Great Pampa Finch

Embernagra platensis (Gmelin)

To the few records previously reported (Friedmann, 1929:117; 1931:58) of this finch as a cowbird victim in Argentina and Paraguay, may be added one more Argentine instance. Señor S. Narosky informs us that he found a parasitized nest at Tapalque, Buenos Aires, on 7 December 1971.

Bronzed Cowbird (Molothrus aeneus)

Discussion

Additional information on this cowbird is geographically “spotty,” as much as the range of the bird has few or no resident, local observers. The data here brought together are dependent on specimen records (of eggs) in collections, on the kind cooperation of a small number of collector-observers, and on the literature.

NEW HOSTS. —Recently one of us (Kiff, 1973) reported 3 new hosts for this cowbird in Costa Rica: Catharus gracilirostris, Amblycercus holosericeus, and Ramphocelus passerinii. We now add 7 more (6 species and 1 subspecies) from Arizona, Mexico, and Guatemala. Since all are new to the total host list of Molothrus aeneus, there is no need to mark them with an asterisk.

New hosts for Molothrus aeneus aeneus: Turdus grayi grayi, Zarhynchus wagleri wagleri, Icterus wagleri wagleri, Habia fuscicauda salvini, and Melozone biarcuatum biarcuatum.

New host for Molothrus aeneus lloyei: Setophaga picta.

New host for Molothrus aeneus assimilis: Piranga flava dextra.

These bring the known host catalog of the bronzed cowbird up to a total of 71 species, or 86 species and subspecies of birds. One report of a species previously recorded (Friedmann, 1971:252) as a victim, the brown-throated wren, Troglodytes brunneicollis, has since been found to have been erroneous, due to a confusion in the record, which really refers to Thryothorus sinaloa, a species otherwise reported as a victim of the bronzed cowbird.

In the 1963 compendium (Friedmann, p. 175) 15 species of hosts were noted as having been found to rear the young bronzed cowbirds. Only 5 others have since been observed to do so; they are: Cyanocorax yncas, Thryomanes bewickii, Mimus polyglottos, Piranga bidentata, and Ramphocelus passerinii.

DUMP NESTS AND MULTIPLE PARASITISM. —Before proceeding to the comments on individual species of hosts, it should be mentioned that there is a slowly growing mass of evidence suggesting that the bronzed cowbird occasionally makes use of what may be termed dump nests, in which excessive numbers of eggs of multiple hens are deposited. Since these nests are either old ones, already deserted by their builders, or since the unacceptably large numbers of parasitic eggs cause the hosts to desert, these eggs are wasted. A case in point, recently described to us by F. F. Nyc, Jr., is a dump nest he found at Brownsville, Texas—a small platform-like nest in a crotch about 7 feet up in a tree, containing 14 eggs of the bronzed cowbird and no eggs, or any other sign, of a host. It may be recalled that the South American shiny cowbird, Molothrus bonariensis, is even more addicted to the use of such dump nests, in which from 15, 17, 20, 25, 26, to, in one instance, an incredible maximum of 37 of its eggs have been reported, and as many as 13 female cowbirds have been found to lay their eggs in one of these nests (Friedmann, 1929:86–87).

In these instances the circumstantial evidence suggests that the eggs, dissimilar enough to suggest they were laid by 13 hens, were those of year-old hens still living in loose flocks late in the season and without individual breeding territories. It now appears that a similar situation may be present in the case of the bronzed cowbird. Aside from Nyc's record of 14 eggs of this species in a deserted nest, we may call atention to a record, given in our account of the green jay, below, also in southern Texas, of a nest containing 10 of the parasitic eggs together with 2 eggs of the host.

It is true there have been some records of excessive, multiple parasitism by the brown-headed cowbird as well, but the number of such cases is extremely small compared with the many thousands of records of more limited, “normal” degrees of parasitism. The number of these cases is so insignificant statistically that it is not possible to look upon them as evidence for the dump nest habit in that cowbird species—a total of only 21 nests with 5 or more (in one case up to 12) eggs of the brown-headed cowbird, against nearly a thousand times that many “normal” nests, each with 1 or 2 eggs of the parasite (Friedmann, 1963:12–13).

Another aspect of the matter of host selection by the bronzed and the brown-headed cowbirds that is becoming even more clearly discernable from the total mass of records is that there are significant differences between the two parasites that serve to divide the available host species in a way to lessen competition for victims in areas where the two cowbirds are sympatric and numerous. Vireos and wood warblers are reported as victims far more often for the brown-headed cowbird than for the bronzed species. This does not mean that the overlapping of the host choices of the two is insignificant; more than half of all the species known to be parasitized by M. aeneus are also molested by M. ater. For that matter, it is not at all uncommon to find individual nests containing eggs of both species of cowbirds. However, most of these hosts are used significantly more frequently by one than by the other of the two species of brood parasites. The trends are not as pronounced as they are in the sympatric species of parasitic Cuculus in Africa, for which the term “alloxenia” was proposed to describe the situation where related species of parasites affect thfferent species of hosts (Friedmann, 1967a; 1967b).

Data on Individual Host Species

GREEN JAY

The Pampa finch (Embernagra platensis), also known as the great Pampa-finch, is a species of bird. It was traditionally placed in the family Emberizidae but molecular phylogenetic studies have shown that it is a member of the tanager family Thraupidae.