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, postur- 

 ing 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 scat- 

 tering 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, cinna- 

 mon 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 



