Monarchs: are they ever bird food?
It's all a matter of timing.
(Listen to the radio version here.)
I learned a little about monarch butterflies in high school—that their eggs are always laid on the underside of a milkweed leaf; that the caterpillars eat milkweed exclusively; that the milky substance in milkweed leaves and stems contains cardiac glycosides which are very toxic; and that as a result, monarch caterpillars and adults are toxic. Birds who eat monarchs get sick, so many of them avoid orange butterflies innately or quickly learn never to eat anything that looks like them. Another orange butterfly with monarch-like markings, the viceroy, gets similar protection.
I learned in high school that viceroys aren’t toxic, but oops—turns out they are. So this is not a case of Batesian mimicry (a harmless species mimicking a toxic one for protection) but of Müllerian mimicry (two similar-looking species mutually benefitting because a predator need sample just one of either before avoiding both).
Another group of bright orange butterflies, the fritillaries, are also toxic due to the caterpillars feeding on passionflower vines. Birds usually avoid eating them, too.
Some animals avoid brightly colored or boldly patterned insects instinctively, but we and our fellow intelligent species often learn by experience. When I raised baby Blue Jays as a rehabber, I noticed that while the birds were nestlings and young fledglings, they eagerly swallowed every food item I put in their mouths.
But when they got a little older and started picking up their own food items, they grew extremely selective—even suspicious. At this point, even though they’d learned to trust me, when I offered them any new food item, including wonderfully tasty treats, they took just a tiny bite and then waited a good 15 minutes before eating the rest. And they seem to retain this habit for life—when I cared for adult Blue Jays, I noticed that they always tested novel food items and waited about 15 minutes before eating more.
This made complete sense to me, because Joel Carl Welty, author of my college ornithology textbook The Life of Birds, published in 1975, discussed Lincoln Brower’s famous studies on monarchs, showing Brower’s dramatic photos of a Blue Jay tasting a monarch for the first, and presumably last, time, and then vomiting. Based on that 15-minute “waiting period” I'd consistently observed between a Blue Jay tasting a novel item and eating more, I felt a flash of recognition when Dr. Brower, in an interview on Science Friday (“The Case of the Barfing Blue Jay”) mentioned that the Blue Jay barfed about 12 minutes after ingesting the monarch.
Adult monarchs feed on nectar, no longer ingesting milkweed toxins, so their eggs are not toxic, nor are the tiny caterpillars that emerge from them.
As they grow, feeding exclusively on milkweed, the caterpillars take in more and more of those cardiac glycosides, growing increasingly toxic.
I’ve always presumed that birds avoid eating both monarchs and their caterpillars entirely, but learned the hard way a few weeks ago that House Wrens do eat them—at least the early instars. (Between hatching and pupating, monarch caterpillars have five growth stages, or instars.)
This year Russ and I planted a bunch of swamp milkweed in our yard, and so far it seems to be thriving. One day I was thrilled to find more than a dozen monarch caterpillars, all in the second or third instar stage (the first instars are so tiny that they’re hard for my old eyes to see). A few times I’ve spotted, but not been able to photograph, a female monarch in the leaves, not the nectar-rich flowers, presumably depositing an egg.
Within a few days of first discovering the caterpillars this year, every one of them vanished. I was mystified. I’d had a few caterpillars on some milkweed plants several years ago, but they all died after an airplane sprayed Bt over the neighborhood. That “harmless” insecticide used to kill gypsy moths and spruce budworm is indiscriminate about which moth and butterfly larvae it kills, but so far this year I’d not noticed any pesticide applications. The mystery was cleared up when I spotted one of my adult House Wrens meticulously searching through the milkweed for caterpillars.
Despite their aggressiveness in taking over chickadee nest cavities, I love House Wrens, and have been thoroughly enjoying seeing a lot of thriving fledglings this year. But how can I not feel ambivalent about what their parents have been feeding them?
Of course, that also brought to mind the horrible day in 2020 when I was thrilled to discover a whole batch of newly-fledged House Wrens in our raspberry patch until I realized a family of Blue Jays had discovered the same thing, and were using them as baby food. I wrote about it:
I watched one [adult Blue Jay] drop into the raspberries and come up with a little brown puffball. That was all that I had the stomach for. As I walked into the house, I could hear the baby jays calling and begging … When that parent flew in, I could hear one hungry jay stop whining suddenly to eat its breakfast.
What goes around comes around, I guess.
Back in the late 90s and early 00s, I was a writer for the Webby-Award-winning website “Journey North,” serving as their American Robin and Whooping Crane “expert” (I put that in quotes because I’m an excellent researcher and have spent a lot of time watching both species but am hardly an authority on either). Journey North focused on the migrations of many animals, from insects to whales, but had a primary focus on monarch butterflies. The monarch pages included a lot of information from and about Lincoln Brower, one of the world authorities on monarch butterflies, whose fame had gone far, far beyond photographing barfing Blue Jays.
When I got a good camera, I started photographing and video-recording monarchs at different stages of life.
I photographed some eggs and tiny caterpillars, but even with a macro lens couldn’t capture any detail.
But as our grandson Walter grew increasingly intrigued with nature and obsessed with the Kratt brothers and their show Wild Kratts, he learned about tardigrades—microscopic creatures nicknamed moss piglets. We had some mosses growing between our patio blocks, so we decided to buy a digital microscope. Russ and Walter have had a lot of fun searching for and examining tardigrades and other tiny things. Now suddenly I had a way to get a lot more detail looking at monarch eggs and first-instar caterpillars.
So last week when I looked through the milkweed and found more eggs, I started searching for new caterpillars. It took a few days for me to find a few tiny caterpillars; I brought one inside to raise so at least one of them would survive my House Wrens—this was a second instar.
Then I brought in a milkweed leaf with an egg.
On Wednesday night, the egg looked about to hatch.
I stayed up watching the tiny caterpillar within squirming about more and more as I got sleepier and sleepier. Finally, about 1 am, I collapsed into bed. But I woke up a bit after 3 and got up to see what was happening—without a moment to spare. The following video starts very soon after I started watching, as the tiny thing chews its way out of the egg shell, properly called the chorion.
After it chewed its way out, it returned to eat the chorion. It was about 2 mm long.
It was breathtaking to see this all so up close and personal. I stayed up for four hours taking photos and videos, but then collapsed into bed.
By the time it hatched, I regretted bringing the egg into the house while it was still white. Had I waited a couple more days until the egg started getting darker, the leaf would have been moister when the egg hatched. As it was, the leaf was growing a little brittle, so when the caterpillar started moving away from the mostly-eaten chorion, I painstakingly got it onto a fresh young milkweed leaf. I was pretty sure all would be well when I finally fell asleep again, but when I woke up about 11:30 am, I could not find the tiny thing at all, even using the microscope to scan every square millimeter of the leaf. To my dismay, I did find a tiny spider who looked contentedly full. I still feel horribly guilty.
The caterpillar I originally brought into the house is still doing fine, and I want to give Walter a chance to see it pupate and emerge, but after that there will be no more reason for me to ever bring any caterpillars into the house again. Yes, there’s predation in their natural world, but as my poor microscopic hatchling learned, there’s predation indoors, too.
You can watch the entire 45-minute video showing the caterpillar eating its way out of the egg, then returning to eat more of the chorion on this video. I’m clearly not very experienced at this sort of thing, so refocusing and zooming in and out is pretty sloppy, plus a couple of times I moved the leaf. But I didn’t want to edit out the bad parts because I wanted it to accurately show those 45 minutes.
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Oh, Laura, this article is so timely for me. I am composing a substack article about an elementary-level milkweed nature journaling class I taught to at a local library. Out of curiosity, yesterday I asked AI, "What birds eat monarch butterfly larva?"The response was this: "Most birds generally avoid eating monarch caterpillars and butterflies because they sequester toxins, called cardenolides, from milkweed, which cause illness or vomiting. However, species like cardinals, orioles, grosbeaks, sparrows, and house finches have adapted to eat them by stripping the toxic insides or eating only specific less-toxic body parts. Smart birds! While most bird species learn to avoid monarch caterpillars—recognizing their bright yellow, white, and black striped warning coloration—several birds are known to actively hunt and consume them." YOU, my dear, have given me a more information... with great photos! I love how you observe, investigate and document! I'll be sharing your brilliant article!