| Summary: | Winter irruption refers to an unpredictable increase in the number of individuals usually dominated by one age class, either juvenile or adult, into a given area as a response to fluctuations in the food supply. Irruptions are well documented for avian predators breeding in the Arctic and Subarctic regions whereby 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 class composition 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 the Monte Desert (semiarid grassland and shrub steppes) of the Andean foothills at Mendoza province in central Argentina (33°S). We measured the overall abundance per winter (combining the five age classes) and calculated the relative abundance per age class for each winter of both species. We used the ratio of total number of individuals observed in a single winter/the total hours of observation per winter as an index of abundance, because sampling effort was not equal across winters. We found that (1) both species exhibited winter irruptions, (2) the irruptions were largely driven by a marked increase of Basic I individuals (the youngest 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 the two species, with a joint irruption event in the austral winter 2016 and a second irruptive event only recorded for Black-chested Buzzard-Eagles in the austral winter of 2018. We posit that the marked increase of Basic I individuals during the irruptive winters cannot be fully explained by successful breeding of the local population, and is likely largely a result of individuals born elsewhere, presumably at southern latitudes, and overwintering at our study site in ...
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