4. Some fish, as the glaucus, conceal themselves in summer time; for this fish hides itself for sixty days in the summer time. The onus and the chrysophrys hide themselves. The reason for supposing that the onus hides itself for a long while appears to be that it is captured at long intervals; and the influence of the stars upon them; and especially of the dog-star, appears to be the cause of their hiding themselves in summer time, for the sea is then disturbed. This is most conspicuous in the Bosphorus; for the mud is thrown up, and the fish are thus brought to the surface; and they say that, when the bottom is disturbed, more fish are often taken in the same cast the second than the first time; and after much rain animals make their appearance which before were either not seen at all or but seldom. CHAPTER XVIII. 1. Many kinds of birds also conceal themselves, and they do not all, as some suppose, migrate to warmer climates; but those which are near the places of which they are permanent inhabitants, as the kite and swallow, migrate thither; but those that are farther off from such places do not migrate, but conceal themselves; and many swallows have been seen in hollow places almost stripped of feathers; and kites, when they first showed themselves, have come from similar situations. Birds with crooked claws, and those also with straight claws, conceal themselves indiscriminately; for the stork, blackbird,[217] turtle dove, and lark hide themselves, and by general agreement the turtle dove most of all, for no one is ever said to have seen one during the winter. At the commencement of hybernation it is very fat, and during that season it loses its feathers, though they remain thick for a long while. Some of the doves conceal themselves; others do not, but migrate along with the swallows. The thrush and the starling also conceal themselves, and among birds with crooked claws the kite and the owl are not seen for a few days. [217] Κόττυφος, Turdus merula, _Strack_, blackbird, but probably more than one kind of bird is included under the same name. Compare 9, 36, 2. CHAPTER XIX.