Earth Notes: Spiderling Traps

Jul 10, 2019

Northern Arizona University biologist Jason Wilder made a grisly discovery recently: a plant that ensnares and kills birds.

Released MacGillivary warbler
Credit Jason Wilder

Wilder was hiking in a remote dry wash in the volcanic field east of Flagstaff last summer, when he caught the killer red-handed. Two live birds were fully entwined in the stems of a Torrey’s spiderling—and would have died if Wilder, a keen birder, hadn’t freed them.

The spiderling is an annual flowering plant in the four o'clock family, native in Arizona and New Mexico. It grows in disturbed soils of dry washes and canyon bottoms—the same locations that often attract birds searching for food and temporary shelter as they migrate through.

Spiderlings germinate with the arrival of summer monsoon rains, and bloom in late summer and early fall. They develop sticky rings on the stems, and as individual plants mature they often spread and form large patches of cagelike networks of those sticky entwined stems—all timed to coincide exactly with fall bird migration.

During several visits to the site, Wilder found the feathery remains of 36 unlucky entanglements—including a dusky flycatcher, a mourning dove, and an unidentified owl. He successfully released two MacGillivray’s warblers and a Brewer’s sparrow.

Large numbers of insects were also trapped on the plants’ sticky rings—many alive and struggling to free themselves, creating a tempting lure for hungry birds.

Torrey’s spiderling is the only native North American plant known to kill birds by entanglement. And these findings suggest it’s likely responsible for a fair bit of bird mortality across the Southwest every fall.