ARGENTINE COW-BIRD 93 



have evidence that this semi-parasitical habit does 

 tend to eradicate the nest-making one* The Synal- 

 laxes build great elaborate domed nests, yet we have 

 one species (S. aegithaloides) that never builds for 

 itself, but breeds in the nests of other birds of the 

 same genus* In some species the nesting-habit is in 

 a transitional state. The Tyrant-bird, Machetornis 

 rixosa, sometimes makes an elaborate nest in the angle 

 formed by twigs and the bough of a tree, but prefers, 

 and almost invariably makes choice of, the covered 

 nest of some other species or of a hole in the tree. It 

 is precisely the same with our Wren, Troglodytes 

 furvus. The Yellow House-Sparrow (Sycalis pelzelni) 

 invariably breeds in a dark hole or covered nest. The 

 fact that these three species lay coloured eggs, and 

 the first and last very deeply coloured, inclines one to 

 believe that they once invariably built exposed nests, 

 as M. rixosa still occasionally does. It may be added 

 that those species that lay coloured eggs in dark 

 places construct and line their nests far more neatly 

 than do the species that breed in such places but lay 

 white eggs. As with M. rixosa and the Wren, so it is 

 with the Bay-winged Molothrus ; it lays mottled eggs, 

 and occasionally builds a neat exposed nest ; yet so 

 great is the partiality it has acquired for large domed 

 nests that whenever it can possess itself of one by 

 dint of fighting it will not build one for itself. Let 

 us suppose that the Cow-bird also once acquired the 

 habit of breeding in domed nests, and that through 

 this habit its original nest-making instinct was com- 

 pletely eradicated, it is not difficult to imagine how 



