Everyone knows to watch their step around a black widow—including predatory birds. A new study shows that birds know to avoid the spiders based on their bright red markings. Meanwhile, the spiders' prey has a much more difficult time making out the markings, leaving them unaware of the lurking danger. The findings of the study were published last week in the journal Behavioral Ecology.

The species collectively known as black widows hold a special place in the human imagination. The pop mythology surrounding the female black widow’s potent venom and characteristic red hourglass markings have inspired songwriters, comic book creators, and even video game designers—a fact that the authors of the current study exploited. 

“Video game developers love to put black widows in their games because they’re scary,” lead author Nicholas Brandley said in a press release. Brandley and his colleagues bought a digitally rendered black widow (in this case, Latrodectus mactans) from a video game developer. They adapted the code for a 3D printer, and made black widow models in the lab. Four of the eight fake spiders were painted all black. The other four got the full killer-spider treatment in "Heavy Body Mars Black" and "Berry Red"—commercially available colors that closely match the coloration of live black widows. 

Actual spider belly. Image Credit: Nicholas Brandley

The scientists started by testing spider-eating birds. They set up seven bird feeders around Durham County, North Carolina, and replenished them daily with seeds so local birds would get used to visiting. Then, on test day, they took all the seeds away and replaced them with a single fake spider, laid belly up. 

They watched as the birds (Carolina chickadees, northern cardinals, tufted titmice, white breasted nuthatches, and two kinds of woodpeckers) approached the feeders. They found that the birds were 2.9 times more likely to attack the plain black spiders than the faux black widows. Smaller birds were especially likely to take off at the sight of the red hourglass. “The birds would see a spider model with red markings and get startled and jump back, like ‘Oh no man, get me out of here,’” Brandley said in the press release. 

Clearly, the birds could see the spiders’ markings and understood what they meant.

Next, the researchers measured the wavelengths of light given off by the markings of two black widow species. Bird eyes, human eyes, and insects all pick up different wavelengths, which affects how we all perceive color. They found that the black widows’ red markings are half as visible for insects as for birds. 

The scientists then wondered if viewing angle had anything to do with it, since black widows like to hang belly-up beneath their webs. This position would display the red markings to spider-eating birds flying overhead while conveniently concealing them from insects below. 

To find out, they set up 20-inch-tall “widow towers” in the lab, brought in two real black widow species (L. variolus and L. mactans), and let them set up house. Time and time again, L. variolus spiders, which also had red marks on their backs, built their webs higher in the tower than spiders from the other species, which did not. This suggests that the L. variolus spiders were raising their vivid markings even farther from insect view, making them harder to see.