A century later, resolving Joseph Grinnell's “striking case of adventitious coloration”

ABSTRACT In 1921, The Auk published an unresolved mystery submitted by Joseph Grinnell, founding director of the Museum of Vertebrate Zoologyat the University of California, Berkeley. He had collected two Oak Titmice ( Baeolophusinornatus) whose breast feathersappeared to be yellow; the featherswere covered in microscopic particles. He invited readers to further investigate the nature of these particles. Nearly a century later, we have determined that Grinnell's mysterious grains are willow(Salix) pollen, a diagnosis he rejected because the size of the particles had been mismeasured, and because he lacked a plausible explanation for how the birds would become coated in pollen. We investigated this case using Grinnell's own specimens, field notes, and correspondence, a testament to Grinnell's dedication to scientific rigor through carefully documented and curated natural history collections.