Alejandro G. Di Giacomo

Chapter 3 – Host Use by Generalist and Specialist Brood-Parasitic Cowbirds at Population and Individual Levels

Abstract We analyzed host use at population and individual levels in the host-generalist shiny cowbird ( Molothrus bonariensis ) and in the host-specialist screaming cowbird ( M. rufoaxillaris ). At the population level, shiny cowbirds were less generalists than previously assumed and host use varied considerably among regions. There was, however, no clear preference for host species, since we found no associations between frequency of parasitism and hosts body mass, type of nest, or genetic distance between host and parasite. Regarding screaming cowbirds, frequency and intensity of parasitism varied among host populations. Parasitic eggs and chicks were equally successful in the primary and secondary hosts, but they experienced high mortality rates when transferred to other suitable, but unused hosts. Screaming and shiny cowbirds overlapped little in host use and had no apparent effect on each others success when reared together in a common host. At the individual level, there was an association between mtDNA haplotypes of shiny and screaming cowbird chicks and the hosts in whose nests they were found, indicating that nest choice by parasitic females is not random. We discuss how host imprinting by females can lead to the formation of host-specific races and how the occurrence of recognition errors and social learning can lead to the colonization of new hosts.

Parental Care in Tawny-bellied (Sporophila hypoxantha) and Rusty-collared (S. collaris) Seedeaters

Abstract The genus Sporophila (Emberizidae) comprises species of small finches characterized by marked sexual dichromatism, which in birds is positively associated with extent of female bias in parental care. We analyzed differences in parental care in Tawny-bellied (S. hypoxantha) and Rusty-collared (S. collaris) seedeaters. We video-recorded nest activity during incubation and when young were 2–4 and 7–9 days of age. Females of both species built the nest and incubated the eggs alone. Female Tawny-bellied Seedeaters: (1) incubated 59% of the time, (2) had a higher frequency of nest visits than males when chicks were 2–4 days of age, and (3) their visits were longer because after feeding they remained in the nest brooding the chicks. There were no gender differences in frequency of nest visits when chicks were 7–9 days of age, but visits of females were longer than those of males. Female Rusty-collared Seedeaters: (1) incubated 51% of the time and (2) had a higher frequency of nest visits when chicks were 7–9 days of age. Both males and females brooded chicks and there were no gender differences in frequency and length of nest visits when chicks were 2–4 days of age. Parental care in both species is female biased, but the extent of male care is slightly higher in Rusty-collared than in Tawny-bellied seedeaters.

Effects of grassland burning on reproductive success of globally threatened Strange-tailed Tyrants Alectrurus risora

Summary The north-east of Argentina contains a high number of globally threatened grassland birds and is increasingly managed for livestock, with annual burning in remaining natural grasslands. The Strange-tailed Tyrant Alectrurus risora, a globally threatened grassland specialist, has suffered a 90% contraction in its original range. From 1996 to 2008 we monitored a breeding population in Formosa, north-east Argentina, and explored the effect on breeding of four accidental and one prescribed fires. The plant most frequently used for nesting was Imperata brasiliensis, but the frequency of use was lower after a fire than before. In years with a fire, the height of the plant used for nesting and the height of the nest were lower than in those without a fire. Females avoided nesting in the burned plot in the breeding season immediately after the prescribed fire, but they started to return to the burned plot by the second breeding season and did not discriminate between plots by the third breeding season after the fire. Movements of females after the prescribed fire did not affect nest survival, clutch size, hatchability or chick survival indicating this species was adapted to regular fires. However, our results also show that annual burning in the grassland negatively affects the settlement for reproduction of Strange-tailed Tyrants and that the intervals between burns in the same grassland should be longer than two years.

Utilization of a new host in the screaming cowbird Molothrus rufoaxillaris, a host specialist brood parasite: host switch or host acquisition?

The screaming cowbird Molothrus rufoaxillaris has been long known as a host specialist brood parasite. However, in the past years, the utilization of two new hosts has been documented. We examined the variation in mitochondrial control region sequences from screaming cowbird chicks found in the nests of two hosts, the bay-winged cowbird (Agelaioides badius), which is its regular host, and the chopi blackbird (Gnorimopsar chopi), which is a new host, in Formosa Province, Argentina. If a group of females switched to this new host, we expected to find an association between host use and haplotype frequency distribution, indicating the presence of host-specific female lineages, whereas we expected no such association if the cowbird population incorporated this new host and females use both hosts simultaneously. Haplotype frequency distributions differed between cowbird chicks from the nests of both hosts. This indicates that nest choice by females of this brood parasite is not random and that they preferentially parasitize the nests of the same host species.

Breeding biology and conservation of the Marsh Seedeater Sporophila palustris

Summary The Marsh Seedeater Sporophila palustris is one of the most endangered and least known of the capuchino seedeaters. Breeding populations are patchy, occurring in north-east Argentina, southern Brazil, and Uruguay. We present data on the breeding biology of the species, and describe nests and eggs, behaviour (including courtship, nesting, incubation and parental care), breeding sites and food items. Marsh Seedeaters breed in well-preserved grasslands with wet soils and tall vegetation. In general, these habitats are used for extensive livestock ranching. Main threats are overgrazing, widespread use of fire, conversion of grasslands to pastures of exotic grasses and rice fields, afforestation, and illegal trapping. Most breeding sites are located in Important Bird Areas, but only one in Brazil and one in Argentina are protected. We propose a series of actions to promote the conservation of Marsh Seedeaters and other endangered birds that coexist in grassland habitats. Resumen El Capuchino Pecho Blanco Sporophila palustris es uno de los capuchinos menos conocidos y uno de los mas amenazados. Sus poblaciones reproductivas son locales, fragmentadas y ocurren en el nordeste de Argentina, sudeste de Brasil y en Uruguay. Presentamos datos reproductivos describiendo nidos y huevos, comportamiento (incluyendo cortejo, nidificacion, incubacion y cuidado parental), sitios de anidacion y datos sobre alimentacion. El Capuchino Pecho Blanco nidifica en pastizales altos y humedos en buen estado de conservacion. Estos habitats en general se han convertido en areas ganaderas. Las principales amenazas son sobrepastoreo, uso generalizado del fuego, drenaje de humedales, reemplazo total del habitat por pasturas y forestaciones y captura ilegal. La mayoria de los sitios se localizan en AICAs, pero solo uno en Brasil y otro en Argentina se encuentran protegidos. Finalmente, proponemos una serie de medidas que contribuyen a la preservacion del Capuchino Pecho Blanco y de otras aves amenazadas que conviven con este.

Screaming Cowbird Parasitism of Nests of Solitary Caciques and Cattle Tyrants

Abstract The Screaming Cowbird (Molothrus rufoaxillaris) is one of the most specialized brood parasites with only three known hosts: Baywing (Agelaioides badius), the main host throughout most of its range, and two alternative hosts in some areas of its distribution, Chopi Blackbird (Gnorimopsar chopi) and Brown-and-yellow Marshbird (Pseudoleistes virescens). We studied Screaming Cowbird parasitism in northeast Argentina where this parasite uses Baywings and Chopi Blackbirds as hosts. We monitored 69 nests of Baywings, 251 of Chopi Blackbirds, 31 of Solitary Caciques (Cacicus solitarius), and 30 of Cattle Tyrants (Machetornis rixosa). The frequency of Screaming Cowbird parasitism on Baywing nests was 80% and was 46% for Chopi Blackbirds. We recorded one event of Screaming Cowbird parasitism on one nest of Solitary Caciques and three events of Screaming Cowbird parasitism on one nest of Cattle Tyrants. The identities of parasitic eggs in both hosts were confirmed by sequencing the mtDNA control region. We propose these events of parasitism resulted from recognition errors by Screaming Cowbird females that regularly parasitize Baywings and Chopi Blackbirds. The nest of Solitary Caciques had been frequently visited by a pair of Baywings before Screaming Cowbird parasitism occurred, and the nest of Cattle Tyrants was near an active Chopi Blackbird nest that had been previously parasitized by Screaming Cowbirds.

Reproductive success of the specialist brood parasite Screaming Cowbird in an alternative host, the Chopi Blackbird

ABSTRACT The Screaming Cowbird (Molothrus rufoaxillaris) is the most specialized brood-parasitic cowbird, relying almost entirely on the Bay-winged Cowbird (Agelaioides badius) as host. Recently, Screaming Cowbirds have expanded their range to areas where Bay-winged Cowbirds are absent, and they are exploiting the Chopi Blackbird (Gnorimopsar chopi). Interactions between Screaming Cowbirds and Chopi Blackbirds are largely unexplored, as is the reproductive success of the parasite in this host. Screaming Cowbirds, Chopi Blackbirds, and Bay-winged Cowbirds coexist in northeastern Argentina, providing an ideal system to explore interactions between a specialist brood parasite and an alternative host and to compare the reproductive success of the parasite in its main host and in an alternative host. Screaming Cowbirds parasitized both hosts throughout their breeding seasons (Chopi Blackbirds, mid-October to mid-January; Bay-winged Cowbirds, mid-November to mid-March). Frequency of parasitism was lower in Chopi Blackbirds than in Bay-winged Cowbirds (46% vs. 74%). Nest survival was higher in Chopi Blackbirds than in Bay-winged Cowbirds (37% vs. 15%). In successful nests, survival of Screaming Cowbird eggs and chicks was high and relatively similar in both hosts (Chopi Blackbirds: eggs, 99%; chicks, 90%; Bay-winged Cowbirds: eggs, 93%; chicks, 93%), but hatchability was lower in Chopi Blackbirds than in Bay-winged Cowbirds (52% vs. 92%). Considering (1) nest survival and (2) egg survival, hatchability, and chick survival in successful nests, the reproductive success of Screaming Cowbirds (i.e. proportion of eggs that resulted in fledglings) was 0.17 in Chopi Blackbirds and 0.12 in Bay-winged Cowbirds. Our results indicate that the Chopi Blackbird is a frequent host of the Screaming Cowbird, and parasitism of this alternative host may help explain the range expansion of this parasite in areas of Brazil where the Bay-winged Cowbird is absent.

COOPERATIVE BREEDING OF THE YELLOW-RUMPED MARSHBIRD (ICTERIDAE) IN ARGENTINA AND PARAGUAY

Abstract We found helpers at eight of nine nests of Yellow-rumped Marshbirds (Pseudoleistes guirahuro) from northeastern Argentina and Paraguay. From one to four helpers were observed during the incubation and nestling periods, but during nest building we saw solitary pairs. The helpers, of unknown sex and age, provisioned food to the chicks and defended the nest against several raptors. The Yellow-rumped Marshbird had a similar breeding system to its congener the Brown-and-yellow Marshbird (P. virescens) except that the former did not nest in colonies. According to a recent molecular phylogeny both Pseudoleistes marshbirds plus the Saffron-cowled Blackbird (Xanthopsar flavus) form a well-defined clade. All three of these species have helpers. This result suggests that cooperative breeding in the Icteridae has a phylogenetic component. Cría Cooperativa de Pseudoleistes guirahuro en Argentina y Paraguay Resumen. Encontramos ayudantes en ocho de nueve nidos de Pseudoleistes guirahuro presentes encontrados en el noreste de Argentina y en Paraguay. Los ayudantes, de uno a cuatro, fueron observados en nidos durante los períodos de incubación y con pichones, pero durante la construcción del nido vimos parejas solitarias. Los ayudantes, de sexo y edad desconocidos, aportaron alimento a los pichones y defendieron el nido contra diversas aves rapaces. Pseudoleistes guirahuro tiene un sistema de cría social similar al de su congénere P. virescens, aunque no nidificó en colonias. Según la reciente filogenia molecular de la familia los dos Pseudoleistes más Xanthopsar flavus forman un clado definido, y estas tres especies tienen ayudantes. Este resultado sugiere que la presencia del sistema de cría cooperativa tiene un componente filogenético entre los ictéridos.

No evidence of genetic variation in microsatellite and mitochondrial DNA markers among remaining populations of the Strange-tailed Tyrant Alectrurus risora , an endangered grassland species

The Strange-tailed Tyrant Alectrurus risora (Aves: Tyrannidae) is an endemic species of southern South American grasslands that suffered a 90% reduction of its original distribution due to habitat transformation. This has led the species to be classified as globally Vulnerable. By the beginning of the last century, populations were partially migratory and moved south during the breeding season. Currently, the main breeding population inhabits the Ibera wetlands in the province of Corrientes, north-east Argentina, where it is resident all year round. There are two remaining small populations in the province of Formosa, north-east Argentina, and in southern Paraguay, which are separated from the main population by the Parana-Paraguay River and its continuous riverine forest habitat. The populations of Corrientes and Formosa are separated by 300 km and the grasslands between populations are non-continuous due to habitat transformation. We used mtDNA sequences and eight microsatellite loci to test if there were evidences of genetic isolation between Argentinean populations. We found no evidence of genetic structure between populations (Phi(ST) = 0.004, P = 0.32; Fst = 0.01, P = 0.06), which can be explained by either retained ancestral polymorphism or by dispersal between populations. We found no evidence for a recent demographic bottleneck in nuclear loci. Our results indicate that these populations could be managed as a single conservation unit on a regional scale. Conservation actions should be focused on preserving the remaining network of areas with natural grasslands to guarantee reproduction, dispersal and prevent further decline of populations.