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October 31st, 2011 | Meera
Happy Halloween! Today seemed like an excellent day to make this post.
Entering the bird lab this past Thursday morning, I found Mary, who usually works at the sink, sitting on a stool beside the large metal prep table that dominates the room. In front of her were two plastic trays; on each, several tidy rows of specimens were arranged. The birds that made up this small collection represented three different species: Dark-eyed Juncos (Junco hyemalis), Nashville Warblers (Vermivora ruficapilla), and White-throated Sparrows (Zonotrichia albicollis).
All three are extremely common birds in the Chicago area at this time of year, either because they’re migrating through on their way to warmer southern climes, or because they spend the winter here.
And all three are known to me personally from morning walks through the Wooded Island in Jackson Park, the treed and windy urban oasis by the lakefront where I saw the distempered raccoon earlier this spring. I love the sight of dozens of dark gray Juncos against patches of snow on the ground, like a fireplace’s worth of cinders someone has rolled up into cozy little balls. Nashville warblers make jaunty little tail flicks as they forage through low trees and shrubs (often that’s all I see of them, an olive whisk-whisk-whisk before they rustle away through the leaves). And the mustard-yellow smudges next to each eye on White-throated Sparrows always make me imagine these fluffy, familiar creatures having just feasted messily on a stash of abandoned hot dogs.
The fact that Mary was working on these birds wouldn’t, in and of itself, have been of much note except that the specimens were in a form that I’d never seen before in the lab. Normally, these are species that Dave chooses to preserve as skeletons. One of the advantages of doing so is that there are many measurements it’s possible to take from a skeleton that it’s impossible to take from a study skin. To prepare a specimen for being skeletonized by the dermestid beetles, volunteers must first remove all its feathers and skin, a process called “roughing out.”
But the specimens Mary was working with seemed to have gone only part-way through this process. On the Nashville Warblers and the White-throated Sparrows, the feathers from their bodies had been removed, but those on their heads had been left in place. And on the Juncos, tail feathers remained as well.
In this state the specimens appeared, I confess, both fascinating and a little macabre. The juxtaposition of intact, feathered crowns, their plumage still beautifully soft and many-colored, with the dark red muscle of de-feathered bodies, created an incongruity—the likeness of life next to the unmistakable sign of death—that forced me to stop.
Why had some feathers been left on these birds?
Mary soon explained that in each of these species, subtle but significant differences in plumage coloration can be observed. Such variations raise a host of scientific questions (Are the disparities related to sex, age, or region? Can they be traced to genetic differences? Is one form of coloration more common than another, and if so, why? Does the prevalence of each pattern change over time?).
To document these variations, Mary was collecting caps from all three species, as well as tail feathers from the Juncos—because these were the parts of the birds’ bodies where the differences occurred. This way, the caps and tail feathers could become part of the museum’s collections and potentially help to answer some of these questions.
But when she was finished collecting what she needed, the beetles would go on to skeletonize the rest of the specimens’ bodies as usual, thus preserving the ability to take bone measurements from them in the future. The fact that I’d seen the birds’ bodies in this state was a coincidence: an accidental glimpse at a bit of scientific frugality.
Here are some of the variations this kind of data will hopefully help to quantify:
Juncos can have crowns that vary from a light gray to a deep black, sometimes tinged with brown—and while all Juncos have white outer tail feathers and black inner tail feathers, there can be considerable variance in the amount of white and black on the intermediary feathers. This photo doesn’t show the subtle differences in the shades of the crowns very well, but you can clearly see how much more black than white there is in the tail feathers of the bird in the foreground, and how much further out the black extends to the edges of its tail.
Nashville Warblers can have a patch of wonderfully rich chestnut-colored feathers in the center of their crowns, something I’ve never noticed when birding because the tiny flecks of red are all but impossible to see amidst or underneath their otherwise gray head feathers. Adult males all have some red in their caps, but the amount can vary widely; and some adult females have a little ruddiness there, too, while others have none. These differences are unfortunately very hard to see in the photo I took, but if you squint you might be able to see some red stippling in the third specimen from the left.
Finally, White-throated Sparrows actually have two well-documented morphs, or variant forms. You can see these quite clearly in the photo above: one morph has distinct black and white stripes running vertically down its crown, while the other has black and tan stripes arranged in the same pattern. Both morphs can be found in both sexes.
DNA analysis has shown that this polymorphism in White-throated Sparrows arises from genetic differences. Both White Stripe and Tan Stripe birds, as they are usually called, show a slight preference for mating with individuals from the other morph. This opposites-attract tendency (which goes by the unwieldy name of “disassortative mating”) keeps the approximate proportion of each morph in the overall population stable, so that neither morph disappears or becomes dominant.
Most fascinating of all, at least in the case of White-throated Sparrows, the morphological variations we see in their crowns are also associated with clear behavioral differences. White Stripe males are more aggressive and more showy—they’re more likely to engage in “spiraling,” a wonderful-sounding behavior that involves singing as they ascend the branches of a tree by circling it. They’re also less dedicated providers of parental care, and less monogamous than Tan Stripe males. As for White Stripe females, they are almost as bold and selfish as their male counterparts. (This may explain why both WS males and females seek out calmer, more reliable partners from the opposite morph.) For more on this subject, I’ll point you to this excellent post by GrrlScientist, who explains the genetics behind these behavioral variations far better than I could.
What I love about my own experience of all this is that it illustrates so clearly a principle I’ve always felt to be true about the study of natural history. That is, the macabre (like beauty) is not a thing that exists as an inherent property of the world, not something with a palpable presence in time and space. Instead it arises out of the complex interaction between ourselves and the world. Even if disquiet is our first reaction to a memento mori, it need not be our last.
But to the extent that encounters with the macabre invite curiosity—like the curiosity that struck me so forcibly when I walked into the lab and saw those unusual-looking specimens on Mary’s trays, and led me to learn some of the things I’ve shared with you today—I think it’s an extraordinarily useful quality in science.
I would be remiss if I didn’t tell you about two additional things I hope you will investigate:
1) Flinchy, the t-shirt company co-founded by my favorite fellow bird lab volunteer, Diana Sudyka, has several new designs available for purchase. I own one of them, and can testify to its quality and attractiveness. And greater luminaries than me endorse Flinchy shirts, too.
2) I wrote a piece for the Scientific American Guest Blog this week about my rather extraordinary friend Nina and her Field Museum project, LinEpig. You can find it here, under the curiosity-provoking (though not macabre) title “Internet Porn Fills Gap in Spider Taxonomy.” Nina picked the title, because she knows even better than I do that first you catch the eye, and then you tell the story.
Till next time, dear readers-mine. I hope it won’t be so long again.