SHORT COMMUNICATIONS 
417 
in a tree) to deliver food. The blackish plumaged 
cowbird flew at least 100 m southward at a 
considerable height on 16 December and was no 
longer observed. Only the brown plumaged 
cowbird was then detected and apparently aban¬ 
doned the territory of the hosts by 21 December, 
when it flew to the south. No cowbirds were 
detected following the Orange-breasted Thorn- 
birds in subsequent days. 
DISCUSSION 
A pair of Orange-breasted Thombirds. suppos¬ 
edly the same reported here, has been monitored 
in this area lor several years, its territory covering 
a narrow band of —140 m along the wooded 
marsh edge (GNM. unpubl. data). However, while 
provisioning the pair of cowbird fledglings the 
fumariids extended their territory —100 m in a 
perpendicular line from the main axis, crossing a 
relatively busy road. The Orange-breasted Thorn- 
bird is fairly common in the study area, but the 
above example of parasitism was the only one 
involving the Shiny Cowbird observed during 
>15 years of continuous ornithological studies. 
Another marsh fumariid. the Yellow-chinned 
Spinetail (Certhiaxis cimumometis), has been 
observed feeding young cowbirds in the same 
area but, in the only reported case, the young 
parasite died before fledging (Dias and Mauncib 
1997), The bird species most frequently observed 
leeding Shiny Cowbird young at the site were 
Rufous Hornero (Furnarius rufus) and House 
Wren (Troglodytes aedon), which apparently 
constitute usual hosts (GNM. unpubl. data). 
It is known that adults other than the hosts may 
occasionally feed out-of-nest cowbird fledglings 
and the observation of a certain species feeding 
cowbird fledglings may not actually designate an 
effective host (Lowther 2010). However, it is 
reasonable to assume that the pair of Orange- 
breasted Thombirds. which were the only indi¬ 
viduals observed feeding the young cowbirds 
during 20 days, represented the true host, i.e., the 
birds which received the cowbird eggs and 
incubated them. The observations reported here 
show that thombirds are capable of rearing 
cowbird fledglings until independence. 
It is important in the study of brood parasitism 
to document species capable of successfully 
rearing young cowbirds (Lowther 2010). Thus, 
documentation of fledged young until indepen¬ 
dence. where the only evidence of parasitism of a 
certain species is from cowbird eggs or nestlings, 
is crucial. Other Phacellodomus species are not 
known to have reared cowbird young (Lowther 
2010). and the information that the Orange- 
breasted Thombird is an effective host is impor¬ 
tant as it suggests that further studies may 
document congenerics as effective hosts. It is 
also important to note that among the 29 fumariid 
species previously known as victims of the Shiny 
Cowbird. only five are known as effective hosts 
(Lowther 2010). 
ACKNOWLEDGMENTS 
I am grateful to R. M. Fraga and R. A. Dias for comments 
and corrections to the manuscript. This report was also 
improved from reviews by Peter Lowther and Nacho Areta. 
LITERATURE CITED 
Dias, R. A. and G. N. MAURfctO. 1997. Certhiaxis 
cinnamomea como hospcdciro de Molothriis honar- 
iensis no sul do Brasil. Atualidades Ornitologicas 79:9. 
Fraga, R. M. In press. Family lcteridae. Handbook of the birds 
of the world. Volume 16. (J. del Hoyo. A. F.lliott, and J. 
Sargatal, Editors). Lynx Edicions, Barcelona, Spain. 
Friedmann, H. 1929, The cowbirds: a study in the biology 
of social parasitism. C. Thomas, Springfield, Illinois. 
USA. 
Lowther, P. E. 2010. Lists of victims and hosts of the 
parasitic cowbirds ( Molaihrus ), Version 22 September 
2010. The Field Museum. Chicago, Illinois, USA. 
http://fm 1 .fieldmuseum.org/aa/Files/lowther/CBList. 
pdf 
Lowther, P. and W. Post. 1999. Shiny Cowbird 
(Molothrus bonurieiisis ). The birds of North America. 
Number 399. 
Simon. J. E.. J. F. Pacheco. B. M. Whitney, G. T. de 
Mattos, and R. L. Gaguardi. 2008. Phacellodomus 
ferrugineigula (Pelzeln. 1858) (Avcs: Fumariidae) € 
uma cspecie vdlida. Revista Brasileira de Ornitologia 
16:107-124. 
