Why do some birds roll themselves in dust ?

Because it may free them from annoyances, or prevent the bites of insects. - Knapp.

Why has it been asked whether Mahomet and his followers might not take one method of purification from these pulveratrices?

Because it is found from travellers of credit, that if a strict Mussulman is journeying in a sandy desert, where no water is to be found, at stated hours he strips off his clothes, and most scrupulonsly rubs his body over with sand or dust. - G. White.

Why are certain birds of a flock called sentinels?

Because they observe approaching danger, and speedily communicate it to the whole flock. In some cases these sentinels are deceived by false appearances. Dr. Edmonstone, in his " View of the Zetland Isles," gives a very striking illustration of this neglect of the sentinel, in his remarks on the Shag. " Great numbers of this species of the cormorant are sometimes taken during the night, while asleep on the rocks, and the mode of accomplishing it is very ingenious. Large flocks, sit, during the night, on projecting rocks of easy access, but before they commit themselves to sleep, one or two of the number are appointed to watch. Until these sentinels are secured, it is impossible to make a successful impression on the whole body; and, to surprise them is therefore the first object. With this view, the leader of the expedition creeps cautiously and imperceptibly along the rock, until he gets within a short distance of the watch. He then dips a worsted glove in the sea, and gently throws water in the face of the guard. The unsuspecting bird, either disliking the impression, or fancying, from what he considers to be a disagreeable state of the weather, that all is quiet and safe, puts his head under his wing and soon falls asleep. His neck is then immediately broken, and the party dispatch as many as they choose."

Why is the new arrangement of birds called "quinary"?

Because it proposes to arrange them in groupes of Jives, thus: Raptores, or birds of prey ; Insessores, or perching birds ; Rasores, or gallinaceous birds; Grallatores, or wading birds ; Natatores, or web-footed birds. Each of these is divisible into Jives; and again into Jives. The details of the arrangement would occupy too much space in the present work; and for them the reader is referred to several papers on the subject by Mr. Vigors, the ingenious secretary of the Zoological Society, who thinks the proposed arrangement to be strictly in accordance with the natural varieties of birds. It may be observed here, as a curious fact, that " by far the greater number of the pie and sparrow tribe in this country, and perhaps elsewhere, generally lay Jive eggs ; the rook, the crow, the hedge-sparrow, goldfinch, blackbird, thrush, etc. The advocates of the quinary arrangement will doubtless advance this in corroboration of their system."* Why are some birds called monogamous ? Because they pair, (from monogamia, one marriage.) Why are other birds called polygamous ? Because they never, unless compelled, confine themselves to individual association. - (From poly-gamia, many marriages.)

The whole number of birds enumerated by Linnaeus, specifically; is only 960, while those described by Dr. Latham, in his recently published work, amount to about 5,000.

* Jennings' Ornithologia.