Planet Earth

Eating Spiders Can Fix a Bird Brain

Blue tits raise smart, brave chicks by feeding them arachnids.


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Forget Baby Einstein. Birds have hit upon a cheaper, more reliable method for enriching their offspring's minds: spider snacks.

The blue tit, a European relative of the chickadee, feeds its chicks huge, heaping helpings of spiders peaking when the chicks are five days old. This behavior has long puzzled biologists because spiders are rarer and harder for tits to catch than are caterpillars, the chicks' other main dish.

Kathryn Arnold, an ornithologist at the University of Glasgow, recently found out why a bird might favor scrawny spiders over juicy caterpillars: They contain far more taurine. This amino acid is known to be a key input for a growing mammal—it's essential for the proper development of visual acuity, overall intelligence, and resistance to anxiety—but Arnold didn't know what it did for birds. Since the spider feast begins when the chicks open their eyes for the first time, she suspected that taurine might affect avian development in much the same way it does mammals.

Arnold studied the phenomenon by feeding taurine supplements to blue tits living in nestboxes along Loch Lomond in Scotland. Blue tits, family-oriented birds that they are, lay clutches of seven or eight eggs. Arnold fed taurine-laced gelatin gummy worms to one chick and plain gummy worms to another in each nest, in addition to the spiders and caterpillars provided by their parents.

Once the birds were ready to fledge, Arnold gauged the intelligence and courage of the hand-fed birds. In the land of birds, courage means not running away from something new that appears in your cage. In this case, Arnold used a “really, really hideous bright pink plastic frog” and a metal nut. Birds supercharged with taurine were noticeably braver than their siblings.

To test the birds’ facility for spatial learning, the researchers drilled a series of holes in a piece of plywood, hid sunflower seeds in some of the holes, and covered each with a felt flap. Once the birds poked around behind the flaps and found the seeds, the scientists took the board away and returned it five minutes later with seeds in the same holes. The birds got points for how well they remembered where the food was the second time around. “The blue tits love doing this because in the wild they’d be poking into crevices,” Arnold says. “They’re very into it, they’re very curious.”

Again, for the most part the taurine birds were smarter than their kindred, with one exception: Some of the bravest birds, those that approached the distressing pink frog the quickest, failed the seed test. “The very bold birds were kind of crazy,” Arnold says. “They’d go down and pull all the flaps off without really thinking about it. They were quite hyperactive, really.”

So just enough taurine—but not too much—seems to create the ideal blue tit. Of course, bird parents don’t mark their tiny calendars “Day Five: Feed chicks lots of spiders.” (This would be difficult as they lack hands. Also, calendars.) Instead, natural selection most likely favors parents who will put in the extra effort to hunt spiders and thereby boost their chicks’ taurine consumption when it will do the most good. Arnold says this type of optimized eating may be a common behavior in the animal kingdom.

No one yet knows whether adult birds also skew their diets toward spiders, because most blue tits do their eating at the top of trees, where it’s hard for biologists to follow them. “The only data we have [on adult diets] is from the 1950s, when biology was done with a shotgun,” Arnold says.

In recent decades, biologists have abandoned the shotgun in favor of more humane research tools—like the hideous plastic frog.

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