SPECIES ACCOUNT. Barred Owl/Chouette rayée (Strix varia)
Mitchel Daniel1 and Cory Toth2
1 – Department of Biology, Queen’s University, Kingston, ON Canada K7L 3N6
2 – School of Biological Sciences, University of Auckland, Auckland, New Zealand 1010
Taxonomy: Class: Aves. Order: Strigiformes. Family: Strigidae. Genus: Strix. Species: Strix varia.
The Barred Owl is known by numerous other common names, such as the hoot owl, rain owl, round-headed owl, swamp owl, and wood owl. Its scientific name, Strix varia, means “a variegated, strident owl” in Latin, in reference to its streaked colourations, and to the fact that it is highly vocal ( Jobling 1992; Odom and Mennill 2010). It has been speculated that the Fulvous Owl (S. fulvescens), a common inhabitant of the cloud forests of Central America, may be part of the same species (Am. Ornithol. Union 1998). The Barred Owl belongs to the same genus as the Spotted Owl (S. occidentalis), and the two may be part of the same superspecies (Mayr and Short 1970).
Description: A medium-large owl, the Barred Owl measures between 50 – 61cm long (wings 312 – 376mm), and weighs from 630 – 1050g. Females are both larger and heavier than males, but are otherwise indistinguishable (König et al. 1999). Adult plumage is generally brown or greyish-brown, with the head, neck and underparts marked with the namesake barring pattern of dark brown and pale buff. The wing coverts and scapulars are similarly barred (but with broader bars than the rest of the body), and each feather is tipped with white. The head is large and round, lacking ear tufts, with a rimmed, greyish-brown face marked with dark concentric rings. The collar is horizontally barred, while the breast and under parts are vertically streaked. Bill colour is a pale yellow, and the eyes are a blackish-brown. The toes and tarsi are yellow in colour and bare of feathers (Figure 1; König et al. 1999, Johnsgard 2002).
Barred Owl nestlings are initially covered with white down that is replaced within a few week’s time with a second, longer coat of down that is buff basally and white terminally. Upon fledging, juvenile Barred Owls acquire flight feathers that closely resemble those of adults, and the white of their body feathers is generally replaced with more adult-like buff tones (Johnsgard 2002).
As they are predominately nocturnal birds (Taylor and Forsman 1976), Barred Owls can be hard to spot during the day. They are typically highly cryptic during daylight hours, sitting motionless high in the canopy where they are concealed by dense foliage and can avoid detection by mobbing birds. They may sometimes hunt during the daytime, but generally only do so during the nesting season or on days when the sky is dark and cloudy (Terres 1980).
Barred Owl calls are loud and persistent during the fall, and in the late summer and spring (Dunstan and Sample 1972). Barred Owls have numerous types of calls, but the most common is the hoot, “hoo, hoo, too-HOO; hoo, hoo, too-HOO-waah” in which the last syllable drops off in both pitch and volume. This call is commonly described by the pneumonic “Who cooks for you? Who cooks for you ALL?” A Barred Owl will often call 8 times consecutively, followed by a silence as it waits for a conspecific response (Elderkin 1987). Shorter versions of the call also exist, such as “hoo-hoo, hoo-WAAH” and simply “hoo-WAAH.” These are used in courtship, and mates can often be heard in duet with one another (McGarigal and Fraser 1985). Males have noticeably deeper call types than females (Johnsgard 1988). In addition to these calls, Barred Owl vocalisations also include shorter yelps, barks, and squalls (McGarigal and Fraser 1985). Barred Owls respond aggressively to song playback, suggesting that calls play a role in territorial aggression (Mosher et al. 1990).
Distribution: Traditionally, the Barred Owl’s range has spanned forested regions across the eastern half of southern Canada and the United States, as well as some parts of Mexico (Figure 2; Am. Ornithol. Union 1998). Over the past century the species has spread west through a narrow stretch of boreal forest across Manitoba, Saskatchewan, and Alberta (Semenchuk 1992). More recently the species has extended its range throughout British Columbia, expanding as far north as the southern Yukon, and as far south as northern California (Dark et al. 1998). The cause of the Barred Owl’s extension into the west is unknown, though there are currently several non-mutually exclusive hypotheses. The Barred Owl has historically inhabited deciduous forests in the east, and adaptation of some populations to coniferous forest conditions may have led to expansion into more coniferous-dominated forests in the west (Boxall and Stepney 1982). Additionally, Barred Owl range expansion may have been enabled by tree planting and fire suppression efforts in the west leading to the forestation of historical grasslands (Dark et al. 1998). It has also been suggested that ecological effects associated with climatic changes, such as increases in mean temperature and summer rainfall in parts of western Canada, may have contributed to this forestation and facilitated the Barred Owl’s expansion (Johnson 1994).
The westward expansion of the Barred Owl has caused its range to overlap with that of the Northern Spotted Owl (S. occidentalis caurina), a subspecies of the Spotted Owl that inhabits the west coast of the United States from northern California to the southern edge of British Columbia. Coincident with the Barred Owl’s expanding distribution in this area, the Northern Spotted Owl’s range has been receding southwards, likely due in part to negative interactions between the species (Kelly et al. 2003).
There are four subspecies of Barred Owl, differing in their distributions and possessing subtle differences in their morphologies. Strix varia varia is the most widespread, inhabiting the northern region of the species’ range and extending as far south as Oklahoma (Bishop 1931). In this subspecies the toes are basally feathered, although these feathers are typically scant or bristled in the middle region. Both darker and paler morphs of this subspecies exist. Strix varia georgica is common in the south-eastern United States. This subspecies appears similar to S. v. varia, except that its toes lack feathers on all but the outer side of the middle toe (Stevenson and Anderson 1994). Strix varia helveola inhabits southern and central Texas and possesses upperparts that are pale brown with bolder white barring. The feet and bill of this subspecies are also larger (Ridgway 1914). Strix varia sartorii, the least common subspecies, inhabits montane regions of Mexico (Ridgway 1914). The basal toe-feathers of this subspecies are darker and its underparts are striped with black or dark drown, while its upperparts possess brighter and more numerous white markings. The bill, feet, and body of this subspecies are also larger (Ridgway 1914).
Barred Owls are year-round inhabitants of temperate and boreal forests, including upland stands, wooded swamps, and riparian habitats. They are known to prefer mature and unfragmented forests (Van Ael 1996), particularly of hardwood or mixed deciduous-coniferous composition (Nicholls and Warner 1972), though they are capable of occupying a broad range of forest conditions including fragmented and immature stands (Kelly et al. 2003). They can frequently be seen – or more likely heard – in the forests surrounding the Queen’s University Biological Station (QUBS).
There are a number of potential explanations for the Barred Owl’s preference for old-growth forests. It is possible that the lower density of tree stems in mature forests may allow for easier hunting (McGarigal and Fraser 1984). The closed canopy of old-growth forests may afford some protection from thermal variation or songbird mobbing, and the structural complexity of mature forests may support a greater variety of prey (Mazur et al. 1998). One factor believed to strongly influence Barred Owl habitat choice is the availability of nesting sites; as Barred Owls are not nest-hole excavators, they must instead occupy suitable tree cavities or the abandoned nest-holes of other birds, both of which are more plentiful in mature forests (Nicholls and Warner 1972). Nesting sites are believed to be a major variable limiting Barred Owl population size (Mazur et al. 1997). Moreover, studies have shown that Barred Owls will inhabit early-growth forests if they are provided with artificial nesting sites, such as tree-side nesting boxes, further suggesting that nest site availability is a primary cause of the Barred Owl’s association with old-growth forests (Johnson and Follen 1984; US Fish and Wildlife Service 1987).
Barred Owls are highly territorial and not nomadic, probably due in part to the value of acquired suitable nesting sites (Nicholls and Fuller 1987). More northerly populations are a partial exception to this trend however, as individuals may make small southward migrations during the late fall as prey becomes scarce (Elody 1983). Radio-tracking studies have also indicated that the home ranges of individual Barred Owls often increase in size during the winter, probably in response to reduced prey availability (Mazur et al. 1998). Individuals have been observed fighting at territory boundaries, typically engaging in aggressive hooting and chase flights (Mazur et al. 1998).
The Barred Owl is an opportunistic predator with a catholic diet. Mammals (particularly small ones, such as cricetid and sciurid rodents) comprise much of their diet year-round, and they will regularly take birds up to the size of a grouse (Marks et al. 1984). Amphibians, reptiles, and invertebrates make up a large portion of the Barred Owl’s diet seasonally (although the latter may be underestimated, as many invertebrates do not leave any detectable traces in pellets; Livezey et al. 2008). Fish may also be eaten on occasion (Smith et al. 1983). As they are opportunistic, Barred Owls will take most prey that they can overpower. This sometimes includes rather formidable prey; remains of Eastern Screech Owls (Megascops asio) and Long-Eared Owls (Asio otus) have been found in the stomachs of Barred Owls (Johnsgard 2002).
Barred Owls are semi-nocturnal-to-nocturnal hunters and usually select an elevated perch to scan for prey (Bosakowski and Smith 1992), including locations over water to hunt for fish (Smith et al. 1983). Numerous other hunting strategies have also been observed. For example, Barred Owls have been reported to give chase to prey (such as amphibians) on the ground (Elderkin 1987), flush night-roosting birds, and catch bats on the wing (König et al. 1999). Owls in Nova Scotia have been observed sorting through leaf litter with their beaks to prey on earthworms, while Barred Owls in Washington have been observed to forage through grass to find slugs (Livezey et al. 2008). Catches are generally consumed right away. Larger prey items, however, may be cached in one of numerous places, including in the nest, as well as on or in the crooks of branches (Elderkin 1987). Generally meals are consumed whole if the item is small enough, but for larger prey the head is consumed first, followed by the body. One pellet is generally cast for each meal, although more pellets will be cast for prey items consisting of larger proportions of indigestible material (Duke and Rhoades 1977).
The Barred Owl lifespan is believed to average 10 years, although the oldest known individual in the wild reached over 18 years of age (Klimkiewicz 1997). Known predators include climbing mammals (such as weasels and raccoons), and several species of large owls and hawks, which prey on Barred Owl eggs and nestlings (Elderkin 1987). The predominant predator of adult Barred Owls is the Great Horned Owl (Bubo virginianus; Bent 1938), although the two species regularly co-exist (Fuller 1979). Radio-telemetric monitoring data has indicated that when the home ranges of males from both species overlap, Barred Owls and Great Horned Owls rarely come within 400 m of one another (Fuller 1979). These findings suggest that Barred Owls may mitigate the threat posed by co-occurring Great Horned Owls by moving to other parts of their own home range to avoid the current location of the predator (Laidig and Dobkin 1995). Human actions are an additional cause of Barred Owl deaths, primarily via shootings or collisions with vehicles, although these are believed to be rather minor contributors to overall mortality (Jackson and White 1995).
Barred Owls reach sexual maturity at two years of age. The breeding season runs from March to August, but courtship and mate-selection typically begin in February (Figure 3; Nicholls and Warner 1972). As part of their courtship display, males chase after females while emitting hooting and screeching calls. Males also display to females by fanning their wings and swaying or sidling along a branch (Bent 1938; Figure 1). Barred Owls are socially monogamous, keeping their mates for life (Nicholls and Fuller 1987). Additionally, Barred Owls tend to maintain the same territories and nesting sites across multiple years (Nicholls and Fuller 1987).
As noted above, Barred Owls most often nest in tree cavities, such as the hollows resulting from broken off treetops or limbs (Mazur et al. 1997). As these hollows must be large enough to fit the body of adult females, suitable cavities can only be found in forests with sizable trees, a likely explanation for the Barred Owl’s association with old-growth forests (Johnsgard 1988). Barred owls are known to also nest in the abandoned stick nests of other large birds, such as raptors and crows (Johnsgard 1988), although this may be a response to limited availability of suitable tree cavities (Mazur et al. 1997). The re-use of nesting sites over multiple years is fairly common (Elderkin 1987). Barred Owls have also been documented nesting on the bare ground (Robertson 1959). Barred Owls often hold territories containing multiple available nesting sites, which may be advantageous due to the ephemeral nature of these sites (Mazur et al. 1997). Tree cavities are susceptible to rot, while stick nests come apart over time. Consequently, maintaining multiple potential nesting sites may provide Barred Owls with alternatives should one site become unusable (Mazur et al. 1997).
Barred Owls are single-brooders, raising a single successful clutch per season. Should a clutch fail, the Barred Owl’s long breeding season enables pairs to re-attempt with a second – or even third replacement clutch (Bent 1938).
First-egg dates typically fall between March and April, although the breeding schedule is often shifted earlier in more southern populations (Howell 1932). Eggs are white, slightly oval, and have a somewhat rough surface texture. They are laid at 2-3 day intervals and clutches typically total two or three eggs, although as many as four or five eggs may occasionally be laid (Elderkin 1987; Murray 1976). Incubation takes place over a 30-day period and is performed exclusively by the female. Upon hatching, nestlings almost immediately begin making begging calls (Elderkin 1987). Both parents hunt food for their young, but the male typically delivers what he catches to the female, who tears the items into smaller pieces before providing them to nestlings. Roughly four to five weeks after hatching the flightless young leave the nest and begin using their talons and beak to move along nearby branches (Dunstan and Sample 1972). Young fledge by six weeks of age, and begin flying by week 10. However, they continue to receive parental care until they are at least four months of age. At this time, usually in late summer or early fall, young disperse from the area (Dunstan and Sample 1972).
Interactions with the Northern Spotted Owl
Northern Spotted Owl populations are in significant decline in regions where they co-exist with Barred Owls, believed to be due in part to detrimental interactions between the two. The Barred Owl and Northern Spotted Owl are close phylogenetic relatives, and occupy similar niches (Peterson and Robins 2003). Both are associated with mature forests, and occupy stands of similar composition. They are alike in that rodents and small mammals comprise the core of their diets (Livezey et al. 2008), and both owls nest in tree hollows or the abandoned nests of other species (Hamer et al. 1994). Thus, where they co-occur, these two owls compete for many important resources.
Interactions between these two species are believed to have strong negative impacts on Northern Spotted Owl populations because they are out-competed by the Barred Owl in several respects (Dark et al. 1998). Direct observations suggest that Barred Owls consume more varied prey compared to the Northern Spotted Owl, including far more invertebrates (Livezey et al. 2008). This is believed to confer a competitive advantage to the Barred Owl, as it helps them to endure seasonal changes in food availability, and because it allows them to provision their young with softer, more palatable food items (Livezey et al. 2008). In addition, Barred Owls are larger and more aggressive, allowing them to displace Northern Spotted Owls from their territories and nesting sites (Hamer et al. 1994).
Evidence has also suggested that Northern Spotted Owls are less responsive to conspecific vocalizations when Barred Owls are nearby, possibly as a means of avoiding aggressive encounters (Crozier et al. 2006). Since vocalizations are an important component of intra-pair communication and territory announcement (Ganey 1990), this provides another means by which the presence of Barred Owls may be damaging to the breeding success of Northern Spotted Owls.
Northern Spotted Owls and Barred Owls have been known to hybridize, presenting an additional threat (Kelly and Forsman 2004). F1 hybrids, colloquially known as “Sparred Owls,” have morphologies intermediate between the two parent groups (Hamer et al. 1994). The lower breast and abdomen are marked with a checkerboard of white patches and brown vertical and horizontal streaks. Hybrids are intermediate in colour, appearing darker than Barred Owls but lighter than Northern Spotted Owls. Most hybrids are the result of a male Spotted Owl mating with a female Barred Owl (Kelly and Forsman 2004). F1 hybrid pairs have been known to produce viable offspring (Hamer et al. 1994), and subsequent generations are more difficult to distinguish as hybrids (Kelly and Forsman 2004). Extensive long-term banding studies have detected relatively few hybrids, suggesting that while hybridization does exist, it is a relatively minor threat to Northern Spotted Owl populations (Kelly and Forsman 2004).
Conservation status: Overall, Barred Owls are increasing in number, and their range is expanding (Sauer et al. 1996). Consequently the species is categorized as “Least Concern” by the IUCN. However, it is important to note that these descriptors address the species as a whole, and while Barred Owls are generally doing well, individual populations are in decline in many regions of eastern Canada and the United States, probably due to deforestation (Cadman et al. 1987). Efforts to conserve these declining eastern populations should focus on protecting contiguous old-growth forests. In locations where old-growth forests are being lost, creating artificial nesting sites and maintaining even a small number of mature trees for canopy complexity within younger forests may supply the essential ecological factors for Barred Owls to persist in these areas (US Fish and Wildlife Service 1987).
The primary conservation concern regarding Barred Owls is their negative interaction with the Northern Spotted Owl. Prior to the Barred Owl’s range expansion into the west, Northern Spotted Owls were already experiencing significant population declines, due primarily to logging activity (Noon and McKelvey 1996). Thus, the Barred Owl’s invasion of the American Northwest acts as an additional threat to a species already at risk. Notably, these declines have been most severe in the northern regions of these subspecies’ ranges, where Barred Owls have been present longest. In 1990, the subspecies was listed as federally endangered in the United States (US Fish and Wildlife Service 2004). In 1994, the American government installed the controversial Northwest Forest Plan as part of an effort to protect the Spotted Owl and several other rare species that are dependent on old-growth forests in the region. By reducing logging activity, this plan was intended to prevent habitat destruction and thereby help the Northern Spotted Owl recover (USDA and USDI 1994). More recent research has indicated that habitat protection alone is an insufficient means of protecting the Northern Spotted Owl, as Barred Owls, too, readily inhabit protected old-growth forests and will tend to exclude or out-compete Spotted Owls in these areas (Pearson and Livezey 2003). For instance, an old-growth forest reserve was established in Gifford Pinchot National Forest in Washington State, and Barred Owls now outnumber Spotted Owls in the region (Pearson and Livezey 2007). In 2004, the Northern Spotted Owl’s status as Threatened was sustained, further validating concerns that these management strategies have been ineffective (Courtney et al. 2004).
The United States government has since discussed the controversial approach of removing Barred Owls, by lethal or non-lethal measures, from within the range of the Northern Spotted Owl (Verhovek 2007). This is regarded as the most effective potential approach for protecting Northern Spotted Owls from the Barred Owl threat, although there are several difficulties associated with removal methods (Buchanan et al. 2007). Translocating Barred Owls away from regions inhabited by Spotted Owls would be impractical and expensive, given the large scale at which this would need to be done, while the alternative of shooting Barred Owls has raised ethical concerns (Buchanan et al. 2007). Moreover, both of these options would be poor long-term strategies since, to be effective, they would likely require continual, active removal of Barred Owls for an indefinite period of time (Buchanan et al. 2007). Critics have argued that Barred Owl removal would only be an effective management strategy if it is performed alongside strong habitat-protection efforts (Bernard 2009).
Northern Spotted Owl populations levels continue to decline, and it is believed that there are only 3000 – 5000 breeding pairs left in the wild, most of which are in Washington, Oregon, and California (Knickerbocker 2007). In Canada, there are fewer than 100 pairs remaining (Government of Canada 2002).
Research Needs: Due to the relatively hidden and inaccessible nature of Barred Owl nests, little research has focused on reproduction in this species and their breeding ecology is poorly understood.
Characteristics of the Barred Owl’s habitat, its diet and foraging habits, and its territorial behaviours have been well studied. However, researchers should continue to investigate the ecological factors that cause Barred Owls to be reliant on old-growth forests, as this will aid conservation efforts in areas where deforestation is causing population declines.
Increasing understanding of the nature of competition between Barred Owls and Northern Spotted Owls should be a priority, especially with regards to whether the two owls are competing primarily for space and nesting sites, or food. The US Fish and Wildlife Service is currently conducting small-scale experiments to determine if Barred Owl removal would have the desired positive effect on Northern Spotted Owl populations (US Fish and Wildlife Service 2011). Such studies will provide the information necessary for the future development of effective management strategies.
Literature and Further Reading
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Reviewers: Paul Handford (Univ. Western Ont.) and Shelley Arnott (Queen’s Univ.)