606 REPORT OF NATIONAL MUSEUM, 1893. 



most advantageously in the nest for receiving heat from the parent bird, 

 while those of the Cowbird contained living embryos, even when 

 under all the other eggs, and as frequently happens, glued immovably 

 to the nest by the matter from broken eggs spilt over them. 



The following instance of extraordinary vitality in an embryo Molo- 

 thrus seems to show incidentally that in some species protective habits, 

 which will act as a check on the parasitical instinct, may be in the 

 course of formation. 



Though birds do not, as a rule, seem able to distinguish parasitical 

 eggs fnmi their own, however different in size and color they may be, 

 they often do seem to know that eggs dropped in their nest before they 

 themselves have began to lay ought not to be there? and the nest, even 

 after its compljetion, is not infrequently abandoned on account of these 

 premature eggs. Some species, however, do not forsake their nests; 

 and though they do not throw the parasitical eggs out, which would 

 seem the simplest plan, they have discovered how to get rid of them 

 and so save themselves the labor of making a fresh nest. Their method 

 is to add a new deep lining, under which the strange eggs are buried 

 out of sight and give no more trouble. The 8iso2)ygis ieterophrys, a 

 common Tyrant Bird in Buenos Ayres, frequently has recourse to this 

 expedient, and the nest it makes being rather shallow the layer of 

 fresh material, under which the strange eggs are buried, is built 

 upward above the rim of the original nest, so that this supplementary 

 nest is like one saucer placed within another, and the observer is gen- 

 erally 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 2 half-fledged young birds besides 3 

 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 Avere quite rotten, but the third con- 

 tained a living embryo, ready to be hatched, and very lively and hungry 

 when 1 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 sliells and 

 began to grow, the little heat from their bodies jDenetrating 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 Cowbird makes so great a mistake 



