The population size was monitored during the peak song period (March), except for 1999 and 2000 when it was monitored in February and May, and in April, respectively.

Cowbird parasitism on Atlapetes pallidiceps was first noted by Krabbe and coworkers in 1999 (Krabbe 2004), but it was not until the study by Oppel and coworkers in 2002 (Oppel et al. 2004b) and the continuing studies of the breeding biology of Atlapetes pallidiceps by Mery Juiña, that the extremely high rate of parasitism was documented.

As a consequence, cowbird control by shooting commenced in 2003, and has been undertaken highly successfully by Aldo Fernando Sornoza during all following breeding seasons, with the result that parasitism has been reduced to only one or two nests anually.

The natural history of the Shiny Cowbird (Molothrus bonariensis) appears to be much like that of the better-studied Brown-headed Cowbird (Molothrus ater) of North America. Females of Brown-headed Cowbirds are known to be able to lay up to 80 eggs in a breeding season. Therefore, female cowbirds are shot preferentially at Yunguilla, where the natural sex ratio is 4-5 males per female.

The Shiny Cowbird has been reported to parasitise more than 100 different species of birds, virtually any open-nest passerine species within its range.

All cowbirds collected at Yunguilla are weighed, sexed, investigated for stomach contents, and carefully prepared as study skins. See attached file below (cowbird data.xls) for details. Two morphs of females occur, one (the commonest) beginning with streaked plumage and terminating with uniform dull black plumage, one (less common) with unstreaked pale underparts.

96 cowbird stomachs from Yunguilla contained corn (65%), ticks (Ixodes) (34%), insects (18%), seeds (14%) and blackberries (Rubus sp.) (3%).



Photos by Aldo Fernando Sornoza


Photo by Niels Krabbe


Photo by Aldo Fernando Sornoza