Hummingbirds are tiny and delicate, but don’t be fooled: They are among the most aggressive birds in the avian kingdom. Their territorial fury is especially aimed at other hummingbirds. Competition over a patch of flowers or a mate often results in high-speed aerial chases, divebombing and beak jousting.

So when Gustavo Cañas-Valle, an ornithologist and birding guide, stumbled across a cave full of hummingbirds nesting and roosting together in Ecuador’s High Andes, he could hardly believe it.

“I thought, ‘This looks like a colony,’” Mr. Cañas-Valle said. He added, “They were like bees.”

He documented 23 adult birds and four chicks,all of the subspecies Oreotrochilus chimborazo chimborazo, commonly known as the Chimborazo hillstar.

Mr. Cañas-Valle’s discovery, described in the journal Ornithology in November, may be the first documented example of hummingbirds that nested and roosted communally. It is also notable that he found the birds engaging in both these behaviors in the same space — something that even highly social species from other bird families tend not to do.