Laura Erickson's For the Birds

(Recording of a Cedar Waxwing)

One of the most satisfying elements of ornithology is that there are still mysteries about birds yet to be solved, like how exactly do migrants find their way? The more researchers learn, the more questions arise, and the more complex–even miraculous–migration seems. Some birds have a sense of geomagnetism–does this mean that they literally have a magnet in their brain tissue? Apparently yes–a high iron content has been measured in some bird brains. Some birds use the stars for navigation–baby birds in their nests notice the rotation of stars around Polaris, and orient toward or away from it by season. Ingenious experiments in planetariums taught us that. As more is learned, questions become more refined, and the most creative ornithologists come up with novel experiments to find answers.

One ornithological mystery which is being solved right now is why waxwings have those tiny red tips on their secondary wing feathers. These decorations are found on both males and females, adults and occasionally even nestlings, and in every season. Yet a good many waxwings don’t have them at all. Some ornithologists have speculated that they may have something to do with age, but most concede that they have no idea what the function of these beautiful markings is.

But thanks to two Canadian ornithologists, we now have a plausible explanation for them. The main problem with researching waxwings in the field is that they lack philopatry–meaning an individual bird is unlikely to return to the same nesting place from one year to the next. So when an ornithologist bands a nestful of baby waxwings, chances are he won’t be able to find them again the next year to see how their plumage has developed. The year waxwings hatch out, they’re noticeably streaked, but it’s hard to know exactly how old a bird without these streaks is.

What James Mountjoy and Raleigh Robertson, of Queen’s University, did was to examine the total number of red feather tips on samples of birds of known age. They used museum specimens as well as living birds examined in the field, and came up with an elegant statistical analysis to show that even though some individual young birds have the red secondary tips, overall the tips are larger and much more numerous in birds that are two years old or older. They also discovered that waxwings breed assortatively–that is, guided by the number of tips, experienced birds tend to choose experienced mates, and young birds tend to choose young mates.

A few other songbirds show delayed plumage maturation– like male American Redstarts, which are dully colored until they’re almost two years old, but waxwings are the only known species of songbird with delayed maturation in both males and females. Significantly more babies are successfully raised by parents with a high number of red feather tips. So even though there is enough variation to confuse bird-watchers, the wing tips apparently help waxwings to identify and court suitable mates with a minimum of aggression within a flock.

You might not think the beautiful markings of a waxwing have anything to do with dry statistics, but when mathematics merges with the study of birds, new and beautiful patterns emerge, renewing faith in the order of nature. And it’s particularly pleasing to me to know that at least in one species, the older you get, the more beautiful and desirable you become.

(Recording of a Cedar Waxwing)

This is Laura Erickson, and this program has been “For the Birds.”