In the course of nesting microclimatic studies of the Broad-tailed Hummingbird (Selasphorus platycercus), I observed three cases of intraor interspecific piracy of nesting materials. These observations were made in the vicinity of the Rocky Mountain Biological Laboratory, Gothic, Gunnison County, Colorado, at an elevation of 2920 m, where this is apparently the only breeding trochilid. On 17 and 18 June 1971, Broad-tailed females were observed building nests # 1 and 3 approximately 10 m above the ground in quaking aspen trees (Populus tremuloides) approximately 40 m apart on a hillside. A storm with rain, hail, and gusty winds occurred the night of 18 June. The following day, nest 1 was apparently abandoned, while a female was removing nest material from nest 1 to nest 3. On 20 June, a Warbling Vireo (Vireo gilvus) was removing large beakfuls of material from the walls of this same hummingbird nest 3. The damage was repaired on 21 June. On 18 July, after at least 6 days during which the female was observed feeding young, the nest walls were again suddenly diminished in height, far more extensively than occurs from normal spreading of the nest cavity as the young grow to fledging size. From 18 July until fledging, 31 July, there was no evidence of major addition to the nest walls, although the female did attempt some repair or adjustment of what remained. The young birds completed their development in a noticeably shallower nest than others under observation, but were on schedule with other Broad-tailed Hummingbirds that came from nests begun at the same time. On 20 July, a female hummingbird was removing fine material from the nest of the Western Wood