Abstract:

These observations were made in Estancia Curupicay, a cattle ranch, some 12 miles from Mercedes, and situated nearly in the centre of the province of Corrientes. The ranch consists of open pasture land, with few native trees except along the streams. It has many exotics such as casuarinas, Eucalyptus and paraísos planted round the administration buildings and in scattered groups to give shade to the cattle. The Shiny Cowbird (Molothrus bonariensis), is abundant in all the district. The Crowned Slaty-Flycatcher (Empidonomus aurantio-atro-cristatus ), was recorded once feeding a fledgling Shiny Cowbird (7 January 1975). The Hornero (Fumarius rufus ), is also here in numbers, nine nests being the aproximate number built in the gardens of the ranch each year. Horneros have been observed feeding young Cowbirds since 1973, in November of each year. Three pairs were seen with young parasites each spring, with two or with one, but never with any of their own brood and the young Cowbird at the same time. Great Kiskadees ( Pitangus sulphuratus ), have also been seen feeding young Cowbirds, so it would seems that covered neasts are no protection against brood-parasites. I have actually seen a female Cowbird go into an Horneros nest on two oceasions, and found a fallen Horneros nest with broken eggs of the owners and one Cowbird´s egg that was mottled and typical. The Rufos-collared Sparrow ( Zonotrichia capensis ), in common in the gardens, and nest in the privet hedges, but I have never seen them followed by young Cowbirds. The Bay winged Cowbirds come to my bird table, in company with young Screaming Cowbirds, but I have never seen a young Shiny Cowbird in their small flocks. So it would seen that brood-parasitism by the Shiny Cowbird on the Hornero is relatively common in the province of Corrientes, also on the Great Kiskadee, and that birds victimized vary according the zone. The species being different in the province of Buenos Aires to the ones that are preferred here.