Red Eyed Vireo

Vireo olivaceus (Linnaeus)

Of 27 nests from Saskatchewan, Manitoba, and Alberta reported to the Prairie Nest Cards files at the Manitoba Museum, 5 had cowbird eggs in them, a lower incidence of parasitism than obtains farther south and east. We may recall that in the Douglas Lake area of Michigan, Southern (1958) reported cowbird eggs in 72.1 percent of the nests of this vireo (75 parasitized out of 104 active nests studied). Other sizeable samples reported in the 1963 compilation (Friedmann, pp. 88, 89) showed 36.4 percent parasitism in Ohio and 41.3 percent in southern Quebec. The nest records files at Toronto show 65 cases of parasitism out of 240 nests in Ontario (27.1 percent).

Although this vireo often deserts its nest if a large number of cowbird eggs are laid or if all the vireo eggs are removed by cowbirds (Southern, 1958), there are several cases known in which vireos continued to incubate clutches consisting only of cowbird eggs. Incubation of 5 cowbird eggs continued at 1 of the nests studied by Southern (1958:195) after the 2 vireo eggs that were present were removed (presumably by cowbirds). Harrison (1975:172) cites a bird incubating at a Pennsylvania nest that contained 4 cowbird and no vireo eggs. One of us (L.F.K.) found that incubation of 3 cowbird eggs continued after he removed the only vireo egg from a nest in Cabell County, West Virginia. Another of us (S.I.R.) studied a comparable nest at the University of Michigan Biological Station, Pellston, Michigan. This last nest had a number of unusual features. Two cowbird eggs were laid on one morning and the 6 cowbird eggs the nest eventually held were laid over a span of at least 9 days. When discovered on 18 June 1963, the nest held I vireo egg and 2 cowbird eggs. The vireo egg disappeared on 25 or 26 June, but incubation of the 6 cowbird eggs then present continued until at least 8 July. The nest was probably abandoned on 9 July, and the cowbird eggs were removed (presumably by a predator) by 10 July. Thus, incubation continued for about 21 days, during the last 13 or 14 of which the nest held no host eggs. It is likely that the cowbird eggs failed to hatch because their combined mass was too great to allow successful incubation by a bird as small as a vireo (see Friedmann, 1963:21–22 for a discussion of the incubation capabilities of hosts).

PHILADELPHIA VIREO

Vireo olivaceus (Linnaeus)

The South American populations of the red-eyed vireo, formerly separated as Vireo chivi, have at least 3 subspecies that are known to be parasitized by the shiny cowbird, but apparently the species is not parasitized nearly so frequently in South America as is its North American nominate race by the brown-headed cowbird. The southernmost subspecies, V. olivaceus chivi, was known as a victim of M. bonariensis bonariensis in Argentina (Friedmann, 1929:110; 1938:43) and of M. bonariensis melanogyna in southeastern Brazil (Friedmann, 1934:344); the race V. olivaceus griseobarbatus is parasitized in southwestern Ecuador by M. bonariensis aequatorialis (Friedmann, 1963:209), and V. olivaceus vividior has been found to be parasitized by M. bonariensis minimus in Trinidad (Friedmann, 1949:156) and may be now stated to be similarly affected by M. bonariensis venezuelensis near Caracas, Venezuela, according to information received from Paul Schwartz.

RUFOUS-BROWED PEPPERSHRIKE