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I was on my way home from school (fourth grade?) when this treeful of dapper little birds stopped me in my tracks. I’d never seen anything like them: backswept crests, black masks, subtly colored brown and yellow plumage with vivid red markings on their wings. They were carrying on in high-pitched sibilant voices, ignoring me completely. 

They were cedar waxwings, and they’d probably been on a berry binge. That was years ago in Arkansas, but they’re Bay Area regulars as well, arriving in September and hanging around until May or June before flying to Humboldt County and points north to nest. They’re fond of their berries: they visit the juniper in front of my house and the pyracanthas, and will feed on mistletoe, madrone and peppertree fruits. Sometimes they overdo it, gorging themselves into a stupor on fermented fruit; mass deaths from ethanol poisoning have been documented. 

Berries are important in their social lives. Courting pairs will ceremoniously pass a berry back and forth, and there are unsubstantiated reports of a whole line of birds doing this. 

The waxwing’s diet may be responsible for the color of the wax on the wings. Unique among birds, the three species of waxwings — our cedar, the far northern Bohemian and the Japanese — have flattened waxy tips on some of their flight feathers. Chemists have determined that the color comes from the pigment astaxanthin, similar to the substance that makes flamingos pink, but as far as I know they haven’t traced the metabolic pathway from food to feather. 

Why red wax, though? In many birds, brightly colored plumage is explained as a product of sexual selection, Darwin’s other Big Idea, propounded in The Descent of Man. Female birds are supposed to be attracted to colorful males. Selective mating spreads the gene coding for bright feathers in males and for the female preference for brighter males through the population. Give this enough generations and you get baroque extravagances like the peacock’s tail or the plumes of the birds of paradise. 

It’s not just about esthetics. There’s evidence that females use the hue of a male’s plumage — or the complexity of his song, or the vigor of his dance routines — to assess his general health. Brighter males have been found to have fewer parasites, and it may be that only the fittest can schlep all their adornments around without getting nailed by a predator. Anyway, the idea is that more colorful males are the best potential mates. 

That doesn’t work for waxwings, though, since both sexes have red waxy feather-tips. Their function stumped ornithologists for years. Alexander Wilson, Audubon’s contemporary and rival, wrote in 1832 the tips preserved the feathers “from being broken and worn away by the almost constant fluttering of the bird among thick branches of the cedar.” Nice try, but there’s no empirical support for the idea. 

It wasn’t until the 1980s that Canadian biologists James Mountjoy and Raleigh Robertson came up with a more plausible explanation. Bird banders, who logged the age and sex of every bird they handled, had been aware that older waxwings had more wax-tipped feathers than younger ones. Looking at the birds’ mating patterns, Mountjoy and Robertson discovered that May-December relationships are rare among waxwings: older birds pair up with older birds. They also found older pairs nested earlier and fledged more chicks than younger pairs. 

So the wax may allow waxwings to select experienced, competent mates, good at the all-important business of producing more waxwings. The wax seems to be a badge not of vigor but of maturity, the avian equivalent of that touch of gray. 

Joe Eaton lives in Berkeley. He’s nature editor at Faultline Magazine (http://www.faultline.org) and writes a column for Terrain, the Ecology Center’s quarterly.