216 THE ZOOLOGIST. 



taken up the migrations of birds on the American Continent as 

 its first study, has mapped out its vast territories into districts, 

 has invited observations from everyone interested in natural 

 history, and is prepared to most carefully tabulate and compare 

 all results which may be thus obtained. 



It may be said that the students of bird-life of the present 

 day are fully awake both to the interest of these vast seasonal 

 bird shiftings and to the unsolved problems which as yet lie 

 behind them. If on a map of the globe lines could be drawn 

 accurately setting down the forward and backward movements of 

 birds, we should not only be astonished at the greatness of the 

 phenomenon, but we should be possessed of clues which might 

 disclose to us important secrets concerned with the physical 

 geography of the earth in past ages. 



There can be little doubt that the primary cause of bird 

 migrations is tempei'ature. We well know tliat the isothei'mal 

 lines have been over and again deflected; and it is at present a 

 problem whether the movements of birds indicate the lines of 

 ancient temperatures, or have been changed in direction as the 

 temperatures of the countries frequented by them have been 

 modified. 



On their return journey in the spring many birds appear to 

 take a diff'erent route to the one they had observed in the autumn. 

 The Knot and the Bar-tailed Godwit, which in the autumn may 

 be found in large numbers upon the ooze and sandflats along the 

 coast, are rarely seen on their passage back in the spring ; whereas 

 the Whimbrel, so common along the shore in April and May on 

 its way back to the north, is equally rare in the autumn — proving 

 that these birds have one route for their spring migration and 

 another for their autumnal journey. But the avifauna of a dis- 

 trict becomes varied tlirough other causes than migration. 

 Pembrokeshire used to be in a special degree the winter abode of 

 the Snipe, the Woodcock, and the Wild Duck. Drainage and 

 improved cultivation ; the wanton destruction of the birds at 

 their northern breeding-stations ; the efficiency of modern sport- 

 ing guns and ammunition, with the increased numbers of sports- 

 men, have all conspired to reduce these birds to a tithe of their 

 former numbers ; but while these have decreased, many other 

 species have found the county yearly becoming more adapted to 

 their requirements, and have proportionately multiplied. Of 



