MOLOTHRUS BONARIENSIS. 77 



and the observer is generally able to tell from the thickness of the 

 whole structure whether any parasitical eggs have been entombed in 

 it or not. Finding a very thick nest one day., containing two half- 

 fledged young birds besides three addled eggs, I opened it, removing 

 the upper portion, or additional nest, intact, and discovered beneath it 

 three buried Molothrus eggs, their shells encrusted with dirt and glued 

 together with broken egg-matter spilt over them. In trying to get 

 them out without pulling the nest to pieces I broke them all ; two 

 were quite rotten, but the third contained a living embryo, ready 

 to be hatched, and very lively and hungry when I took it in my 

 hand. The young Tyrant-birds were about a fortnight old, and as they 

 hatch out only about twenty days after the parent-bird begins laying, 

 this parasitical egg with a living chick in it must have been deeply 

 buried in the nest for five or six 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 very far advanced, it still has a chance of being hatched 

 before or contemporaneously with the others ; but 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 learn. 



5. To all these circumstances favourable to the Molothrus may be 

 added another of equal or even greater importance. It is never engaged 

 with the dilatory and exhaustive process of rearing its own young ; and 

 for this reason continues in better condition than other species, and, 

 moreover, being gregarious and practising promiscuous sexual inter- 

 course, must lay a much greater number of eggs than other species. 

 In our domestic fowls we see that hens that never become broody lay a 

 great deal more than others. Some of our small birds rear two, others 



