Feeding strategy and predation of the Barn owl (Tyto alba) and the Burrowing owl (Speotyto cunicularia) on rodent species, sex, and size, in agrosystems of central Argentina

Se examinó la estrategia alimentaria y la predación de la lechuza del campanario (Tyto alba) y de la lechucita vizcachera (Speotyto cunicularia) por especie, tamaño y sexo de roedores, comparando consumo y abundancia relativa de la presa en agrosistemas de Argentina central. La lechuza del campanario mostró una mayor preferencia que la lechucita vizcahera por los bordes de campos de cultivos como habitats de caza. Ambas especies predaron significativamente más Oligoryzomys flavescens que lo esperado a partir de lo capturado en trampas. La lechuza del campanario consumió más Akodon azarae y menos Calomys laucha, mientras que la lechucita vizcachera consumió más C. laucha y metros A. azarae que lo esperado. La selección de juveniles por la lechucita vizcachera puede ser explicada por una alimentación oportunista. Contrariamente, la selección de la lechuza del campanario sobre los roedores de mayor tamaño dentro de cada especie, puede explicarse por un comportamiento de alimentación selectivo. Se registró un solo caso de predación diferencial por sexo: la lechuza del campanario consumió más hembras que machos de Calomys laucha.Feeding strategy and predation of the Bartz owl (Tyto alba) and the Burrowing owl (Speotyto cunicularia) on rodent .species, size, and sex was studied by examining consumption and abundance of prey categories in agrosystems of Central Argentina. The Bartz owl foraged in borders of cultivated fields more than did the Burrowing owl. Both owl species preyed on Oligoryzomys flavescens significantly more than expected based on trap captures. The Barn owl consumed atore Akodon azarae arad fewer Calomys laucha whereas the Burrowing owl took fewer A. azarae and more C. laucha than expected. The Bartz owl selected the largest individuals of every prey species, while the Burrowing owl preyed on juveniles more than expected based on trap captures. The under-representation of large rodents in the diet of the Burrowing owl may be explained by opportunistic feeding behaviour. In contrast, the choice of large-sized rodents by the Barn owl may be a result of selective feeding behaviour. The Bartz owl preyed more on female Calomys laucha than on reales, the only case showing significant difference between sex of prey chosen by owls and the estimated sex ratio

Practical and Theoretical implications of perch use for avian predators on rodent populations

We hypothesized that the vertical structure of the habitat is a determining feature of avian predation intensity on rodents, other habitat conditions being equal.Two trap grids for rodents were set up in a field with com stubble at two times, before and after setting up perches in the experimental plot; one of the trap grids served as a control. There was no significant difference in captures between control and experimental grids before setting up perches (control validation). However, more rodents were captured in the control than in the experimental plot after the treatment. Predation lowered rodent density by roughly 40%. Calomys laucha was the affected species. Differential predation was not observed according to size or sex. We conclude that habitat vertical structure causes significant changes in rodent abundance, and it should be considered in rodent control and raptor management.We hypothesized that the vertical structure of the habitat is a determining feature of avian predation intensity on rodents, other habitat conditions being equal.Two trap grids for rodents were set up in a field with com stubble at two times, before and after setting up perches in the experimental plot; one of the trap grids served as a control. There was no significant difference in captures between control and experimental grids before setting up perches (control validation). However, more rodents were captured in the control than in the experimental plot after the treatment. Predation lowered rodent density by roughly 40%. Calomys laucha was the affected species. Differential predation was not observed according to size or sex. We conclude that habitat vertical structure causes significant changes in rodent abundance, and it should be considered in rodent control and raptor management

Ecología de las comunidades de roedores involucradas en la fiebre hemorrágica argentina

Fil: Kravetz, Fernando O.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Patrones de abundancia relativa en ensambles de pequeños roedores de la región pampeana

Se estudió la composición específica de 8 ensambles de pequeños roedores de la región pampeana en dos tipos de habitat: bordes de cultivo y pastizales. Akodon azarae fue la especie numéricamente dominante en 7 casos. Los ensambles muestreados en los bordes de cultivo contaron con mayores índices de diversidad y de equitatividad que los de pastizales. Los bordes presentaron una mayor representación de Mus y Calomys, en detrimento de Akodon y de Oxymycterus. Se ensayaron pruebas de bondad de ajuste entre los patrones de abundancia relativa de los ensambles y los modelos de “barra partida” (BP), expresión biológica de una distribución idealmente uniforme, y de la “serie geométrica”, que caracteriza a ensambles con estructuras jerarquizadas. Los ajustes fueron rechazados en todos los casos. En 7 de los 8 casos los ensambles presentaron una distribución más jerarquizada que la predicha por el modelo BP. Se proponen diversas interpretaciones de las causas de los desajustes observados: 1) una combinación de los factores de competencia y de variabilidad temporal en el tamaño de los nichos de las especies, 2) el número de recursos por los que las especies compiten, 3) el papel de la interferencia de las especies socialmente dominantes sobre la apropiación de los recursos, y 4) el reclutamiento en los bordes de cultivo de especies procedentes de los campos de cultivo (Calomys, Mus). La representación actual de Calomys en los agroecosistemas superaría a la que habrían tenido originalmente en los pastizales de la llanura pampeana.We studied the composition of 8 small-rodent communities in the Pampean region of Central Argentina in two different habitats: crop field borders and grasslands. Akodon azarae was the most abundant species in 7 communities. Species diversity and evenness were higher infield borders than in grasslands. Borders had higher proportion of Mus and Calomys and lower proportion of Akodon and Oxymycterus than grasslands. We fit the data to species-abundance patterns expected according to the “broken stick” model (BS), which is the biological expression of an ideally uniform distribution, and to the “geometric series “ model, which is related to communities with hierarchical structures. The goodness-of-fit tests were rejected in all cases. Moreover, in 7 of 8 cases, BS was rejected because the communities had a stronger hierarchical structure than predicted by the model. Wepropose several interpretations for this: 1) a combination of both competition and temporal variations of niche breadths in determining the species- abundance curves, 2) the number of resources for which the species are competing, 3) the role that interference competition may play in confering the possesion of resources to socially dominant species, and 4) the recruitment of species (Calomys, Mus) from crop areas to field borders. The present proportion of Calomys spp. in agroecosysterns probably exceeds its original proportion in Pampean grasslands

Dispersión invernal de Oligoryzomys flavescens (Rodentia: Muridae) en bordes de campos de cultivos de agroecosistemas pampeanos

Se estudió el patrón de dispersión invernal de Oligoryzomys flavescens en bordes de campos de cultivos mediante la técnica de remoción. Las áreas en las cuales se efectuaron las remociones exhibieron, tras 15 días de haber finalizado las mismas, abundancias de O. flavescens que no difirieron de las abundancias observadas en las áreas de captura, marcado y recaptura. La recuperación de las áreas removidas se debió a la dispersión de ejemplares hacia las mismas. Los dispersores colonizadores no parecen presentar características en cuanto al peso, el largo del cuerpo y la proporción de sexos que los diferencien claramente de los residentes de esta especie. Las tasas de recuperación de O. flavescens fueron superiores a las previamente encontradas en A. azarae, especie competitivamente dominante, aunque estas diferencias fueron significativas en uno solo de los inviernos estudiados. Los movimientos dispersivos en O. flavescens serían fundamentales para el mantenimiento de la abundancia local en los bordes de campos de cultivo en la estación invernal.We studied winter dispersal pattern of Oligoryzomys flavescens in cropfield borders using the removal technique. Fifteen days after removal, O. flavescens abundance in removal areas did not differ from that observed in capture-mark-recapture areas. The recovery in removal areas was due to the dispersal of individuals. Colonizing dispersers do not seem to present differential body mass, body length, and sex proportion compared to resident individuals. Recovery rates were higher than the observed in a previous work in A. azarae, a competitively dominant species, although there were significant differences just in one of the studied winters. Dispersal movements in O. flavescens would play a main role to maintain local abundantes in cropfield borders in winter

Copulatory Pattern And Mating System Of Akodon Azarae (Rodentia, Muridae)

Volume: 84Start Page: 133End Page: 14

Productivity of <i>Tyto alba</i> (Bam owl) in nest boxes in Pampean agrosystems

En este estudio se examina la productividad de Tyto alba (lechuza de campanario) en nidos artificiales instalados en agrosistemas pampeanos. Se instalaron nidos artificiales en Diego Gaynor, provincia de Buenos Aires, en los cuales T. alba presentó un tamaño medio de puesta de 5,6 ± 0,5 huevos,una media de eclosiones de 4,4 ± 1,1 y un éxito reproductivo de 4,4 juveniles/puesta. La mortalidad de pichones fue nula. La producción en nidos artificiales fue mayor que en nidos naturales (esta última obtenida en base a datos presentados en este estudio y otros de bibliografía). El 29% de los pichones nacidos en los nidos naturales considerados murieron debido a la baja calidad de los sitios de anidación. La calidad de los sitios de anidación puede ser manipulada a traves de la instalación de nidos artificiales, incrementando la producción de T. alba en agrosistemas pampeanos.We examined the productivity of Tyto alba in nest boxes in Pampean agrosystems of Argentina. Nest boxes were setup in trees in Diego Gaynor, Buenos Aires Province. Mean clutch size was 5.6 ± 0.5 eggs,mean hatching per nest was 4.4 ± 1.1, and reproductive success was 4.4 fledges/clutch. No mortality of chicks was recorded. Productivity in nest boxes was greater than in natural nests (the latter based on results obtained from this study and from past studies). Twenty nine percent of the mortality which occurred in the natural nests was caused by the low quality of nest sites. Thus, tbe quality of nest sites rnay be manipulated, by setting nest boxes, to increase reproductive success of <i>T. alba</i> in Pampean agrosystems

Reproductive performance of capybaras (Hydrochoerus hydrochaeris) in captivity under different management systems in Argentina

The capybara (Hydrochoerus hydrochaeris Linnaeus, 1766) is a wild rodent of great economic interest and is easily domesticated. Variations in reproductive parameters for the capybaras depend on both intrinsic and extrinsic factors, which modulate the different stages of its reproduction either in captivity or wildlife. In captivity, an intensive production system is feasible, offering an economical profit under certain reproductive conditions. This work evaluates the reproductive performance of capybaras under different management systems in captivity, interpreting the results biologically and giving management guidelines to optimize their intensive production. The study was carried out at the Capybara Breeding Station, located in the Agricultural Experimental Station "Delta del Paraná", belonging to the National Institute for Agricultural Technology (INTA; Campana, province of Buenos Aires, Argentina; 34°08'S; 58°12'W), from August 1997 to December 1998. Three different management systems were tested: System I "Isolated pens" (IP), composed of three different types of outdoor enclosures for "reproduction", "maternity" and "growth". System II "Isolated females" (IM), formed by a central enclosure ("reproduction pen"), communicating with 12 paddocks ("maternity pen") and "growth pens". System III "Mixed pens" (MP), an enclosure of 35 $\times$ 10-m divided in "reproduction area" and "maternity pens"; the offspring were moved to "growth pens" elsewhere. A temporary weaning technique was tested for the "mixed pens" system. The variables analyzed for all systems were litter size at birth and at weaning, nursing and interbirth periods, and survival rates (at birth, weaning and total). The reproductive efficiency of each adult female was calculated as the total number of individuals weaned alive in a one-year period. During the study, 348 young were born in 104 births, from a total of 59 adult females. We conclude that the system that performs the best for capybara reproduction is the "mixed pens" system, including temporary weaning. Under this management strategy the following parameters were registered: mean litter size of 3.8 newborn per birth; 85% of live offspring at weaning; two births per year per female; all resulting in a reproductive efficiency of 6.5 young weaned alive per year $\times$ female. These reproductive parameters provide the necessary conditions to obtain economical profit of captive-bred capybaras

Male-female interaction during breeding and non-breeding seasons in Akodon azarae (Rodentia, Muridae)

Dyad encounters between male and female adults of Akodon azarae (Fischer, 1829) were analyzed by means of observational techniques in a natural closure during the breeding and non-breeding seasons. The animals were held in observation during 21 days, with daily 15-minute recordings of interindividual separation distance, relative displacements, characteristics of the male-female interaction, copulation, and construction and exclusive or shared use of nests by each pair. The couples, which bred successfully, showed, on average, the longest separation distance between male and female allowed by the closure. During the first two weeks of gestation the females exhibited more displacements than their respective mates did. The male-pregnant female encounters were significantly more aggressive than those recorded between pairs which did not breed successfully. During the non-breeding season a shorter average distance between individuals and a frequent use of nests shared by the pair were recorded. The results obtained are discussed within the framework of the social system of A. azarae