OCR Text |
Show 154 MR. W. H. HUDSON ON THE [Mar 3, Nests of Mil-vulus violentus. 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th Eggs in each. 9 3 2 7 5 3 4 6 4 4 47 Of Milvulus violentus. 1 3 0 2 1 3 0 1 0 1 12 Of Molothrus bonariensis. 8 0 2 5 4 0 4 5 4 3 35 Thus of forty-seven eggs found in ten nests thirty-five were parasitical ! 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 discovered that these eggs had disappeared, others newly laid being found in their places. The large number of Scissor-tails' nests containing 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 |