MIGRATION OF BIRDS. 133 



is building, there is a new occupation altogether ; and 

 the nests even of very small birds are constructed with 

 so much care, that that and the finding of subsistence 

 demand more than the average power of industry. 

 When the female begins to sit on the eggs, the feeding 

 of her partially depends upon the male ; and when the 

 young are hatched, their support, till they are in a 

 condition for supporting themselves, requires a consi- 

 derable portion of the time and industry of both parents. 

 When the young are fledged, the parent birds still 

 require long days : the operation of moulting, by which 

 their tattered plumage is replaced by a new supply, 

 exhausts them : thus they have long days, and also 

 food in abundance, when they are least able to make 

 exertions in search of it ; and by the time that the 

 decreasing supply warns them that it is time to seek 

 more southern climes, they are in prime feather and 

 vigorous health, and able to sustain the fatigues of the 

 voyage. The return, too, is, generally speaking, after 

 the autumnal equinox, so that in their migration south- 

 ward they have the same advantage of a longer day 

 than in places northward. Thus, even in this com- 

 mon-place matter, a matter which is so common-place 

 that few take the trouble of heeding it, and almost 

 none inquire farther than saying that it is the instinct 

 of the birds, we may trace as perfect a succession of 

 antecedent and consequent, or as we say, of cause and 

 effect, as in any other part of the works or economy 

 of creation. We ought, indeed, to guard very care- 

 fully against stopping at the word instinct, or indeed 

 at any other word which is so very general that we 

 cannot attach a clear and definite meaning to it. Those 



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