For decades, entomologists assumed that birds didn't eat monarch butterflies because they didn't like the taste. Until in 1957, one scientist decided to test that theory, using the same logic that parents of recalcitrant kids use to coax them into eating their spinach. You don't know you don't like it until you try it!
The mystery of why birds didn't snack on monarch butterflies was only made more complicated by the fact that birds are perfectly happy to eat another type of butterfly, the viceroy. And in fact, the viceroy attempts to copy the monarch so assiduously, it's been held up as the standard of animal mimicry.
The first problem with this notion was uncovered in a study that was meant to prove the monarch's inedibility. Butterflies trapped by scientists using a pair of tweezers were offered to birds. The birds gobbled down the swallowtails and many of the viceroys, but left the monarchs alone. That seemed to confirm the birds' distaste for monarchs, but not the idea that viceroy mimicked the monarch to attain its status as inedible. If that was the goal of the viceroy, it was hardly successful. If it wasn't remotely successful, why do it?
But more importantly, why would the monarchs be inedible? Doctor Frederick Urquhart was perplexed by this. The theory was that they grazed on milkweed, which was a bitter plant and made the butterflies' flesh too bitter to eat. But birds, Urquhart pointed out, were known for their relative lack of taste or smell. In any case, if the alternative is starving, surely any bird would choke down something distasteful. Urquhart realized, finally, that he hadn't even put the most basic assumption to the test. Were monarchs actually bitter? Since he followed them along their migration route, he had the chance to find out, and grabbed a couple of monarchs to snack on. They had no bitter taste at all. Urquhart announced that they tasted a bit like dry toast, but had no other flavor.
So why didn't birds eat them? Researchers got right on it, in a series of experiments based in scientific curiosity but flavored with just a bit of sadism. First the scientists picked the wings off monarchs and put them out on plate near a birdfeeder for birds to eat. Although the birds had other options when it came to food, they ate the plate clean every day. Clearly, if anything about the monarchs put the birds off their food, it wasn't their bodies.
Other researchers began getting birds accustomed to wingless monarchs, and then switched them to chilled, winged monarchs, which were too cold to make an effective getaway. The birds gobbled them down, but another clue to mystery was revealed when one of the monarchs got warmed up. The bird pursued the fleeing butterfly across a meadow but couldn't catch it and finally gave up. To keep such a thing from happening again, the researchers damaged the wings of the remaining butterflies, and the birds were happy to eat them all.
As far as researchers could understand, monarchs make fine food for birds, when they are seen as food and when they can be caught. It's possible that monarchs hide in plain sight — that their bright colors and distinctive shape mean that birds don't recognize them as food until they are habituated to them. It's also possible that the birds couldn't catch them.
Whatever the birds' reasons for abstaining from monarch butterflies, they seem to be decent eating for entomologists.
Viceroy Image: Public Domain Image