(YELLOW-BREASTED MARSH-BIRD.)
+Pseudoleistes virescens+, _Hudson, P. Z. S._ 1870, p. 549, et 1874,
p. 156 (Buenos Ayres); _Scl. et Salv. Nomencl._ p. 37; _Durnford,
Ibis_, 1877, p. 175, et 1878, p. 59 (Buenos Ayres); _Gibson,
Ibis_, 1880, p. 31 (Buenos Ayres); _White, P. Z. S._ 1882, p. 602
(Buenos Ayres); _Barrows, Bull. Nutt. Orn. Cl._ viii. p. 135
(Entrerios); _Scl. Cat. B._ xi. p. 352. +Leistes anticus+, _Burm.
La-Plata Reise_, ii. p. 491 (Paraná).
_Description._--Above and below dark olive-brown; lesser upper
wing-coverts, under wing-coverts, and middle of the abdomen yellow;
bill black; feet dark brown: total length 9·5 inches, wing 4·6, tail
3·8. _Female_ similar.
_Hab._ S. Brazil, Paraguay, Uruguay, and Argentina.
In both sexes in this species the plumage is deep olivaceous brown,
the breast pure yellow. It is active, strong on the wing, sociable and
noisy; and being moreover a pretty and elegant bird, very common in
settled districts, and with a preference for man's neighbourhood, it
is familiar to every one, and has won amongst many competitors the
vernacular name of _Pecho-amarillo_ (Yellow-breast), for with us
yellow-breasted species are somewhat numerous. It remains all the year,
invariably going about in flocks of from twenty to thirty birds, and
feeds on the ground in the fields or on the open plain. While they are
feeding, one bird takes up a position on a stalk or thistle-top to keep
guard; when he flies down another bird takes his place; if a person
approaches, the sentinel gives the alarm, and all the birds fly off in a
very close flock, making the air resound with their loud ringing notes.
After feeding, they repair to the trees, where they join their robust
voices in a spirited concert, without any set form of melody such as
other song-birds possess, but all together, flinging out their notes at
random, as if mad with joy. In this delightful hubbub there are some
soft silvery sounds. Where they are never persecuted they have little
fear of man, but they invariably greet his approach with a loud vigorous
remonstrance.
In October the birds break up their companies to pair. Sometimes they
breed on the open plain in a large cardoon thistle, but a thick bush or
low tree is preferred. The nest is like that of a Thrush, being deep,
compactly made of dry grass and slender sticks, plastered inside with
mud, and lined with hair or soft dry grass. It is, however, deeper and
more symmetrical than the Thrush's nest, and it is sometimes plastered
with cow-dung instead of with mud. The eggs are four, very long, white,
and abundantly spotted with deep red, the spots becoming confluent at
the large end.
The Yellow-breast is never seen to quarrel with its fellows or with
other birds, and it is possibly due to its peaceful disposition that it
is more victimized by the parasitical _Molothrus_ than any other bird. I
have frequently found their nests full of parasitical eggs, as many as
fourteen and in one case sixteen, eggs in one nest. In some seasons all
the nests I found and watched were eventually abandoned by the birds on
account of the number of parasitical eggs dropped in them. I have also
so frequently found parasitical eggs on the ground under the nest that I
believe the Yellow-breast throws out some of these foreign eggs, and in
one instance I was quite sure that this had happened. The nest was in
a cardoon bush, and contained five eggs--two of the Yellow-breast
and three parasitical. These three were of the variety most thickly
mottled with red, and consequently closely resembling the eggs of the
Yellow-breast. I was surprised to find five more eggs of the Cow-bird
on the ground, close together, and about three feet from the bush; and
these five eggs were all pure white and unspotted. Naturally I asked,
How came these eggs in such a position? They had not fallen from the
nest, which was very deep, contained few eggs, and was scarcely thirty
inches above the ground. Then they were all white, while those in the
nest were mottled. That the eggs had been laid in the nest I felt
certain; and the only way I can account for their being in the place
where I found them is that the Yellow-breast itself removed them, taking
them up in its bill and flying with them to the ground. If I am right,
we must believe that this individual Yellow-breast had developed an
instinct unusual in the species, which enables it to distinguish, and
cast out of its nest, eggs very different from its own--an instinct, in
fact, the object of which would be to counteract the parasitical habit
of _Molothrus_. What would be the effect of such an instinct should the
species acquire it? Doubtless it would be highly prejudicial to the
parasitical birds laying white eggs, but favourable to those laying
mottled eggs. This would be natural selection operating in a very
unusual manner; for the Yellow-breast, or other species, would improve
another to its own detriment, since the more the parasitical eggs
assimilated to its own, the greater would be the likelihood of
their being preserved. The perfect similarity of the eggs of _M.
rufoaxillaris_ to those of _M. badius_ perhaps was brought about in this
way. But, it may be added, if besides the Yellow-breast some one other
species laying very different eggs (a _Zonotrichia_ or _Tyrannus_, for
instance) should also acquire this distinguishing habit, and eject all
eggs unlike its own from its nest, the habit in the two or more species
would ultimately cause the extinction of the parasite.
It might throw some light on this obscure subject to examine, for
several successive summers, a large number of nests, to ascertain
whether the nests of the Yellow-breast are often found without any
white unspotted eggs, or if the same proportional number of white
(parasitical) eggs are found in the nests of the Yellow-breast,
Scissor-tail, Song-Sparrow, Pipit, and other species.