LL Wolf 1964 Thryothorus plumbeus July 21 Cañon de Lobos, Pri. E. Cuenavaca, Morlos, Mexico 7800 ft. Found pair not-building 4½ feet up at top branching of nearly bare Acacia. Both sexes build, but one does much more than other. Juice of sang while I was in nest. Averaged 1½ trips to nest per min. Nest structure nearly completed + all materials during 15 min watched (0855-0910) were placed inside. Materials gathered within 30±yds of nest & usually closer. Adult carrying material flew to one of several used perches 10-15 feet from nest & paused before flying to nest. Several times flew up to nest then turned in air & came back to perch. Perches in shade I approx 2 ft above ground. On leaving nest usually flew straight off to out of sight, but sometimes stopped at one of perches for 5-10 sec. before flying off. P seemed to be gathering most of his material in stream gulley to NE of nest. Song of ♂ - --- - first part can be fast to drill a slow so each note stands out. last part is upward inflected whistle. May also introduce song with an upward inflected note. July 22 Cañon de Lobos, Pri. E. Cuenavaca, Morlos Mexico saw adult - presumably the ♀ carrying nesting material at 0730. Most of trips were to W & she seemed to be plucking plant fiber for lining. July 24 Cañon de Lobos, Pri. E. Cuenavaca, Morlos, Mexico 7800 ft. not probably of this species. 7+ put up in Opuntia . Two birds in nearby (20 feet) thicket. ♂ singing occasionally. 0844 - another bird began