A monarch butterfly on common milkweed. (Courtesy Maine Audubon)
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It doesn’t feel like there’s much in our lives we can control right now. But you can control the plants in your backyard.

Growing certain plants in your backyard is the best thing you can do to help birds during the nesting season. You want plants that host native caterpillars, the most important food for baby birds.

More than 95 percent of all songbirds in Maine rely on insects for their diet or to feed their young. Caterpillars – fat, nutritious, and easy to catch – are far and away the most important. They’re basically little protein burritos. A single pair of black-capped chickadees must catch at least 7,500 caterpillars per year to raise a clutch of young.

Birds need insects, even the birds that we don’t think of as insect-eaters. Northern cardinals, our regular feeder visitors and whose bills are specially evolved to crush seeds, get at least half their diet from insects. We all know that our ruby-throated hummingbirds love to drink nectar from flowers or our feeders, but between 60 and 80 percent of a hummingbird’s diet is made up of insects, mostly spiders.

So where do all these caterpillars come from? They all grow and feed on plants. But plants don’t love to be eaten and defend themselves, most often by filling their leaves with distasteful chemicals. But caterpillars still need to eat, so they have developed ways to digest or avoid the chemicals. Plants respond by evolving additional defenses, and the evolutionary arms race rages.

One of the results of this warfare is that the effort and time it takes to break a plant’s defenses gives caterpillars little time to focus on other plants, meaning that many caterpillars are only able to eat a small number of plants.

A popular example is the monarch butterfly, which is completely dependent on a few species of milkweed. Monarchs can’t eat anything else, so some of them have dropped 97 percent or more from historical levels as milkweed has been cut from the landscape.

When new plant species arrive, their own natural defenses, similarly honed after centuries of battle with insects on their home turf, usually completely baffle local caterpillars.

Gingko trees are popular in cities and yards, for example, but they support literally zero species of caterpillar. They are wildlife dead zones. Oaks, on the other hand, support more than 500 species of caterpillar, black cherry supports 450, native viburnum supports more than 100, and so on.

(By the way, I should point out that these caterpillars are not pest insects, but rather moths and butterflies that don’t bother humans. Browntail moth caterpillars, on the other hand, can live on a bunch of hardwood species.)

So plant native plants, if you can find them. Most nurseries do not put an emphasis on native plants, focusing instead on showy nonnatives or cultivars. Maine Audubon is expanding its project to grow and sell native plants to support native birds, and the Maine-based Wild Seed Project has information about what plants will work in your garden and seeds to help you get started.

Take back control of your backyard, and do some real good.

Nick Lund of Cumberland is outreach and network manager at Maine Audubon. He has written about nature for the National Audubon Society, Down East, National Parks Magazine, The Washington Post, and others. He can be found online at TheBirdist.com and on Twitter @TheBirdist.

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