An all-caterpillar diet does not have to be boring.
That’s what I concluded as I watched Baltimore Oriole parents feed their nestlings during my recent nest-watch. They brought wiggly larvae in all colors, sizes, shapes, and textures: green, white, black, skinny, plump, smooth, striped, bumpy, and hairy. Aside from the occasional dragonfly and moth, the nestlings’ diet seemed pretty predictable.
Until they were about four days old.
That’s when I noticed the parents, especially Lady Baltimore, began varying the kids’ diet. The new entrée on the menu was spiders, usually the kind that carry their egg sacs with them as they roam. Lady Baltimore grasped the bulging round sacs with her bill and delivered the meal, spidery legs and all, to her nestlings.
Here she is in a meal-delivery montage.
On one occasion, I saw her retrieve and swallow the spider’s body after the nestlings presumably ate the egg sac. Here’s that incident, captured in a video I recently shared on social media.
Perhaps the nestlings preferred the squishy part of the meal, a texture that must be like the caterpillars they’d become accustomed to. After two days of eating lots of spiders, the nestlings were again fed mostly caterpillars. I didn’t give their menu any more thought.
Then I began reading a wonderful new book.
An interesting comment caught my eye: “Chickadees seek out spiders (high in the essential nutrient taurine) for the first week or so that they are feeding [their] young.”
Taurine? A few Internet clicks later, I learned that when taurine, an amino acid, is included in a nestling’s diet, it helps ensure proper development of the bird’s brain. And—here’s the best part—it makes the bird grow into a courageous adult.
Scottish researchers fed some nestlings extra taurine and compared their development with a control group whose diet included typical amounts of the nutrient. They put an alien object—a pink plastic frog—in the nestlings’ cage. The birds fed extra taurine ignored the scary frog and passed a series of find-the-hidden-seed tests. The control group was spooked by the frog (my words) and largely failed those tests. The researchers concluded that the taurine-fed nestlings grew to be smarter and braver adults, a developmental key to finding food and avoiding predators.
In my reading, I also learned that mammals need taurine for vision and brain development. It’s made by the liver and kidneys and is abundant in scallops, mussels, clams, the dark meat of turkey and chicken, and the energy drink Red Bull.
Nothing I read, however, suggests that consuming spiders—or any source of taurine—makes us any braver.
(Here’s the nestling taurine study if you want to read more. It’s pretty interesting!)