From phys.org
This Journal of Raptor Research (Note: I'll share this in the comments section) issue focuses on movement ecology—how and why raptors move. This can include classic movements like migration, as well as nomadism.
Short-eared owls (Asio flammeus) fall into the latter category—they have a penchant for small mammals that breed in "boom and bust" cycles approximately every four years. Therefore, these owls must travel great distances to find prey in sufficient numbers. This requires a high degree of nuanced perception, yet we know very little about the actual mechanisms that allow an owl to accurately locate these peaks in prey cycles.
A feature article in the new issue, titled "Irruptive Movements by Short-eared Owls and Concepts of Habitat Use: Commentary and Suggestions to Address Information Gaps," emphasizes the need for more understanding about this phenomenon, and proposes eight research questions to guide future work.
Changing climate regimes can severely impact where and when small mammals show up on the landscape, especially in the tundra, where many predators rely on their presence. There is an increased need for conservation leaders to understand the decision-making of species whose survival is linked to these ephemeral pulses in prey, and owls are a strong choice for future investigation.
Short-eared owls are medium-sized raptors with speckled golden plumage and bright yellow eyes. They are widely distributed across the globe and hunt in open country, typically following the presence of voles, lemmings, shrews, mice, and other small mammals. They cannot afford to settle down in one place year after year like many other raptors because of the shifting patterns of their prey.
"There appears to be a great deal of complexity associated with decisions made by irruptive and nomadic species about whether to settle in a territory or move elsewhere," says author Joe Buchanan.
Short-eared owls hatch asynchronously, a strategy towards countering changes in food supply—the bigger, older chicks can eat the younger ones. Short-eared owls, however, specialize in prey that can be highly unpredictable and can exhibit high degrees of nomadism to locate areas of high prey availability to maximize their chances of rearing large broods.
Short-eared owls have a short lifespan and the size of their brood shifts depending on how much food is available—when there are many small mammals available, the owls lay up to 10 eggs. In years of scarcity, maybe only two eggs. This gives them a life history strategy atypical for vertebrate predators, which are usually long-lived and consistently raise few young.
The authors of the Letter propose more research on the relationship between short-eared owl movements, behavior, and habitat selection. They acknowledge that the dovetailing of these topics is complex but underscore the importance of a deeper research dive.
"How this predator navigates the erratically undulating tapestry of food abundance that spans thousands of miles remains a mystery," says the author of the Letter, Travis Booms, "yet it is an integral part of their life history and a key component to their conservation and persistence." This is likely a truism for other nomadic species linked to vole and lemming cycles, such as the snowy owl (Bubo scandinavius).
Short-eared owls, like other top predators, are bioindicators. They are sensitive to environmental change and provide cues on the health of their surrounding environments. Booms, Buchanan, and their co-authors propose that the density of short-eared owls could provide a strong indication that a habitat is of high quality because the owls only settle if there is a reliable abundance of prey.
As denizens of open landscapes, this species is also an important ecological link in quickly vanishing habitats like grasslands, which have shrunk to a fraction of their former range worldwide, forcing the species to, as co-author John Callidine describes it, "Travel Far, Breed Hard, and Die Young."
...occur near or at the peak in the cycle of small mammal abundance, and certainly when prey abundance declines, causing local breeders and their offspring to search for more productive landscapes. Tracked Short-eared Owls have been recorded undertaking “loop movements” to areas 50 km or more from their occupied home range. Such flights may function to assess conditions in other areas, facilitating decisions to remain or settle elsewhere nearby (Calladine et al. 2024a; see Dale and Sonerud 2023).
The level of potential competition for resources (and the change in prey abundance) that occurs during both the increasing and decreasing phases of the prey cycle are largely unknown but may result in negative consequences of density-dependent habitat selection. This was documented in Snowy Owls (Bubo scandiacus) in eastern North America where immature owls used areas where they experienced higher levels of mortality (McCabe et al. 2022), suggesting the areas were ecological traps (Dwernychuk and Boag 1972, Battin 2004). With notable exceptions (e.g., Village 1987, Korpimäki and Norrdahl 1991), the uses of space by Short-eared Owls through time during the small mammal cycle are poorly understood in most areas. However, the occurrence of prey and the periodicity or magnitude of their cycles can change over time (e.g., Brommer et al. 2010) and space; for example, Short-eared Owls responded to a recently established subpopulation of voles by colonizing and breeding in Spain (Luque-Larena et al. 2013, Mougeot et al. 2022) and in the lower latitudes of North America, where small mammal prey cycles appear to be less pronounced (Miller et al. 2023). All such variations will influence the dynamics of how Short-eared Owls use space, but their “travel far, breed hard, and die young” life strategy appears adapted to take advantage of such spatially and temporally dynamic conditions (Dale and Sonerud 2023).
Relating Short-eared Owl movement to concepts of habitat selection and behavioral ecology may help focus future research. Of the following research questions, not all are mutually exclusive. Although most research on Short-eared Owls occurs in the northern hemisphere, the following questions may also be relevant in other parts of their range (Mikkola 2014, Enríquez 2017). (1) Assuming the existence of a source-sink dynamic, how long do source landscapes function as sources, and what is the temporal and spatial pattern of variation in habitat quality (e.g., source vs. sink), reproductive output, and subsequent survival and recruitment? (2) Do Short-eared Owls of source origin have a greater likelihood of survival than those from sinks or less productive locations, and does this influence survival rates for owls that travel great distances to new breeding areas? (3) Assuming there are differences among landscapes in the density of colonizing Short-eared Owls during the increasing part of the prey cycle, what factor(s) other than prey abundance (e.g., predators, human disturbance, prior experience at the site) distinguish between landscapes with differing densities of owls? (4) Do certain colonizing Short-eared Owls (e.g., recent recruits to the population) experience density-dependent influences in their use of habitat and potentially occupy areas that might function as ecological traps? (5) What is the threshold prey encounter or intake rate that triggers levels of competition through space and time, or the decision to remain or move? (6) Are there vigilance benefits associated with high densities of breeding Short-eared Owls, and as prey availability declines, do these benefits change? (7) Where there are differences in the magnitude of small mammal prey cycles across latitudes (e.g., less pronounced cycles at lower latitudes), do decisions to stay or disperse reflect local knowledge (e.g., staying at a known but marginally productive area at a lower latitude but departing from a similarly productive site farther north as a hedge bet that a higher quality area will be encountered)? (8) Due to the short life expectancy of Short-eared Owls (at least in some areas) and associated limited opportunities to acquire knowledge, are there differences in search behavior (e.g., at nearby versus distant locations) or other aspects of knowledge transference, including conspecific attraction?
Given the tremendous distances traveled by irruptive/nomadic species from one breeding location to another, sometimes in successive years, there is substantial uncertainty inherent in this movement strategy. The location of source or otherwise high-quality landscapes for Short-eared Owls varies through time in many regions, and the density of owls fluctuates in response to prey resources or competition. This uncertainty is consequently transferred to the conservation and management arena, illuminating the need for a better understanding of behavioral ecology and the variation in patterns of habitat use at multiple spatial and temporal scales across landscapes and regions (see Miller et al. 2023). Protecting specific places for the sake of irruptive/nomadic species may be ineffective because of the potential mismatch between place-based conservation and the movement ecology of irruptive species (Cottee-Jones et al. 2015). The questions we posed above should augment recommendations presented by Booms et al. (2014) and may enhance future research and subsequent development of conservation strategies for this and other irruptive/nomadic species.