Page:The Zoologist, 1st series, vol 4 (1846).djvu/222

1388 numbers of the Raptores, especially of those which nest in the district, the same cause has greatly increased the numbers of the smaller spe- cies of perchers, which find in the extensive covers a safe retreat from the depredations of birds of prey, and from the still more mischievous attacks of bird's-nesting boys.

The closer weeding of the fields, consequent upon agricultural im- provements, has interfered a little with some of the ground-nesting birds, but this probably does not apply to many species.

Of the regular migratory birds of Norfolk, those which arrive in spring, nest in the county and depart in autumn, are thirty-three in number, nearly half of them belonging to the family of the Sylviadae. With the exception of certain species which also visit ns in spring, but proceed to more northern breeding places, these are perhaps more regular in their appearance than birds of any other class. We believe that they also travel in small flocks, and chiefly during the night. The Hirundinidæ are well-known to congregate before departure, and we have observed the sand-martins to collect on the coast in immense flocks previous to their autumnal migration, the beach appearing at a short distance perfectly black with their numbers.

The line of division between the winter residents, and those birds which annually pass through this district in spring, on their passage to, and in autumn on their return from, their breeding places, cannot be permanently drawn with correctness, inasmuch as the movements of these divisions depend much upon the temperature, and perhaps in some measure on the prevailing winds of the particular season ; and that, not so much in our own, as in more northern latitudes. Amongst the autumnal migrants to our coasts, the occasional irregularity in the numbers of the common buzzard, and still more in those of the rough- legged buzzard, are curious and worthy of especial attention. These birds are both regular visiters, but not in large numbers. In the au- tumn of 1839, the latter species was unusually plentiful, and we had an opportunity of observing that the same migration extended over part of Germany ; which makes it probable that the birds came from a considerable distance, and that their appearance was not attribut- able to their having been driven from their usual route by any acci- dental cause. In the autumn of 1845 on the contrary, the rough-legged buzzard was hardly seen, whilst the deficiency was to a certain ex- tent counterbalanced by an increased influx of the common species. These variations are at present unaccountable, as we can neither con- nect them with change of temperature, nor with any deficiency