Ill APPENDIX 



MIGRATIONS OF BIRDS. 



Many species of birds leave certain localities in the fall, and, 

 spending the winter at the South, return in the spring. Why they 

 do thus has never been satisfactorily explained. The failure of food 

 in rigorous climates is, without doubt, the main factor in the solu- 

 tion of the problem, but it is by no means the only one. Birds 

 might incubate in warm latitudes as well as in cold ; unless, per- 

 haps, a second factor be found iiv the physical necessity of main- 

 taining a uniform temperature. But individuals of many migratory 

 species remain in the region of their nativity with no apparent 

 inconvenience. Greater freedom from molestation in rearing their 

 broods has been suggested as a third factor. If it be so, then all 

 southern birds should come north. Some birds return, year after 

 year, to the same localities, as proved by tying bits of red silk to 

 their legs; but it can not be positively asserted of many species. 

 The southernmost limits of some individuals of a species may also 

 be the northernmost limits of others, so that the species may be 

 regarded as resident, though the individuals are migratory. 



The males of some of the Thrush family, as well as those of 

 certain species of other families, in migrating northward precede the 

 females by two or three weeks, while the sexes associate in going 

 southward. 



It is not determined whether any of the northern birds migrate 

 as far as the Equator, though many individuals of most of the 

 species are known not to pass beyond the Gulf States, especially the 

 southern half of Florida. The strictly insectivorous birds, ;is swal- 

 lows, martins, etc., collect in flocks and leave earliest in the fall. 

 They are followed by the granivorous when seeds become scarce or 

 covered with snow. 



Extensive districts become gradually depopulated of certain spe- 

 cies, while in other regions HIPV multiply. After a time the former 

 localities are revisited by the species that had become nearly or 



