CONICET | Buscador de Institutos y Recursos Humanos

Winter irruption refers to an unpredictable increase in the number of individuals usuallydominated by one age class, either juvenile or adult, into a given area as a response to fluctuations in the foodsupply. Irruptions are well documented for avian predators breeding in the Arctic and Subarctic regionswhereby individuals irrupt into irregular nonbreeding areas across the Boreal Temperate Region. However,this phenomenon is largely unknown across the Austral Temperate Region. We studied the age classcomposition of Variable Hawks (Geranoaetus polyosoma) and Black-chested Buzzard-Eagles (G. melanoleucus)during six austral winters (nonbreeding season) at an overlapping breeding and overwintering area in theMonte Desert (semiarid grassland and shrub steppes) of the Andean foothills at Mendoza province in centralArgentina (338S). We measured the overall abundance per winter (combining the five age classes) andcalculated the relative abundance per age class for each winter of both species. We used the ratio of totalnumber of individuals observed in a single winter/the total hours of observation per winter as an index ofabundance, because sampling effort was not equal across winters. We found that (1) both species exhibitedwinter irruptions, (2) the irruptions were largely driven by a marked increase of Basic I individuals (theyoungest age class); at the same time, Basic V individuals (the oldest age class) exhibited their (modestly)lowest abundance, and (3) winter irruptions might be species-specific, as the responses differed between thetwo species, with a joint irruption event in the austral winter 2016 and a second irruptive event only recordedfor Black-chested Buzzard-Eagles in the austral winter of 2018. We posit that the marked increase of Basic Iindividuals during the irruptive winters cannot be fully explained by successful breeding of the localpopulation, and is likely largely a results of individuals born elsewhere, presumably at southern latitudes, andoverwintering at our study site in central Argentina.