Bulletin - United States National Museum - si-pddr - Smithsonian ...

BIRDS OF ARGENTINA, PARAGUAY, URUGUAY, AND CHILE 385
At this season when one male joined a little band of others it was
common for all to point the bill directly up and peer about, posturing
thus for several seconds. In display before the female, males
bent the head down, expanded the neck feathers, and spread the
wrings. One observed in this display in a low bush was so demon-
strative that he toppled off his perch backward at short intervals,
but regained an upright position instanth'. This performance was
varied by short circles on the wing about his mate's perch. Males
uttered a bubbling, gurgling song.
Eggs ascribed to this cowbird were found in five sets of eggs that
were collected. They exhibit much variation in size and coloration.
Two taken from the nest of Rhinocrypta lanceolata at General Roca,
Rio Negro, December 3, 1920, are dull white in color, finely, uni-
formlj'^, and somewhat sparsely dotted with vinaceous tawny; they
measure 21.6 by 16.9 and 21.6 by 17 mm. One secured with a set of
Furnarius rufus rufus is heavily spotted throughout with cinnamon
rufous and hazel, the spots confluent at the larger pole, with a scattering
of purplish spots due to concealment of part of the brown
beneath shell structure; this egg, which is much larger than the first
two, measures 24.1 by 20.1 mm. Another, found in a nest of Mimus
triurus at Victorica, Pampa, on December 26, 1920, is white with a
very faint bluish tinge, entirely unmarked. It measures 21.8 by 19
mm. Three eggs from a nest of Turdus rufiventi'is collected at
Lavalle, Buenos Aires, October 30, 1920, are variable. One is pure
white with a very few fine widely scattered dots of cinnamon rufous,
another is dull white evenly and sharply spotted with hazel, cinnamon
rufous, and chestnut brown, with a few spots of a purplish hue,
and the third has the ground color tinted distinctly with pinkish
brown with the entire surface spotted with suffused markings of
cinnamon rufous, hazel, and purplish. Measurements of these three
are as follows : 21.5 by 18.6, 23 by 17.6, and 22.6 by 17.8 mm. The
fifth and last parasitized nest found was that of a Diuca minor, taken
at Victorica, Pampa, December 26, 1920, which contained one egg
similar to the one collected from the nest of Furnm^ius, but less
heavily marked; it measures 23.4 by 18.9 mm.
It is my opinion that the well-known variation in color among eggs
of this cowbird is due to a mimicry similar to that so widely dis-
cussed and debated in the case of certain parasitic cuckoos of the
Old World. The greater number of the tracheophone species which
form so conspicuous an element among the smaller birds that breed
in the area frequented by this cowbird lay white, unmarked eggs.
As it is obviously of advantage for a parasitic egg to resemble that
of the foster parent, it may be supposed that certain gi-oups or indi-
viduals among the cowbirds that persistently parasitize these