A true “migrant trap”: Boerhavia (Nyctaginaceae) entanglement as a recurring cause of avian entrapment and mortality

Abstract

Bird entanglement in plants is a relatively uncommonly documented occurrence, with only a handful of plant species known to recurrently trap birds and cause mortality. Here I describe a large number of bird entanglement events, including 3 confirmed bird deaths, involving Boerhavia torreyana (Torrey's spiderling) in an arid canyon in northern Arizona. Like many species of Boerhavia, B. torreyana possesses sticky rings on its stems that are hypothesized to enhance herbivory defense. I document that these sticky rings also frequently capture birds, and identify 39 separate bird entanglement events involving at least 9 different bird species in a small area where B. torreyana is common. Because B. torreyana is an annual plant that germinates with the arrival of the Southwestern monsoon in mid-summer, it matures and presents an entanglement risk at a time that coincides with the fall bird migration. Small ground-dwelling passerines and insectivores appear to be particularly at risk of being trapped. There are very few known examples of plant species that regularly causes bird entanglement and associated mortality, and the observations presented here document a new case of this phenomenon.