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M.aR. ?9o3[ THE CONDOR 37 tourist may conveniently visit the "Stygian Cave," at the Mammoth Hot Springs, and find there the bodies of many little birds whose spirit passed away as they entered. There are many such spots about the mineral "formation," almost two miles iu extent, around these boiling springs, where Fort Yellowstone is located. Mrs. Pitcher, wife of the present Park Superintendent, showed me some of these "Stygian" caves, in April, t9o2; and, thereafter, I often examined such "bird caves" as I had found, or searched for others. In any hollow capable of holding the heavy gas (supposed to be carbon dioxide) fatal to animal life, dead birds were liable to be found, provided that the usual accompaniment of heat and moisture (from steam), and sulphurous odors (from emanating gases) were found. Most of the dead creatures were birds; but there were, besides, many insects,. and a few small mammals. Doubtless larger animals may have been killed in some of these pockets filled with gas, although the flame of a candle was rarely extinguished be- fore it came near the ground. When picking up dead birds I always took the precaution to hold my breath. Moisture and a distinct warmth to the ground were always felt. The effect on bird bodies was to cause rapid decay, the flesh quickly disappearing, then the bones, and lastly, the feathers. Upon the latter a caustic action was observed, the bases of the feathers being eaten away, until, in some in- stances, only the tips of the feathers remained and retained the form of the bird, at last sinking flat upon the ground and soon leaving uo trace. The remains were sometimes so indefinite that it was impossible to identify all of the species, or to make an exact count. In fact, I made no systematic effort to observe and record regularly the effects of these caves on the bird life of the locality, although my note books contain some data. On first visiting the caves, in April, remains of the magpie, Townsend solitaire, pink-sided junco, pine siskin, and Rocky Mountain nuthatch were found. May ?6, many pink-sidedjmlcos, warbling vireos, and a few Cassin purple finches, and one Macgillivray warbler were among the victims, at the few caves then known to me. At the "Stygian Cave" proper, in dangerous proximity to the deadly gas, a pair of Townsend solitaires were engaged in building a nest in a hollow of the rock. On my next visit the nest had been finished, but both of the little architects were lying dead at the bottom of the cave, pathetically near each other, their outspread wings touching one another. They perhaps descended to gather building materials, or to drink. During June and July dead birds were seen whenever the caves were visited. Among them were always some Townsend solitaires, Audubon warblers, and Louisiana tanagers. It ahnost seemed that the Stygian caves possessed some pe- culiar attraction for the unfortunate birds; but it is probable that a damp and shady nook offered a sufficient allurement, and that curiosity prompted some to follow companions that had preceded them. August ?, t9o2, eleven birds were found dead within the cavern known as the "Stygian Cave," as follows: Two pine siskins, four pink-sided juncos, (all young of the year, with striped underparts), two warbling vireos, two Louisiana tanagers (adult female and young of the season), and one mountain chickadee. During the September migration an unusually large number of birds perished, but I have kept no record of them. The largest number were pink-sided juncos. October x 5, t9oe, dead birds were found in twelve caves. In all fifty-eight birds and and a mouse (Peromyscus texanus subarcticus J. A. Allen) were counted. These were: one Clarke nutcracker, six pine siskins, thirty-five pink-sided jun- cos, one green-tailed towhee, one Louisiana tanager, four red-breasted nuthatches, four mountain chickadees, three Townsend solitaires, and three western robins.