declines to help protect these migratory birds and the places they need throughout their entire life cycle.
