why red?

To my eyes, male cardinals stand out like they want to be seen. Several resident birds have red markings, including several woodpeckers. None are as brilliant as a cardinal, but a pileated crest glares like a tanager from the woods. Red-headed woodpeckers, more common at lower elevations, also have bold red on their head. Red-bellied woodpeckers sport a full red cap, and flickers wear a long red teardrop that wraps around their head. Downy and hairy woodpeckers have a bright red square on the back of their head, males only.



This prevalence of red makes me wonder whether predatory birds have good color vision. Blindness to red is the most common form of color blindness in humans, and red may be more difficult to see than other colors. Color vision requires pigments, proteins that fold into shapes that reflect or transmit certain wavelengths of light, and red pigments could be less common or more fragile than proteins that enable perception of green, blue or other colors.

You don't spot birds by looking for colors; you see their movement, track them and find them where you think they perched. Cardinals, males especially, are much easier to spot than most birds, but they can still hide if they hide still. Their preferred habitat is tangles of brush and hedges.

Accipiters like sharp-shinned and Cooper's hawks eat birds almost exclusively, and both will kill a cardinal given the chance. Dense branches are impenetrable protection against stooping hawks and owls. Accipiters prey on cryptic birds at least as much as birds with red markings, and they surely hunt by detecting movement, not color. We know hawks have powerful eyesight, so adding red to their visual repertoire offers marginal advantage.

Maybe smaller birds use red for gender and species markings because primary predators lack pigments sensitive to red.