Having read of mites swarming in the nests of various birds and being desirous of acquiring information on the subject, I examined the nest of a house wren, Troglodytes aedon aedon, Vieillot, at Sioux City, Iowa, during the summer of 1927. The young birds had the day before left the nest which was built in a small box located on a pillar of my front porch. When the box was opened and the twigs removed, there was a residue of gray powder in the box, amounting to six or seven drams. The microscope showed this powder to be composed of a seething mass of small mites in all stages of development, in countless numbers. Dr. H. E. Ewing, of the Bureau of Entomology, later pronounced them to be the common chicken mite. Dermanyssus gallinae L.