1874.] MOLOTHRI OF BUENOS AYRES. 167 



from it when found, is a contingency for which no provision has 

 been made ; consequently the egg is wasted. In the Rheas all the 

 females in a flock lay in one nest, a male incubating the eggs after- 

 wards. But as each bird lays a large number of eggs, and as they 

 do not begin laying at the same time, long before they have all 

 done laying the male becomes broody and drives them away. This 

 is probably the cause of their wasting so many eggs ; for all the 

 females that are ready to lay when the male begins to incubate are 

 compelled to drop them upon the plain. But how can we account for 

 the habit of occasionally wasting eggs in another species — the Urraca 

 {Cyanocorax pileatus) "for example ; for this bird builds an elaborate 

 nest in which but one female lays ? In a paper on the habits of this 

 species (P. Z. S. 1870, p. 749) I said that they sometimes built such 

 frail nests that all the eggs dropped through them ; but I did not then 

 know that they also wasted a surprising number of eggs. A flock of 

 about sixteen of these birds passed the winter of 1 872 in thetrees about 

 my house ; on the approach of warm weather they began to scatter, 

 incessantly screaming and chattering as their manner is when about 

 to pair and breed. I observed these birds very attentively, but could 

 not detect them building. At last I found three broken eggs on the 

 ground, and on examining the tree overhead discovered an incipient 

 nest, merely a dozen or so of little sticks laid crossways, upon, or 

 rather through, which the eggs had been dropped. This was in 

 October ; and till January no other attempt at a nest was made ; but 

 eggs in abundance were wasted, for during four months I con- 

 stantly found them about the orchard. Early in January another 

 nest was made, but with less materials than a Cuckoo would have 

 employed ; five or six smashed eggs were on the ground beneath it. 

 Towards the end of January two large nests were built, deep and 

 well-lined with green leaves ; in these nests fourteen or fifteen birds 

 were hatched. In this case we see one essential link in the chain 

 of procreant instincts struck out and the whole made abortive ; 

 but the cause of the loss or suspension for a time of the architectural 

 habit seems very mysterious. 



Besides the Rhea, I know of no species in which two or several 

 females unite habitually to lay in one nest ; but there are many spe- 

 cies in which two or more females occasionally lay together. This 

 is often the case with Dendrocygnafulva, Yanellus cayennensis, and 

 Nothura maculosa. I can give but one case of two birds of different 

 species laying together : this was the Teal (Querquedula Jlavirostris) 

 and the common Partridge or Tinamou {Nothura maculosa). The 

 nest was in a thistle-bush at a distance from the water, and contained 

 the full complement of eggs of both birds. 



XII. Conjectures as to the origin of the parasitic instinct in M. bo- 

 nariensis. — The assertion that the " immediate and final cause of the 

 Cuckoo's instinct is that she lays her eggs not daily, but at intervals of 

 two or three days" (' Origin of Species '), carries no great appearance 

 of probability with it ; for might it not just as reasonably be said that 

 the parasitic instinct is the immediate and final cause of her laying 

 her eggs at long intervals ? If it is favourable to a species with the 



