Pulsed resources at tundra breeding sites affect winter irruptions at temperate latitudes of a top predator, the snowy owl

Abstract

Irruptive migration is mostly observed in species specialized on pulsed resources and is thought to be a response to unpredictable changes in food supply. We assessed two alternative hypotheses to explain the periodic winter irruptions of snowy owls Bubo scandiacus every 3-5 years in temperate North America: (a) the lack-of-food hypothesis, which states that a crash in small mammal abundance on the Arctic breeding grounds forces owls to move out of the tundra massively to search for food in winter; (b) the breeding-success hypothesis, which states that high abundance of tundra small mammals during the summer allows for high production of young, thus increasing the pool of migrants moving south the following winter. We modeled winter irruptions of snowy owls in relation to summer food resources and geographic location. Winter abundance of owls was obtained from citizen-based surveys from 1994 to 2011 and summer abundance of small mammals was collected in summer at two distant sites in Canada: Bylot Island, NU (eastern High Arctic) and Daring Lake, NWT (central Low Arctic). Winter owl abundance was positively related to prey abundance during the previous summer at both sites and tended to decrease from western to eastern temperate North America. Irruptive migration of snowy owls was therefore best explained by the breeding success hypothesis and was apparently caused by large-scale summer variations in food. Our results, combined with previous findings, suggest that the main determinants of irruptive migration may be species specific even in a guild of apparently similar species.

Age composition of winter irruptive Snowy Owls in North America

“…These results are consistent with the idea that winter irruptions in this species are probably the result of good breeding conditions on the arctic tundra, as predicted by the ‘breeding success’ hypothesis, rather than a generalized lack of food (Koenig & Knops , Robillard et al . ). These results expand previous reports of a large number of young Snowy Owls detected during winter irruptions at one location in eastern North America over a 15‐year period (Smith ) and in Manitoba in 1985–1986 (13 individuals; Keith ).…”

“…If winter irruptions are indeed a consequence of high reproductive success (Robillard et al . ), then a large proportion of individuals observed during irruptions should be juveniles, as suggested by Smith ().…”

“…However, in accordance with earlier suggestions by Gross (), Keith () and Holt and Zetterberg (), recent evidence now points to the ‘breeding success’ hypothesis to explain such massive movements of individuals (Robillard et al . ). Indeed, the Snowy Owl relies heavily on lemmings ( Lemmus and Dicrostonyx sp.)…”

“…An alternative hypothesis, dubbed the ‘breeding success’ hypothesis (Koenig & Knops , Robillard et al . ), suggests that winter irruptions in some species could actually be the result of a high population density created by highly successful breeding. This could happen when an especially favourable reproductive season due to high food availability creates an unusually large pool of first‐year birds that can migrate south at the end of the breeding season (Koenig & Knops ).…”

Nomadic breeders Snowy Owls (Bubo scandiacus) do not use stopovers to sample the summer environment

“…After reaching the breeding range, Snowy Owls must prospect, apparently nomadically, as they seek out appropriate areas with high food abundance on which to settle (Therrien et al 2014, Curk et al 2020. Hence, movements within the breeding range may be used primarily to prospect for lemming peaks, as Snowy Owls will only settle to breed in areas of high lemming abundance (Gauthier et al 2013, Therrien et al 2014, Robillard et al 2016. In any year, these peaks are separated by large distances, so owls may assess lemming populations within the breeding range via stopovers to sample prey density interspersed by long-distance movements.…”