MOLOTHRUS PECORIS, COW-BIRD. 183 



tions. A difficnlty, not usually recognized, meets us here, in the variable 

 circumstances of incubation. An egg habitually intrusted to numerous 

 difl^rent birds, from the size of a Thrush down to that of the tiny 

 Blue-gray Gnatcatcher, is necessarily subjected to a varying temperature 

 (within, however, narrow limits), hastening or retarding development 

 of the embryo ; even the material of so many different nests, consid- 

 ered as conductors of heat, may exercise a slight disturbing intlueuce. 

 Under these fluctuating circumstances the duration of incubation may 

 yet be proven inconstant ; and it cannot be supposed to bear a single 

 definite relation to that of each of the birds concerned. Of its prior 

 maturity, as a rule, however, there seems to be no doubt, and the result- 

 ing advantage is obvious. The young Cow-bird immediatel,y demands 

 and receives attention, so that incubation of the other eggs may remain 

 unfinished ; and if the latter are hatched, the chances of the younger 

 and weaker birds against the alien are very problematical. They may 

 • be trampled to death, or smothered, even if they are not forcibly ejected ; 

 and they generally fail to receive sufficient nourishment, of which the 

 lion's share falls to the intruder. 



There is another and a very singular element of success for the Cow- 

 bird. However indignant and despairing the nurses may be supposed 

 to feel about it at first, they generally make a virtue of necessity ; and 

 when they have once made up their minds to accept the unpleasant sit- 

 uation, they do their whole duty by it, even to the neglect of their 

 rightful ott'spring. As not unirequently occurs in other cases, they be- 

 come, so far as we can judge by their actions, really fond of the found- 

 ling, and, at any rate, they give assiduous attention to its wants, and 

 care for it as tenderly as if it were their own. 



It does not appear that the Cow-bird ever attempts to take forcible 

 possession of a nest. She watches her chance while the owners are 

 away, slips in by stealth, and leaves the evidence of her unfriendly visit 

 to be discovered on their return, in the shape of the ominous egg. The 

 parents hold anxious consultation in this emergency, as their sorrowful 

 cries an<l disturbed actions plainly indicate. If theii- nest was empty 

 before, they generally desert it, and their courage in giving up a cosy 

 home results in one Cow- bird the less. Sometimes even after there is 

 an egg of their own in the nest, they have nerve enough to let it go, 

 rather than assume the hateful task of incubating the strange one. But 

 if the female has already laid an egg or two, the pair generally settle 

 ihto the reluctant conviction that there is no help for it; they quiet 

 down alter awhile, and things go on as if nothing had happened. Not 

 always, however, will they desert even an empty nest; some birds have 

 discovered a way out of the diiiicultj- — it is the most ingenious device 

 imaginable, and the more Me think about it the more astonishing it 

 seems. They build a two-story nest, leaving the obnoxious egg in the 

 basement. 1 want no better i)roof that birds possess a faculty indistin- 

 guishable, so far as it goes, from human reason ; and such a case as this 

 bears imi)ressively upon the general (piestion of the diU'erence between 

 reason and that faculty we designate by the vague aiul misleading term 

 "instinct.'' The evidence has accumulated till it has become conclnsive, 

 that the dilference is one of degree, not of kind — that instinct is a lower 

 order of reason — the arrest, in brutes, at a certain stage, of a faculty 

 refljching higher development in man. Instinct, in the ill-considered 

 current sense of term, could never lead a Sununer Vdlowbird up to 

 building a two-story nest to let a Cow-bird's eggs addle below. Such 

 "instinct" is merely force of habit, inherited or acquired — a sum of ten- 

 dencies operating unknowingly and uniformly upon the same recurring 



