28 BIRDS OF LA PLATA 
themselves into words ; but unlike the other Swallows 
the Martin has no set song. 
This Martin is never seen to alight on the ground 
or on the roofs of houses, but solely on trees; and 
when engaged in collecting materials for its nest, it 
sweeps down and snatches up a feather or straw 
without touching the surface. It breeds only in the 
clay ovens of the Oven-bird (Furnarius rufus). I at 
least have never seen them breed in any other situa- 
tion after observing them for many summers. An 
extraordinary habit, for, many as are the species 
that possess the semi-parasitical custom of breeding 
in other birds’ nests, they do not confine them- 
selves to the nest of a single species excepting the 
bird I am describing. It must, however, be under- 
stood that my knowledge of this bird has been 
acquired in Buenos Ayres, where I have observed 
it; and as this Martin possesses a wider range in 
South America than the Oven-birds, there is no 
doubt that in other districts it builds in different 
situations. 
On arriving in spring each pair takes up its position 
on some tree, and usually on a particular branch ; 
a dead branch extending beyond the foliage is a 
favourite perch. Here they spend much of their 
time, never appearing to remain long absent from it, 
and often, when singing their notes together, fluttering 
about it with a tremulous, uncertain flight, like that 
of a hovering butterfly. About three weeks after 
first arriving they begin to make advances towards 
the Oven-bird’s nest that stands on the nearest post 
