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2 October 2025

Irruptive Movements by Short-Eared Owls and Concepts of Habitat Use: Commentary and Suggestions to Address Information Gaps

The scope of strategies used by birds during seasonal movements is considerable and has been the subject of comprehensive syntheses (Bertold 2001, Newton 2024). The movements of species that employ irruptive or nomadic strategies are often unpredictable (Cottee-Jones et al. 2015), which can impede comprehensive understanding and subsequent implementation of effective conservation measures (Teitelbaum and Mueller 2019, Gibson et al. 2021). Across much of its extensive range, the Short-eared Owl (Asio flammeus) specializes on small mammal prey whose abundance varies or cycles markedly in both space and time. In response, the owls may travel thousands of kilometers between successive breeding areas to exploit localized prey abundances (Johnson et al. 2017, Calladine et al. 2024a). Research on this species has occurred across the northern hemisphere and elsewhere (Wiggins et al. 2020), and research and management priorities have been proposed (Booms et al. 2014). Here, we discuss aspects of Short-eared Owl colonization within the context of behavioral ecology and habitat selection, applying that framework to identify key questions that should be addressed in future research.

Short-eared Owls engage in irruptive or nomadic movements (for synthesis, see Newton 2024). In response to cyclic changes in small mammal prey abundance, the owls move across large landscapes, departing areas where prey abundance has crashed and settling in areas where prey resources are greater. These movements and the dynamics of predation represent a numerical response by the owls to a pulsed resource (e.g., Therrien et al. 2014). The changes in density of owls and their prey therefore shift through time and space (Newton 2024), as demonstrated for Short-eared Owls in several regions across the northern hemisphere (Village 1987, Korpimäki and Norrdahl 1991, Miller et al. 2023). In most cases (see potential exceptions, below), and acknowledging the uncertainties of site occupancy associated with nomadic birds (Teitelbaum and Mueller 2019, Calladine et al. 2024a, 2024b), the density of this species is likely a reliable indicator of habitat quality (but see Van Horne 1983). Assessing the specifics of source-sink dynamics is challenging due to the cost and effort required to generate demographic information needed to establish such relationships (Wiens and Van Horne 2011). However, it is logical that prey-rich locations are so different from prey-depleted locations that they likely function as sources to some extent (Pulliam 1988, Furrer and Pasinelli 2016).

Sources are commonly defined as areas where reproductive output exceeds mortality, whereas sinks are areas where mortality exceeds reproductive output (Pulliam 1988). By being more productive, sources are important to the population or subpopulation (as defined by Morrison et al. 2020) by enhancing lifetime reproductive success of individuals, resulting in greater recruitment of juveniles. In contrast, sinks require natal or breeding dispersal from sources to be maintained. Sources have traditionally been described as habitats or locations that are largely static or persistent (Pulliam 1988). In this manner, purported source areas used by Short-eared Owls differ in that they are ephemeral—functioning for only a segment of the small mammal prey cycle—and upon declining they lose much of their value and theoretically become sinks. Although emigration may imply the former source area had become a sink, it is also possible that a low density of owls may persist (Miller et al. 2023; and see Newton 2024). If owls' reproduction in those areas remains equal to or exceeds mortality, the area may not function as a sink per se.

Another aspect of assessing source-sink dynamics is related to how sources and sinks are defined. Pulliam (1988) did not include emigration as a demographic metric of source-sink dynamics (Runge et al. 2006). As a generality, including emigration success (e.g., survival and recruitment to the breeding population) might provide an informative assessment of sources by facilitating estimates of lifetime reproductive success. However, because Short-eared Owls can move great distances between successive breeding areas (Johnson et al. 2017, Calladine et al. 2024a), the relevance of emigration as a meaningful metric may be reduced if the probability of survival is inversely related to distance moved between breeding attempts.

A key part of the Fretwell-Lucas (ideal despotic distribution) theory of habitat selection is that early-arriving individuals use higher quality habitat, whereas later-arriving individuals occupy remaining areas, including those of lower quality (Fretwell and Lucas 1970). Consequently, according to the Fretwell-Lucas model, when Short-eared Owls first begin to settle in a region that is undergoing an increase in small mammal abundance, the first-arriving owls will occupy the best locations, and later arriving individuals settle in lower-quality areas. The Fretwell-Lucas model is logical for behavior of obligate migrants (i.e., birds that are hardwired to migrate and that move in consistent ways) and Fretwell-Lucas may apply in certain cases for facultative migrants, those birds that exhibit irruptive movements only as needed to address changes in environmental conditions (Berthold 2001, Newton 2012). There is evidence that distances moved by Short-eared Owls outside of the breeding season could be density dependent (i.e., distances greater when densities are high; Calladine et al. 2012), and similar mechanisms could operate at other times with birds moving to more distant or lower quality habitats when numbers are high, which would be consistent with the ideal despotic distribution concept. A facultative migrant like the Short-eared Owl might move hundreds or thousands of kilometers between breeding areas from one year to the next (e.g., Johnson et al. 2017, Calladine et al. 2024a), and upon arriving at a new breeding area for the first time would have no prior knowledge of the quality of a landscape (Dale and Sonerud 2023). This could be particularly relevant for Short-eared Owls where survival rates can be quite low (e.g., mean annual survival rate of 0.47 reported in Europe), which implies that most individuals are unlikely to encounter good breeding opportunities (a vole outbreak) at the same place multiple times over their lifetime (Calladine et al. 2024a) and thus limits opportunities for individuals to acquire and use site-specific knowledge.

The manner of settlement by Short-eared Owls at a new landscape is likely influenced by local circumstances. One potential situation, as described above, is that first arrivals would claim the best areas, and later arrivals would occupy lesser areas in accordance with the Fretwell-Lucas model of the ideal despotic distribution. Alternatively, some high-quality landscapes may not be saturated by colonizing individuals (Newton 2006), which is a logical expectation given the lack of knowledge by owls of landscape conditions hundreds or thousands of kilometers from previous nesting areas (Dale and Sonerud 2023). In this case, a plausible situation is that later arriving individuals would also occupy high quality patches, because they were still available. Until the best habitat becomes saturated with owls, if that occurs, there is little or no competition.

In some situations, breeding Short-eared Owls occur in high local densities, resulting in aggregations that are considered semi-colonial (Arroyo and Bretagnolle 1999, Keyes et al. 2016). This raises the question about whether high-density areas reflect a numerical response to a superabundant food resource, if some other benefit of aggregation (e.g., reducing risk of predation; see Lockie 1955, Reid et al. 2011) influenced the behavior, or if the latter is simply an important byproduct of the former. Interactions may shift from interference competition to exploitative competition (Holdridge et al. 2016), and this might be anticipated during the transition from moderate or high prey densities to lower prey densities. If territory sizes were smaller during periods of high prey abundance (Village 1987) there would, in theory, be greater vigilance for predators but perhaps more competitive interactions (Lockie 1955, Pitelka et al. 1955). In contrast, during periods of fewer available prey resources, foraging areas might be larger (e.g., extending farther from nest sites), which might reduce vigilance and result in less competition for resources. Moreover, competition can alter growth rate in the local breeding cluster if conspecific interactions detract from breeding activities (Schoener 1973).

In subsequent years, yet another possibility is that the location of the best areas in the source landscape may have changed, from year one to year two, for example, as would be expected in the increasing phase of the small mammal prey cycle. Given such a change, owls from the previous year that remained in the landscape (or that returned from an overwintering area) may select new areas that are better than those of the previous year, which would indicate short-term philopatry to the landscape (i.e., they nested 2 yr in a row) although not to a specific territory. Because the increasing prey abundance continues to enhance the quality of the landscape, it would be expected that second-year arrivals might also encounter high-quality areas (including areas that were not high-quality locations in the first year), in the same manner as the first-year arrivals described above. It is also possible that at some point, the arrival of new individuals coupled with the output of reproduction from the high-quality landscape would result in competition for high-quality territories or in resource depletion (Newton 2006). This seems most likely to...