ARGENTINE COW-BIRD 79 



bird begins laying, this parasitical egg with a living 

 chick in it must have been deeply buried in the nest 

 for not less than five weeks. Probably after the young 

 Tyrant-birds came out of their shells and began to 

 grow, the little heat from their bodies, penetrating 

 to the buried egg, served to bring the embryo in it 

 to maturity ; but when I saw it I felt (like a person 

 who sees a ghost) strongly inclined to doubt the 

 evidence of my own senses. 



3. The comparatively short time the embryo takes 

 to hatch gives it another and a great advantage ; for 

 whereas the eggs of other small birds require from 

 fourteen to sixteen days to mature, that of the Cow- 

 bird hatches in eleven days and a half from the 

 moment incubation commences ; so that when the 

 female Cow-bird makes so great a mistake as to drop 

 an egg with others that have already been sat on, 

 unless incubation be far advanced, it still has a 

 chance of being hatched before or contemporaneously 

 with the others ; and even if the others hatch first, 

 the extreme hardiness of the embryo serves to keep 

 it alive with the modicum of heat it receives. 



4. Whenever the Molothrus is hatched together 

 with the young of its foster-parents, if these are 

 smaller than the parasite, as usually is the case, soon 

 after exclusion from the shell they disappear, and 

 the young Cow-bird remains sole occupant of the 

 nest. How it succeeds in expelling or destroying 

 them, if it indeed does destroy them, I have not 

 been able to discover. 



5. To all these circumstances favourable to the 



