Intuitively, we know that's not true. Some caterpillar species are so well defended by the chemicals they eat that the naïve birds that eat them throw up (think monarch butterflies). Others are covered with dense hairs (e.g., woolly bears) that are irritating to many birds. Still others, like the many species of green geometer inchworms and noctuid owlet moths, are perfect sources of protein and fats.
New research is showing that some caterpillars are unusually good sources of essential carotenoids — provitamins, antioxidants, and pigments that enhance certain colors 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 discriminate between caterpillar species rich in carotenoids over ones with few carotenoids? We don’t know. If caterpillars are scarce, 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 bird nutrition that I am embarking on a long-term research project to document exactly which species of insects are eaten by birds all over the US, particularly when they're feeding nestlings. But I need your help.
My colleagues and I will then identify the order, family and, when possible, the species of the prey items and feed those data back into the site's searchable database. We'll also showcase the photography, when we have permission to do so.
We will also note whether prey are herbivores, predators, detritivores or aquatic, whether they're brown or green, hairy or smooth-skinned. We will learn, for example, whether eastern bluebirds from Wisconsin have the same diet as bluebirds from Georgia. Do chipping sparrows take different insects during their first brood than they do during their second brood? And so on.
Why should we care what birds eat? My primary concern is the conservation and restoration of viable bird habitat. We can't manage habitats for breeding birds without knowing what breeding birds eat while reproducing*. Please pitch in!
Doug Tallamy, PhD
University of Delaware
* — Some plants are far better at producing insect bird food than others. For example, oaks support 557 species of caterpillars in the mid-Atlantic states alone while non-native ginkgo trees support only 4. 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. You can learn more about planting for backyard habitats at Bringing Nature Home.