154 



MR. W. H. HUDSON ON THE 



[Mar. $, 



Thus of forty-seven eggs found in ten nests thirty-five were para- 

 sitical ! 



6. The female M. bonariensis, and sometimes the male, destroy 

 many of the eggs in the nests they intrude into, by pecking holes in 

 the shells, breaking, devouring, or stealing them. This is the most 

 destructive habit of the bird, and is probably possessed by individuals 

 in different degrees ; for sometimes one nest appears exempt whilst 

 others are completely ruined by it. I have often carefully examined 

 all the parasitical eggs in a nest, and after three or four days dis- 

 covered that these eggs had disappeared, others newly laid being 

 found in their places. The large number of Scissor-tails' nests con- 

 taining no eggs of the Scissor-tail, even after incubation has began, 

 shows how many eggs must be removed or devoured ; for the M. 

 bonariensis destroys indiscriminately the eggs of its own species and 

 those of others. 



II. Advantages of this instinct. — After a perusal of the preceding 

 note one might ask, If there is so much that is defective and 

 irregular in the reproduction of the M. bonariensis, how can the 

 species maintain its existence, and even increase to such an amazing 

 extent ? for it is certainly more numerous, over an equal area, than 

 other parasitical species. For its being more abundant than other 

 species with analogous but apparently more perfect instincts, there 

 may be many reasons unknown to us. The rarer species may be less 

 hardy, have more enemies, be exposed to more perils in their long 

 migrations, &c. But for its being able to maintain its existence there 

 is a very obvious reason, viz. in the many circumstances giving its 

 egg and young an advantage over the eggs and young of the species 

 it is parasitical on. Some of these favourable circumstances are 

 derived from those very habits of the parent bird that at first sight 

 appear most defective ; others from the character of the egg and 

 embryo, time of evolution, &c. 



1 . The egg of the M. bonariensis is usually larger, and almost 

 invariably (the one exception I know being the eggs of the Yellow- 

 breast) much harder-shelled than are the eggs with which it is 

 placed. Now the greater hardness of the shell of its own eggs 

 considered in relation to the destructive egg-breaking and -stealing 



