“What Do Birds Eat? We Don’t Really Know!” by Doug Tallamy

Prothonotary warbler. Photo by Doug Tallamy.

Prothonotary warbler. Photo by Doug Tallamy.

This story originally appeared in the March/April 2016 issue of Bird Watcher’s Digest.

Ask any ornithologist, or knowledgeable birder for that matter, what birds eat and he or she will tell you seeds, fruits, and insects. Backyard bird watchers know that sunflower seeds and suet are all you need to keep winter residents happy, and because that’s the only time many of us feed birds, we tend to think of all birds as seed or suet eaters. Many birds do include seeds in their diet during the months in which seeds are plentiful, but only a few groups (e.g., finches and doves) are able to complete their life cycle on seeds alone. Though many seeds in the temperate zone are nutritious, they are depleted by early spring and are not abundant again until late summer or fall. For similar reasons, no North American bird species can make a living eating only fruits. Fruits are even more ephemeral than are most seeds, peaking in abundance in late summer. Fruits are almost entirely absent during spring migration and the breeding season. Temperate-zone fruits also have lower nutritional value than many tropical fruit species, offering inadequate amounts of protein or fats.

That leaves insects. Insects and other arthropods, particularly spiders that themselves eat insects, are essential dietary components for 96 percent of North American terrestrial bird species. Insects are extraordinarily high in protein: They have up to twice as much protein, pound for pound, as does beef. Insects also have organs in their abdomens called fat bodies that are rich in high-energy lipids. Both protein and fat are the stuff of growth and thus make up the bulk of what breeding birds need to feed their nestlings. Like baby birds, adults require protein and fats for muscle tone and energy and will take an insect over seeds or fruit almost every time. Insects are so important in bird diets that when cold weather decreases insect abundance, 350 species (or 54 percent) of the birds that breed in North America undertake the most dangerous adventure of their lives, migration, to move to areas where insects remain abundant during the winter. That’s right: Nearly all of our terrestrial migrants except birds of prey migrate because they are insectivores.

In view of the importance of insects in the diets of our birds, you would think that scientists would know which particular insects birds depend on. In fact, you would think bird lovers everywhere would be managing land in ways that produce copious supplies of the insects birds need most. But you would be wrong. Ornithologists study birds, not insects; entomologists study insects, not birds; and most birders have not thought about the conservation potential of their own properties. Over the years, many ornithologists have recorded the food brought to nests either by neck ligatures on, or stomach pumping of, the nestlings, or by cameras at the nest. However, the data on prey insects are typically categorized at the order level (Coleoptera, Orthoptera, Lepidoptera, etc.). In rare cases, the data are further subdivided into caterpillars, adult moths, grasshoppers, weevils, katydids, and so on. There are so many species of insects that the expertise needed to identify any of them to species is rare, even among entomologists. Ornithologists are understandably overwhelmed with the information they gather and have been satisfied with broad taxonomic categories.

But such broad categories assume that all caterpillars are equally valuable to particular bird species, or that all species of grasshoppers are equally palatable and nutritious. Intuitively, we know this is not the case. Let’s consider Lepidoptera (moths and butterflies) as an example. There are approximately 12,430 named species of Lepidoptera in North America (another 2,000 remained undescribed), which is nearly 5,000 more species than all of the birds in the world! But they are not all eaten equally by birds. Some caterpillar species are so well defended by chemicals produced in their host plants that they cause the naïve birds that eat them to throw up (think monarch butterflies). Others are covered with dense hairs (e.g., woolly bears) that are irritating to most birds. Many caterpillars taste good and have no hairs, but they blend in with their backgrounds so well that it is not worth a bird’s effort to search for them. Still others, like the many species of green geometer inchworms and noctuid owlet moths that can be so numerous exactly when many birds are rearing their young, are perfect sources of protein and fats. Our beautiful warblers, in particular, love these types of caterpillars.

New research is showing that there are other reasons all caterpillars are not equally valuable to birds. Some caterpillars are unusually good sources of essential carotenoids. Carotenoids are provitamins, antioxidants, and pigments that enhance the greens, reds, oranges, and yellows of bird plumage. Bright feathers are a signal of good health, and in many bird species, females choose their mates based on the brilliance of their plumage. Do birds, then, discriminate between insects rich in carotenoids over ones with few carotenoids? We don’t know. If caterpillars are in short supply, can birds get the carotenoids they need from grasshoppers, crane flies, or cicadas? We don’t know. Do birds prefer green and yellow caterpillars over brown and black ones? We don’t know. In fact, there is so much we don’t know about which arthropods are a “must” for healthful bird nutrition that my graduate students and I are embarking on a longterm research project to document exactly which species of insects are eaten by birds all over the country, particularly when they are feeding nestlings. But we need your help.

Digital technology has spurred a national interest in bird photography. In the past, taking quality photos of birds with insects in their bills was beyond the expertise of most birders. No longer. Even casual bird watchers often have digital equipment good enough to capture images of birds carrying insects or spiders in their bills, and these images are often clear enough that the prey item can be identified to species. Many birders have found photography to be so rewarding that they have traded their checklists for making superb images of both common and rare birds. Moreover, cameras at the nest are no longer expensive research tools; they now serve as forms of backyard entertainment. And every image of a bird with food is an unambiguous record of the arthropods that species finds acceptable.

What I hope to do is encourage members of the birding community from all over the country—citizen scientists, if you will—to submit images of birds holding arthropods in their bills to our website (see sidebar). Such images are particularly easy to take when birds are feeding young, because the parent will often hold the prey for minutes before delivering it to the nest. We will then identify the order, family, and, when possible, the species of the prey and incorporate this information into an interactive Excel file on a public website. We will also note whether the prey is an herbivore, a predator, a detritivore, or an aquatic insect, whether it is brown or green, hairy or smooth-skinned. As data come in, we will accumulate hundreds of feeding records for as many species of terrestrial birds as possible. We will learn, for example, whether eastern bluebirds from Wisconsin have the same diet as bluebirds from Georgia or whether birds from dry regions of the country favor arthropods produced from detritus, whereas birds from wetter areas favor arthropods produced by living plants. Do birds take insects according to their abundance in the environment or do they seek particular insect species and ignore others? Do chipping sparrows take different insects in May than in August or different species while feeding young than while feeding themselves? If you want to know what birds eat cutworms in Tennessee, all cutworm records will be available. Conversely, if you want to know what insects are eaten by burrowing owls in Puma, Arizona, that, too can be lifted from the database. The images from which the database has been assembled will also be available for scrutiny (with permission from the photographers). The potential for gathering information previously unattainable to scientists from this approach is enormous and very exciting to those of us who have asked the simple question, “What do birds eat?”

What have we learned so far?

Vermilion flycatchers love cockroaches; eastern bluebirds prefer noctuid cutworms and armyworms; Carolina chickadees love dowdy pinions; and purple martins relish dragonflies.

Why should we care what birds eat? There are many reasons, but my primary motivation is the conservation and restoration of viable bird habitat. It is common knowledge among bird conservationists that if you want to sustain populations of the resplendent quetzal in Central America, you need copious supplies of avocado trees, because avocado fruits are a mainstay of the quetzal’s diet. Could such specificity also hold for breeding birds in North America? We cannot manage habitats for breeding birds without knowing what breeding birds eat while reproducing. Many people think of bird habitat as a place where birds live, and they judge habitat quality by the quality of nesting sites and cover. But nesting sites and cover mean little to birds if there is not enough to eat. What we have overlooked for decades are the plants that make the insects that birds require, particularly while birds are reproducing.

Some plants are far better at producing insect bird food than others. For example, oaks support 557 species of caterpillars (bird food) in the mid-Atlantic states alone, whereas non-native Zelkova trees from Asia support no caterpillars at all.

Ninety percent of the insects that eat plants can only eat specific plants; if those plants are absent from our landscapes, so will be the bird food they produce.

Unfortunately, this is the case in our yards and managed landscapes when we remove native plant communities that are good at making insect bird food and replace them with vast lawns and ornamental plants from other parts of the world that produce few insects in North America. This oversight must end if we want birds in our future.

Want to help?

Upload digital images (reduced file sizes are fine) of birds carrying arthropods (insects, spiders, or crustaceans) at whatdobirdseat.com. Include your name, date, the location the image was taken, and your best guess as to whether the bird was migrating (spring or fall), feeding young, or overwintering. Even a single image will be much appreciated.

Doug Talamy is Professor and Chairman of Entomology and Wildlife Ecology at the University of Delaware, and author of Bringing Nature Home: How Native plants Sustain Wildlife in Our Gardens.