Scarlet-headed Blackbird (Amblyramphus holosericeus): a new effective host for the specialized Screaming Cowbird (Molothrus rufoaxillaris) that is not a cooperative breeder.

DEPALMA, DANIELA M; ZILLI, C.; MERMOZ, MYRIAM E; CHARNELLI, E.M.

Chattanooga

Congreso; Association of Field Ornithologists and Wilson Ornithological Society Joint Meeting; 2018

Association of Field Ornithologists - Wilson Ornithological Society

Hudson (1920) described that Screaming Cowbird (hereafter ScC) was a brood parasite, supposing that they only parasitize the Greywish Blackbird (Agelaiodes badius GB) since their eggs, nestlings, and fledglings being almost indistinguishable. However, in the last twenty years it was found that two other effective hosts are regularly parasitized: Brown-and-yellow Marshbird (Pseudoleistes virescens BYM), and Chopi Blackbird (Gnorimopsar chopi). As the three host of ScC are cooperative breeders, it has been suggested that competition with the more aggressive and highly generalist Shiny Cowbird (M. bonariensis) preclude that other hosts that do not have helpers-at-the nests could rear ScC successfully. During November 2017, we studied nesting success of birds using roadside borders in Argentine Pampas. We found 123 nest belonging to 17 species. Fifty-seven nests were of BYM; 28 of them were parasitized by Shiny Cowbirds, and 4 by both Cowbirds. We also found 4 nests of Scarlet-headed Blackbird (SHB); one was parasitized with one egg of ScC that was successfully reared alongside with two of the host. Eleven nest were of Furnarids that are used as nest-sites by GB, but they do not start nesting until late December. Having helpers-at-the nests is a not condition that a host must fulfill to rear ScC successfully, as all SHB nests (parasitized or not) were attended by two adults. When alternative hosts are sympatric with GB, all of them are available for parasitism by ScC before the main host. Therefore, by start breeding earlier, those ScC individuals parasitizing them should increase their fitness.