Yellow-browed Warbler. With them it must be the nature of the weather when they arrive in Norfolk, Lincolnshire, and Yorkshire which determines whether they halt or pass on.
A certain number of Russian and Eastern Asiatic birds probably pass over Norfolk and the east coast every autumn, for the most part by night, and at so high an altitude as to be beyond the limits of human sight. As they are not seen their presence is never suspected. Migratory phenomena of this sort only become apparent when brought within our ken by unsettled weather and sometimes fog, as clearly demonstrated by Herr Gätke in his long course of observations on Heligoland.
There is no migrant whose movements can be better observed than the Blackbird's. They come from the east, for the most part in "rushes," from October to Christmas Day, first dropping into turnip-fields with an incredible number of Thrushes, and then swarming in plantations. It is in December and January that Norfolk obtains the old yellow-billed cock Blackbirds, which indicates either that the adults are the last to migrate from Scandinavia, or that, owing to dull plumage and brown bills, these old cocks are not recognized as such by English observers in October and November.
By the 1st of February the northward movement has begun again, almost before the southward movement of individuals nesting in the higher latitudes is over; and, under certain circumstances of wind and weather, it is probable the two streams sometimes amalgamate, or actually cross one another. If any ornithologist possessed of keen sight would go to sea in one of our Yarmouth herring smacks, or obtain the Trinity Board's permission for a week's sojourn on such a floating lightship as "The Outer Dowsing," or "The Leman and Ower," in the month of October, he could not fail to identify a number of species in transit, especially if the wind was from the west. A wind which the migrants (nearly always to be seen at Cromer arriving from the east) would have to fly against would delay nine-tenths of them until sunrise, or later, when they could be easily identified. Its velocity must be an important factor, and it would probably be found that they choose a high or low stratum, according as they are thereby enabled to minimize its power. By anchoring a boat at a measured distance of half a mile from
