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U.S. NATIONAL MUSEUM BULLETIN 223 



Figure 2. — Cross section of reflection globule of Erythrura psittacea (after Sarasin, 1913): 

 1, Epidermis thinned out, with a pelucid layer; 2, ring of densely massed pigment cells; 

 3, pigment cells in cellular "padding"; 4, densely massed light-refractive fibers; 5, inner 

 cellular "padding" containing stellate pigment cells (three); 6, transverse section of 

 mandible. 



foster nest mate, even in cases where the two kinds of young grow 

 up together. The one outstanding exception is the case of the 

 screaming cowbird, Molothrus rufoaxillaris,^^ which is parasitic on 

 the bay-winged cowbird, Molothrus badius.** It is almost impossible 

 to distinguish the young of these species until the advent of the post- 

 juvenal molt. Here we are dealing with two extremely closely related 

 species, and it seems quite obvious that the close resemblance is 

 merely a reflection of a degree of relationship, which happens to 

 work out well in a host-parasite situation (Friedmann, 1929, p. 53). 

 The phyletic proximity is much greater in these two cowbirds than 

 in the viduine-cstrildine picture, but the resulting nestling-plumage 

 condition is comparable in the two. 



" Molothrus rufoaxillaris Casstn, Proc. Acad. Nat. Sci. Philadelphia, 1806, p. 23 (Buenos Aires, Argentina). 

 " Agelaius badius Bieillot, Nouveau dictlonnaire d'histoire naturelle, 1819, p. 535 (Paraguay and La 

 Plata River). 



