Not infrequently two tits will engage in what may be called a "responsive exercise," swinging their two-part song back and forth in the woods like a silvery pendulum. Not soon shall I forget a winter day on which I listened with delight to such an antiphonal duet. I was standing in a road that wound along the foot of a steep, wooded bluff, and the two minstrels were in the woods above me, one of them singing very high in the scale, the other responding in the same tune, but almost, if not quite, an octave lower. At first they were about twenty rods apart, but as they swung back and forth, they gradually approached each other until the distance between them was only a few feet. The music seemed like a slender thread of silver which was being wound up at both ends, gradually drawing the little fluters together. Sometimes one of them would miss one note of his dissyllabic song, and at times the refrains were repeated in a leisurely way, at times in quick succession; but the performers never sang simultaneously, each waiting until his fellow minstrel had given his reply. The pleasing duet lasted for many minutes; indeed, it was kept up long after I left the immediate neighborhood, for when I had gone quite a distance the sweet cadenzas still fell rhythmically on my ear. To my mind the two-part aria seemed like a voluntary performance, and I cannot doubt that it was. There was too much of an air of purpose about it to permit of the thought that it was a mere accident or coincidence; but whether it was a musical contest between rival vocalists, or the love song of a tomtit and his mate, I could not determine. Cunning in other ways, it would be strange if the tomtits did not display acuteness in the selection of nesting sites. A cosy hollow in a dead snag or stump is especially acceptable. Sometimes it is a deserted woodpecker's cavity made trig and clean, while quite often, when the wood is soft enough, the tits themselves chisel out a little hole in a tree or stump or fence post. I recall having once watched a pair of chickadees hollowing the upper end of a truncated sassafras tree that was half decayed. They would fly into the cavity, pick off a chip, dash out and away a rod or two, drop the fragment, then dart back to the hollow for another piece. In this way the busy couple worked hour by hour without resting for an instant. Their reason no doubt for carrying the chips some distance away from their nest was that they did not want any telltale fragments to betray their secret to their enemies.