First-ever bluebird twins highlight citizen science’s value in studying rare events

Gerald Clark enjoys birds

Gerald Clark enjoys birds

Last September, Mr. and Ms. Winfield were reported in the news as defying odds of one in a trillion!

Did they win a lottery? Not the usual kind. They found 6 double-yolked eggs in one carton.

It turns out the newspaper’s calculations were a little exaggerated—the Winfields’ odds were actually more like one in a billion—but this was still a remarkable find.

If you eat eggs for breakfast, it is reasonable to have encountered a double-yolked egg by the time you reach your 8th carton of eggs. And that’s for domesticated poultry. What about for wild birds? Do people who monitor wild bird nests ever encounter twinning?

Last year, when Gerald Clark, a retiree who spends time enjoying birds in his backyard, was monitoring the nests of Eastern Bluebirds in Pennsylvania, he suspected a twinning event. Robyn Bailey, project leader for NestWatch, a citizen-science project administered by the Cornell Lab of Ornithology, authored this exciting paper with Clark based on his photographs and field notes of the nesting attempts.

According to Bailey and Clark, there have only been 14 documented cases of twinning for 13 species of wild birds.

Twinning can happen a number of ways, though there are basically two ways to detect twins: dissecting an unhatched egg or finding more nestlings than eggs in a nest. Clark used the second option: by monitoring and reporting his observations to NestWatch, he collected evidence that there were four eggs in the nest, and then five nestlings. Clark even photo-documented the nest during hatching, capturing the moment when there were 4 nestlings and 1 egg in the nest.  Additional support was that one egg was oversized. Federal law prohibits participants from handling eggs, so Bailey and Clark used the images to measure the eggs digitally. The presumed egg with twins was 11% longer and 12% wider than the other eggs.

Among double-yolked eggs in other species, hatching success is reported to be low, probably because the twins are crowded. After all, one chick needs room to be able to pip the shell. Also, both embryos need to be positioned to access the air cell (the space where oxygen comes in after carbon dioxide goes out the pores of the shell). Therefore, the twins of Clark’s Eastern Bluebird in Pennsylvania appear to be an extremely rare event because not only did the egg contain twin embryos, but they were incubated, hatched without assistance, and survived for at least 11 days.

Previously I’ve said that citizen science is like the folktale of stone soup because everyone chips in a few observations so that cumulatively we get a rich mix of observations, a soup of new knowledge. But sometimes citizen science is more like a lottery because we engage many people in making observations, and when so many people pay extra attention, a lucky few, unexpectedly, make unique discoveries.

Rare phenomena, like exceedingly rare ladybugs, comets, a Gray whale in the Mediterranean Sea, and now eggs with twins, are possible to study through the large groups of citizen science volunteers. Together, volunteers contribute an untold, undoubtedly extraordinary number of hours of observations in the field. With thousands of eggs reported to NestWatch every breeding season, what’s next on the horizon? Sign up alone, or set up a group monitoring program, with NestWatch today and see what you might discover in this type of lottery!

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