About a week had passed away; the bridge was completed, during which time the female birds had arrived; and, save a stray one now and then, not a single individual of that numerous host that had gathered round the _Ribes_ was to be seen. They cared nothing for the gun, and would even dash at a dead companion as it lay on the grass; so I did not drive them away, but left them to scatter of their own free will. My next camping-place was on the western slope of the Rocky Mountains, near a lake, by the margin of which grew some cottonwood trees (_Salix scouleriana_), together with the alder (_Alnus oregona_), and the sweet or black birch (_Betula leuta_). My attention was called to the latter tree by observing numbers of wasps, bees, and hornets swarming round its trunk. The secret was soon disclosed: a sweet gummy sap was exuding plentifully from splits in the bark, on which hosts of insects, large and small, were regaling themselves. As the sap ran down over the bark, it became very sticky, and numbers of small winged insects, pitching on it, were trapped in a natural ‘catch-’em-alive-O.’ Busily occupied in picking off these captives were several very sombre-looking hummingbirds. They poised themselves just as the others did over the flowers, and deftly nipped, as with delicate forceps, the helpless insects. I soon bagged one, and found I had a third species, the Black-throated Hummingbird (_Trochilus Alexandri_). Were any proof needed to establish the fact of Hummingbirds being insect-feeders, this should be sufficient. I saw the bird, not only on this occasion but dozens of times afterwards, pick the insect from off the tree, often killing it in the act; and found the stomach, on being opened, filled with various species of winged insects. The habits of the three species differ widely. The Red-backed Hummingbird loves to flit over the open prairies, stopping at every tempting flower, to catch some idler lurking in its nectar-cells. Building its nest generally in a low shrub, and close to the rippling stream, it finds pleasant music in its ceaseless splash. Minute Calliope, on the other hand, prefers rocky hillsides at great altitudes, where only pine-trees, rock-plants, and an alpine flora ‘struggle for existence.’ I have frequently killed this bird above the line of perpetual snow. Its favourite resting-place is on the extreme point of a dead pine-tree, where, if undisturbed, it will sit for hours. The site chosen for the nest is usually the branch of a young pine; artfully concealed amidst the fronds at the very end, it is rocked like a cradle by every passing breeze. The Black-throated Hummingbird lingers around lakes, pools, and swamps where its favourite trapping-tree grows. I have occasionally, though very rarely, seen it hovering over flowers; this, I apprehend, is only when the storehouse is empty, and the sap too dry to capture the insects. They generally build in the birch or alder, selecting the fork of a branch high up.