_^24 Taverner, The Origin of Migration. \^^^ 



such a nest as this bird builds would be as secure from these 

 enemies in the heart of the tropical forests as in the temperate 

 ones. Therefore, safe nesting sites could not be the object of 

 their migrating, — unless the peculiar form of nest was evolved 

 after the migratory habit had been formed. This, however, does 

 not seem to have been the case. Such a likeness is exhibited in 

 the forms of the nests throughout the whole family, that we are 

 forced to conclude that this type of nest was used by the common 

 ancestor of Icterus, which must have been before the Baltimore 

 Oriole became migratory. 



The cuckoos and doves above mentioned, are notoriously care- 

 less nesters, and under this hypothesis, we would expect that 

 migration would have been forced upon the whole of these fami- 

 lies, or at least upon a considerable number of the species com- 

 posing them. Contrary to this, we find that these are peculiarly 

 tropical and subtropical families, and but a very small percentage 

 of them ever get up into northern latitudes. 



It may be held that the above cases are exceptions, caused by 

 varying local conditions, but it still remains to be proved that the 

 generality of tropical nesters take any greater nesting precautions 

 than northern ones of the same class, as would assuredly be the 

 case if the above were the correct solution of the problem. 

 Furthermore, there are grave reasons, to which I will refer later, 

 for doubting that inadequate nesting habits could ever be the 

 cause of migrations. 



A second theory, advanced under the auspices of Mr. Chas. 

 Dixon, refers the movement to a natural desire of the individuals 

 of a species to disperse during the breeding season, and draws 

 attention to the fact that the bird population is more scattered 

 during the breeding season than at other times. He utterly 

 refutes the idea that adverse circumstances of either food, tem- 

 perature, or enemies can force a bird to change its range, and 

 cites instances of the Great Auk, Labrador Duck, and other spe- 

 cies that have sufifered extermination rather than forsake their 

 accustomed habitat. Mr. Dixon evidently regards this dispersal 

 as effecting a reduction in the density of the population. It 

 certainly does result in this among the adult inhabitants, but it 

 is open to question if we assume that the total population is 



