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. exgithaloides) 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 pelzelnt) 
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 
