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 
