Duplex Nest Construction by Hooded Oriole Circumvents Cowbird Parasitism

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On about 24 January 1970, Thomas Young collected an old nest (MLZ NE-574) of the Hooded Oriole (Icterus cucullatus) that he found suspended from a splintered beam of the overhanging eave of a house in Whittier, Los Angeles County, California. He brought this nest to me at the Moore Laboratory of Zoology. The nest is seemingly unique for this oriole in that it has two approximately equal-sized nest chambers and foundations interwoven side-by-side and suspended by a single group of strands attached to a splinter of the roof beam. The nest, as reported for the Hooded Oriole by many authors, is woven entirely from fibers of the Washington palm (Washingtonia filifera) and is sparsely lined with other plant fibers and down. The unusual nest site has been reported once before by Bent (U.S. Natl. Mus., Bull. 211:228, 1958). Usually the nests are suspended from palm fronds. One compartment of the present specimen contains an egg of the Brown-headed Cowbird (Molothrus ater obscurus) and otherwise shows no sign of use. The other compartment has a thicker lining and the outer rim flattened and thickly clotted with a mass of dried defecation, indicating that young were raised in it by the orioles. It is thought that the used second nest represents an unusually complete extension of the tendency shown by many birds to make a new nest lining when interfered with by an external agent, such as a parasitic bird's egg. Friedmann (U.S. Natl. Mus., Bull. 233:36, 1963) summarizes the few instances of reported parasitism of the Hooded Oriole by this cowbird. This oriole is not included in his list (ibid, p. 183) of 29 species of birds that have been known to avoid nest parasitism by burying the eggs of the cowbird beneath new nest linings. Among the 29 is the Baltimore Oriole (I. galbula). There are no previously known instances of the duplex construction as a means of circumventing parasitism. Dr. Friedmann (pers. comm.) knows of no other similar cases. I am grateful to Mr. Young for permission to publish this note.