163 SUBKISTGDOM VEETEBRATA. 



M IGRATIONS OF BIRDS. 



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

 ing the winter at the South, return in the spring. Why they do thus 

 has never been satisfactorily explained. The failure of food in rigor- 

 ous climates is, mthout doubt, the main factor in the solution of the 

 problem, but it is by no means the only one. Birds might incu- 

 bate in warm latitudes as well as in cold ; unless, perhaps, a second 

 factor be found in the physical necessity of maintaining 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 cannot 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 spe- 

 cies 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 biids, as swallows, martens, 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 species, 

 while in other regions they multiply. After a time the former local- 

 ities are revisited by the species that had become nearly or quite 

 extinct, wliile the latter lose their abundance. Hence, to the annual 

 migrations must be added others marked by cycles of years ; and not 

 necessarily in lines of longitude, but as frequently in those of latitude. 



Birds come and go ; but whence they come and whither they go is 

 a matter of conjecture. One morning the trees of Independence 

 Square, in the heart of Philadelphia, were found filled with crows. 

 Not a caw was to be heard nor a movement seen. The birds 

 appeared to be awaiting in silence further instruction. After some 

 time several new-comers glided among them, threading their wav 

 through all the flock, when suddenly the teeming thousands rose 

 simultaneously and departed as mysteriously as they came. 



ae accurately as if a surveyor had been employed ; stones are removed, streets 

 made, a wall is thrown up on the windward side, and sentinels are posted. 



