In every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior in her column “Since You Asked.” Julie is supervisor of avian research at the Rouge River Bird Observatory at the University of Michigan Dearborn and a research associate at the university’s Environmental Interpretive Center. She blogs about her research at net-results.blogspot.com, and she maintains the website Coffee & Conservation, a thorough resource on where coffee comes from and its impact on wild birds. Here’s a question Julie answered in our December 2013 issue:
Does the color red act as a warning sign to predators that a bird might be poisonous or bad-tasting? I have seen raptors take brown birds but never cardinals, House and Purple Finches, or robins. — Donna Evans, Torrington, Connecticut
Although you may not have seen it, hawks, shrikes, owls, and mammals do prey on cardinals, orioles, and other red or orange birds. I have not seen any research indicating that certain songbirds in North America are less palatable than others, and none are poisonous.
For warning (or aposematic) coloration to work, a predator has to learn that a prey item with a particular color pattern tastes bad or is poisonous. An aversion to a warning color is not instinctive. For example, a monarch butterfly or two will likely have to die at the bill of a Blue Jay before the bird learns that the insect is distasteful. Because North America doesn’t have any nasty-tasting birds, Cooper’s Hawks and other predators have no opportunity to learn that red birds taste bad and avoid preying on others.
In a type of warning coloration known as Batesian mimicry, an animal with warning colors is not distasteful itself but mimics another that is. Worldwide, aposematism and Batesian mimicry are most common in invertebrates and far less so in vertebrates.
Only a few birds are known to be poisonous. One is the Hooded Pitohui, a songbird from the island of New Guinea. It derives the poison in its feathers and skin from a beetle that it eats. Two Amazonian bird species — Cinereous Mourner and Brazilian Laniisoma (also known as Shrike-like Cotinga) — are not poisonous, but as juveniles, they are bright orange. The color suggests that the youngsters are Batesian mimics of a group of unpalatable, fuzzy caterpillars.
In many bird species, bright colors send a signal of another sort. Orange, red, and yellow colors in birds are nearly always derived from plant pigments called carotenoids. Since the birds rely on an appropriate diet to form the colors, carotenoid-based coloration is thought to be a signal of health and fitness.
A version of this article appeared in the December 2013 issue of BirdWatching magazine.