ORIGINAL ARTICLE Reproductive success of wild Lesser Rheas (Pterocnemia -Rhea- pennata pennata) in north-western Patagonia, Argentina Fernando R. Barri Monica B. Martella Joaquı ´n L. Navarro Received: 6 December 2007 / Revised: 26 May 2008 / Accepted: 5 June 2008 / Published online: 25 June 2008 Ó Dt. Ornithologen-Gesellschaft e.V. 2008 Abstract We studied the reproductive success of a wild Lesser Rhea population (Pterocnemia -Rhea- pennata pennata) during two reproductive seasons (2004/2005 and 2005/2006) in north-western Patagonia, Argentina. The parameters recorded included population and nest density, clutch size, hatching success, chick survival (up to 3 months of age) and percentage of chicks that reached the juvenile stage after the winter. We also estimated the percentage of males that attempted to nest and of those that were successful (those producing at least one chick), daily nest mortality rates (DNMR) at different stages of the nesting cycle and the probability that an egg that has been recently laid will produce a chick. On average, both years pooled, the density of this population of Lesser Rheas was 1.55 ± 0.2 individuals/km 2 (SE), nest density was 0.17 ± 0.04 per km 2 , clutch size was 20.8 ± 6.4 eggs, hatching success was 74.4% ± 11.3, Mayfield’s proba- bility of an egg that will produce a chick was 0.64, chick survival was 65.4% ± 14.5 and percentage of chicks that reached the juvenile stage was 26.3%. Nearly a quarter of Lesser Rhea males in the population attempted to nest during a breeding season, and the DNMR was significantly higher during the laying stage (most nest failures were due to anthropogenic disturbances related to livestock raising activities). Nesting success, hatching success, and chick survival of Lesser Rheas were higher than those of their most closely related species, the Greater Rhea (Rhea americana), whereas the percentage of chicks that reached the juvenile stage was similar due to high winter mortalities of chicks. We suggest that the increase in reproductive effort is a strategy of this species to overcome environ- mental constraints. Keywords Conservation  Lesser Rhea  Reproductive success Introduction The Lesser Rhea (Pterocnemia -Rhea- pennata pennata) is a large, flightless, cursorial bird which inhabits the steppes of most of Argentinean Patagonia and southern Chile (Folch 1992). Its reproductive season extends from August to January. The mating system of rheas combines polygyny with sequential polyandry, where only males incubate the eggs and care for the chicks (Hanford and Mares 1985; Balmford 1992; Sarasqueta 2003). In species in which only one sex incubates, there is a trade-off between attendance and defence of the nest and maintenance of a favourable energy budget (Sutherland et al. 2004). In Greater Rheas, the energetic cost invested in reproduction and incubation reduces the number of suc- cessful nesting males to those males that are in better condition (Ferna ´ndez and Reboreda 2003). In an earlier report, Ferna ´ndez and Reboreda (2000) also suggested that in this species an early nest desertion has a better cost/ benefit relationship than late nest desertion because in the former the male’s probability of re-mating and re-nesting is higher. Lesser Rhea males compete for and defend a group of four to ten females and copulate periodically with each female of this group (Sarasqueta 1990; F.R. Barri, personal Communicated by P.H. Becker. F. R. Barri (&)  M. B. Martella  J. L. Navarro Centro de Zoologı ´a Aplicada, Universidad Nacional de Co ´rdoba, CC 122, Cordoba 5000, Argentina e-mail: fbarri@efn.uncor.edu 123 J Ornithol (2009) 150:127–132 DOI 10.1007/s10336-008-0327-5