Friday, October 28, 2011

Cedar Waxwings...Continued

Last week I wrote about a dead cedar waxwing that was killed in a collision with our window. As I wrote, the rest of the flock chattered and whistled their high-pitched calls in the chokecherry trees (Prunus virginiana) above me. This week the trees are silent and bare. Did our windows kill them all? No, thank goodness, it’s natural for cedar waxwings to be an exciting presence one day, and gone the next. These large, gregarious flocks are facultative migrants – they move around as their food supply requires. One day you may have several dozen descend on your bushes, the next day they may be gone. These songbirds dine heavily on many kinds of berries, and also the tiny cones of Eastern redcedar trees, hence their name. 

As a kid in Iowa, I remember late winter days when colorful flocks of waxwings gathered in the highbush cranberry hedge (Viburnum trilobum) outside our kitchen window. The tart berries with a high acid content last well through the winter, and provide a much-needed food source when less-hardy berries have dried out or spoiled. How fun it was to watch them pass berries bird-to-bird down a row if the cluster of fruit could only be reached by one at a time! The birds’ lemony-yellow feathers, rakish black mask, and bright red wax spots captured my mom’s heart, too, and helped guide our Christmas shopping for her.

Since cedar waxwings prefer edge habitat like fields and riverbanks, they have adapted well to human-altered landscapes like my yard and the Museum’s Outdoor Classroom. Their population is stable or even increasing despite the heightened dangers of windows and cats in suburban settings. Brown-headed cowbirds share some habitat preferences with waxwings, and are also doing well in fragmented, edgy habitats. The problem is that cowbirds are nest parasites who lay their eggs in the nests of other species, and force other birds to raise their young. The cowbird chicks grow fast, and usually smother or push the host nestlings out of the nest. Cedar waxwings have a simple solution: they eat so few insects that brown-headed cowbirds in waxwing nests die from a lack of protein. [Although waxwings can go through stretches of strict vegetarianism, they are also excellent flycatchers. While paddling various rivers, I’ve admired their aerial acrobatics as they feasted on summer’s abundance of insects.]

Eating berries has other benefits and consequences, too. Overripe fruit may ferment, causing waxwings to become intoxicated, or even die when they eat too many. Perhaps the birds who crashed into our windows last week were a bit tipsy. A more benign outcome is the waxwing’s tail tip, which is usually yellow, may become orange if it eats the berries of Morrow’s honeysuckle, an introduced species, while the feathers are growing. The pigment rhodoxanthin (a red carotenoid pigment) is responsible for the color change.

As the birds fluttered between clusters of chokecherries last week, I caught glimpses of their bright red waxwings. Used to attract mates, the red is actually flattened extensions of feather shafts colored with a carotenoid pigments – similar to the pigments in carrots and autumn maple leaves. The waxwings obtain the color by modifying pigments acquired from their diet of red and orange berries, and the color increases with age. These birds maximize their nesting success by mating with other birds of similar age and experience levels – information gathered at least partially by comparing the number of red wax tips. Yet another reason why color is not just pretty!  

Where is that gregarious flock now? Are they eating YOUR chokecherries? Have they started in on the mountain ash berries and crabapples already? Watch for these year-round residents to visit a yard (and hopefully not a window) near you!

No comments:

Post a Comment