One of the strangest things about the flittermouse is its voice. It is a penetrating, shrill squeak, so high that many people cannot hear it at all. The chirp of a sparrow is about five octaves above the middle E of the piano, while the cry of the bat is a full octave above that. In England there is a saying that no person more than forty years old can hear the cry of a bat. This is founded probably on the fact that the ears of many of us, especially as we approach middle age, are unable to distinguish sounds more than four octaves above middle E. Some naturalists believe that the shrill squeak which most of us do hear is only one of many notes of the bat, and that the various species have different calls, like those of birds, and probably even have a love-song during the mating season, in late August or early September, which can never be heard by human ears. Most bats found in the Eastern States are either large brown house-bats, one of two kinds of little brown bats, black bats, red or tree bats, pigmy bats, or, last, largest and most beautiful of all, hoary bats. The big brown bat, or house-bat, is the commonest. This is the last of the bats to come out in the evening, for each has a certain fixed hour when it begins to hunt, which varies only with the light. When the big brown bat starts, the twilight has almost turned to dark. The two kinds of little brown bat, Leconte's and Say's, cannot be told apart in flight. Both of them are much smaller than the big brown bat, and the ear of a Leconte's bat barely reaches the end of the nose, while that of a Say's bat is considerably longer. All bats have large ears, each of which contains a curious inner ear known as the "antitragus." Both of these little bats are country bats and prefer caves and hollow trees to houses and outbuildings. The black bat can be told from all other American bats by its deep black-brown color touched with silvery white. This bat likes to hunt and hawk over water, skimming across ponds like swallows. Some of the black-bat colonies, or "batteries," are very large, one by actual count including 9,640 bats. Next comes the Georgia pigmy bat, so called to distinguish it from the very rare New York pigmy bat. This little bat can be told by its small size, for it is the smallest of all of our eastern bats, by its yellowish pale color, and especially by its flight, which is weak and fluttering, like that of a large butterfly.