xvi INTRODUCTION. 



been, in comparatively recent times, subject to but little change. On the 

 other hand, the genera in the two arctic regions are crowded with imper- 

 fectly segregated species, which require for their explanation comparatively 

 recent and important changes in the climate. These are to be looked for 

 in the gradual approach of another glacial epoch. In the warm period 

 which permitted the growth of pines almost at the north pole the whole of 

 Europe, North Asia, and North America enjoyed a semitropical climate, 

 and the variation of species throughout the arctic regions was probably 

 very small. As the climate in Lapland and Siberia gradually changed, the 

 birds living there gradually changed also ,• and thus we find now in many 

 Palsearctie species a semitropical form in West Europe which is connected 

 by a series of intermediate forms with an arctic form in Siberia, which 

 again gradually changes in the east until in China the West-European 

 form is reproduced. In some cases the series is completed by tropical 

 species which have evidently been driven south by the glacial period and 

 have never returned. I have endeavoured to interest the reader wherever 

 possible in some of these near relations of our British birds, many of them 

 so closely allied as to be only subspecifically distinct. 



The number of birds included in the British list is about 380. Of these 

 126 are residents, though many of them have only slender claims to be 

 considered so. Some of them are principally known as winter visitants, 

 a few only remaining to breed, chiefly in the north of Scotland ; others 

 really belong to the summer visitants, but a sufficient number remain 

 during the winter to entitle the species to be considered a resident one. 

 Fifty-five species regularly visit our islands every summer for the purpose 

 of breeding ; but many of these are becoming rarer every year, partly in 

 consequence of the persecution to which they are subjected on their 

 arrival, and partly from the destruction of their breeding-grounds by the 

 drainage of marshes and the reclamation of waste land. Forty-one species 

 may be regarded as winter visitants; but many of these wander still 

 further south during midwinter, being principally seen on our shores in 

 spring and autumn. The birds contained in these categories form a total 

 of 222 species which are fairly entitled to be considered British birds. 

 It has, however, been the practice of ornithologists to consider any bird 

 British which has even once been obtained in our islands in a wild state 

 Of the accidental visitors which thus reach our shores many have occurred 

 only once ; but others have been met with much oftener, though some of 

 the recorded occurrences must be accepted with considerable hesitation. 

 Birds often escape from confinement ; a mistake is made in the identifica- 

 tion of the species ; and in very few cases are we able to trace clearly the 

 pedigree of individual examples so as to leave no reasonable doubt of their 

 authenticity, for skins are very frequently changed or transposed either by 

 accident or design. The number of these accidental visitants to the 



