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We report a case of mortality in juvenile Nycticorax nycticorax in a new colony present at an urban reserve adjacent to the Paraná River (Santa Fe, Argentina). From August to November 2016, 138 nests were monitored. In August, there were few nestlings and juvenile deaths. In September, more than 800 nestlings and juveniles were found dead on the ground, as well as eggs abandoned by their parents. There was no evidence of unusual mortality in other egret species present in the area. We collected a sample of six individuals for laboratory analysis and necropsies. Individuals had very low body condition and almost empty stomachs, distended gallbladders intestines without solid material, predominantly filled with biliary liquid (fasting sign), and albumin and other serum proteins were found at very low levels, indicating starvation as the cause of death. Probably, this mortality was due to parental abandonment in ages at which the young birds still depend on external food provisioning. We considered the interaction of multiple causes, such as human intervention, habitat conditions, climatic factors, and population density. The magnitude of the reported waterfowl die-off is unprecedented in the Middle Paraná river valley., Se reporta un caso de mortandad en juveniles de Nycticorax nycticorax en una colonia de reciente formación situada en una Reserva dentro de una matriz urbana lindante con el sistema de islas del río Paraná (Santa Fe, Argentina). Se monitorearon 138 nidos desde agosto a noviembre 2016. En agosto se registraron escasas muertes de pichones y juveniles. En septiembre, se registraron más de 800 pichones y juveniles muertos en el suelo, además de huevos abandonados por sus progenitores. No se evidenció mortandad inusual en otras especies de garzas que nidificaban en el área. Se tomaron muestras de seis individuos para realizar análisis de laboratorio y necropsias. Los individuos presentaron muy baja condición corporal y estómagos con escaso contenido, vesículas biliares distendidas e intestinos sin material sólido, con predominancia de contenido biliar (signo de ayuno), albúmina y otras proteínas séricas muy bajas, indicando muerte por inanición. Probablemente, esta mortandad se debió al abandono parental en edades donde las crías aún dependen de ellos para su alimentación. Se consideraron la interacción de múltiples causas, como la intervención humana, condiciones del hábitat, factores climáticos y densidad poblacional. No se conocen precedentes de tal magnitud de mortandad de aves acuáticas en el valle de inundación del Paraná Medio.

Most hosts, including humans, are simultaneously or sequentially infected with several parasites. A key question is whether patterns of coinfection arise because infection by one parasite species affects susceptibility to others or because of inherent differences between hosts. We used time-series data from individual hosts in natural populations to analyze patterns of infection risk for a microparasite community, detecting large positive and negative effects of other infections. Patterns remain once variations in host susceptibility and exposure are accounted for. Indeed, effects are typically of greater magnitude, and explain more variation in infection risk, than the effects associated with host and environmental factors more commonly considered in disease studies. We highlight the danger of mistaken inference when considering parasite species in isolation rather than parasite communities

Rodents that have multi-annual cycles of density are known to have flexible growth strategies, and the “Chitty effect”, whereby adults in the high-density phase of the cycle exhibit larger average body mass than during the low phase, is a well-documented feature of cyclic populations. Despite this, there have been no studies that have repeatedly monitored individual vole growth over time from all phases of a density cycle, in order to evaluate whether such variation in body size is due to differences in juvenile growth rates, differences in growth periods, or differential survival of particularly large or small voles. This study compares growth trajectories from voles during the peak, increase and crash phases of the cycle in order to evaluate whether voles are exhibiting fast or slow growth strategies. We found that although voles reach highest asymptotic weights in the peak phase and lowest asymptotes during the crash, initial growth rates were not significantly different. This suggests that voles attain larger body size during the peak phase as a result of growing for longer.