Although snow still blankets the ground, the sun’s rays are intensifying just enough to give the impression of warmth, if not the reality—that’s all it takes for black-capped chickadees to get spring fever. Go outside and you can hear the amorous songs of male chickadees. The simplicity of their “hey sweetie” two- or three-note song is charming. But then, almost everything about a chickadee is charming. Their large head, tiny body and gregarious curiosity combine to make them easily perceived as cute.
What’s difficult to perceive about chickadees is which ones are the males. Scientists refer to chickadees as “sexually monochromatic,” which means that males and females display the same color patterns. Female chickadees will even sing occasionally, taking away a clue that works for the similarly black-and-white common loon (only male loons give the yodel call).
If you’re lucky enough to see a chickadee up close, take the time to examine it. The edge between black cap and white cheeks is not smooth, but shows finely divided feathers. The base of the black throat patch where it grades into white belly feathers is also irregular. Their backs are not pure gray, but tinged with warm beige around the neck that spills over onto their sides. But even with a bird in-hand, it’s difficult to tell if it’s a male or female.
The chickadees themselves, of course, have no trouble seeing differences between the sexes. The key lies not in some failure of our observational skills, but in their ability to see in the ultraviolet spectrum. Birds have an extra type of cone cell in their retina that is sensitive to either violet or ultraviolet light, depending on the species, and their filters are set up to allow UV rays in. Humans have a type of cone cell that is sensitive to UV wavelengths, but we also have filters in our lenses that absorb 95 percent of UV light before it reaches our retinas.
The other part of the equation, besides birds being able to see UV light, is that their feathers reflect light in the UV spectrum. One study looked at birds that humans perceive as sexually monochromatic, and found that more than 90 percent of them were sexually distinct from the avian visual perspective. Another study went further and determined that all bird families possess feathers that reflect UV light. Even in species where males look different to us, UV reflectance may provide additional brilliance that we miss.
Chickadees are part of this pattern. To other birds, males appear brighter white and deeper black than females, which results in a greater contrast between plumage regions. Males also have larger black patches, which we could potentially observe if we looked carefully.
The differences are not only apparent between males and females, but between males of different ranks. Higher ranking males (who are preferred by females) display significantly darker plumage and a sharper contrast between white and black patches. Both of those characteristics, as well as larger black bibs, are also associated with greater reproductive success.
The vibrant colors don’t cause reproductive success, though; they are simply an indicator to females about the health of the male. In one study, sick chickadees couldn’t preen their feathers effectively, and the resulting layer of dirt reduced the intensity of their UV colors, which signaled their poor condition to females. A similar effect was found in jumping spiders, where food deprivation decreased UV-iridescent ornamentation on their abdomen, and most likely made them less appealing as a mate.
You might think that with their handsome colors, their amorous songs and their social natures, the spiffiest male chickadees might attract a downy harem to warm the long winter nights. Instead, no matter how far the temperature plummets, even the cutest chickadees find their own little tree cavity and sleep alone.