76 ICTEIIIDJE. 



eggs and young of the species upon which it is parasitical. Some of 

 these advantages are due to those very habits of the parent bird which 

 at first sight appear most defective ; others to the character of the 

 egg and embryo, time of evolution, &c. 



1. The egg of the Cow-bird is usually larger, and almost invariably 

 harder- shelled than are the eggs it is placed with ; those of the Yellow- 

 breast (Pseudoleistes virescens] being the one exception I am acquainted 

 with. The harder shell of its own egg, considered in relation to the 

 destructive egg-breaking habit of the bird, gives it the best chance of 

 being preserved ; for though the Cow-bird never distinguishes its own 

 eggs, of which indeed it destroys a great many, a larger proportion 

 escape in a nest where many eggs are indiscriminately broken. 



2. The vitality or tenacity of life appears greater in the embryo 

 Cow-bird than in other species ; this circumstance also, in relation to 

 the egg- breaking habit and to the habit of laying many eggs in a 

 nest, gives it a further advantage. I have examined nests of the 

 Scissor-tail, containing many eggs, after incubation had begun, and 

 have been surprised at finding those of the Scissor-tail addled, even 

 when placed most advantageously in the nest for receiving heat from 

 the parent bird, while those of the Cow -bird 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 from their own, however different in size and colour 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 completion, is not infrequently abandoned on 

 account of these premature eggs. Some species, however, do not for- 

 sake 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 labour 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 

 Sisopygis icterophrys 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 upwards above the rim of the original nest; so 

 that this supplementary nest is like one saucer placed within another, 



