108 BIRDS OF LS PLATA 
and I looked forward to the next summer to work 
out the rich mine on which I had stumbled by chance. 
Unhappily when spring came round again ill-health 
kept me a prisoner in the city, and finding no im- 
provement in my condition, I eventually left Buenos 
Ayres at the close of the warm season to try whether 
change of climate would benefit me. Before leaving, 
however, I spent a few days at home, and saw enough 
then to satisfy me that my conclusions were correct. 
Most of the birds had finished breeding, but while 
examining some nests of Anumbius I found one 
which Bay-wings had tenanted, and which for some 
reason they had forsaken, leaving ten unincubated 
eggs, They were all like Bay-wings’ eggs, but I have 
no doubt that five of them were eggs of M. rufo- 
axillaris. During my rides in the neighbourhood I 
also found two flocks of Bay-wings, each composed 
of several families, and amongst the young birds I 
noticed several individuals beginning to assume the 
purple plumage, like those of the previous autumn. 
I did not think it necessary to shoot more specimens. 
The question why M. badius permits M. rufo- 
axillaris to use its nest, while excluding the allied 
parasite M. bonariensis, must be answered by future 
observers; but before passing from this very inter- 
esting group (Molothrus) I wish to make some general 
remarks on their habits and their anomalous relations 
to other species. 
It is with a considerable degree of repugnance 
that we regard the parasitical instinct in birds; the 
reason it excites such a feeling is manifestly that it 
