Introduction

One of the worlds most widely distributed owls, the Short-eared Owl is an open country, ground-nesting species that inhabits marshes, grasslands, and tundra throughout much of North America and Eurasia. It also breeds in South American grasslands and on islands such as Iceland, the Hawaiian chain, and the Galápagos. Few data exist for populations other than the nominate race A. f. flammeus in North America, Europe, and Asia. Most other races are considered island endemics.

Reproduction and population dynamics of this species are closely linked to the density of its primary prey, small mammals such as Microtus voles. Like other birds that depend on such a fluctuating food resource, the Short-eared Owl shows considerable local variation in its numbers and reproductive success, and is even nomadic at times. Although the literature on its diet in the nonbreeding season is extensive, few studies have thoroughly addressed other facets of its biology, ecology, and life history.

Active day and night, this owl tends to hunt low above the ground, often quartering an area on slightly dihedral wings or hovering. Although it generally uses acoustical cues to locate prey, it can rely on vision as well. Its conspicuous courtship flights and calls are a distinctive feature of the landscapes it inhabits. In recent decades, Short-eared Owls have declined in many areas of North America, especially the northeastern United States; habitat loss owing to human activities appears to be the major cause. As a ground-nester, it may also be vulnerable to increased levels of predation.

Although Bent’s (1938) summary of Short-eared Owl life history noted that unusual concentrations of owls often occurred in areas of small mammal outbreaks, the strong link between such outbreaks and the species’ ecology was not fully appreciated at the time. Working on the arctic tundra near Barrow, Alaska, Pitelka et al. (1955a, b) showed that large, annual fluctuations in the number of breeding Short-eared Owls was tightly correlated with the abundance of small mammals. The first intensive field study of Short-eared Owls was carried out by Clark (1975), who studied breeding ecology in Manitoba and wintering ecology in New York. Clark showed that Short-eared Owls respond to spatial and temporal variation in small mammal abundance by shifting breeding and wintering sites, and by adjusting the timing of breeding and fecundity in accordance with local prey abundance.  Since then, additional studies have focused on the diet and breeding ecology of this species in the northeastern US, particularly coastal Massachusetts (Holt 1986a, 1992, 1993a, b; Holt et al. 1992).