CHAMPAIGN COUNTY, Ill. – It’s 5:30 a.m. in the tree farms outside Urbana, but the birds have been up for an hour already. I sip my coffee, putting on rubber boots that will be little help against the dewy, waist-high grass.
A couple of brown birds sit on telephone wires above me, and I have a feeling I am being watched. These are brown-headed cowbirds, which lay their eggs in other species’ nests and then let the nest’s owners raise the offspring.
American robins are nearby, warily watching the cowbirds. Robins are one of a few species to have cracked the “cowbird problem.” Their solution is simple: When they recognize a cowbird egg, they throw it out of the nest. To do this, they must override their natural instinct to care for the egg, however.
I am here to study the robins’ decision-making. How do they know to throw a cowbird egg out of the nest, but keep their own eggs?
To find out, I act the part of the cowbird: I spy on robins to find their nests and slip a foreign egg into each one. I don’t make it easy for them, however. I have tweaked the colors of my 3D-printed fake eggs to make their decisions much harder. Some robins decide to keep the fake eggs, while others toss them out. Some seem not to be able to make up their minds what to do with the fake eggs.
I’m working in the lab of animal biology professor Mark Hauber, who loves these types of questions. Why do some robins accept fake eggs, while others toss them out? By challenging the robins with new types of eggs, we are essentially going back in time to when robins first encountered cowbirds. They likely walked the same line of indecision back then, too.
But robins cannot explain to us why they do what they do. We must coax the answers from them indirectly, by studying their physiology.
In the shadow of a tree, I open up a fine black nylon net designed to be invisible to robins. I catch a female on the way back to her nest, and take a small blood sample. The chemistry of this sample, and others like it, will offer clues to the birds’ experiences with my artificial eggs.
When I let this robin go, she poops on me, flies to the nearest tree and scolds me loudly for what I’ve done. Then she spots a cowbird nearby and chases it away. I think she hates them more than she hates me.