Spotted owl (Caurina subspecies) COSEWIC assessment and status report: chapter 6

Biology

The Northern Spotted Owl is one of the most intensely studied birds in North America. It is a high-level predator with specialized habitat requirements, and is at the top of the food chain in some late-successional coniferous forest ecosystems on the west coast of North America. Northern Spotted Owls in British Columbia have adapted to harsher climatic conditions than populations further south; therefore, they may have more genetic capacity to adapt to changing conditions and may play an important role in maintaining the species if large-scale environmental changes occur (SOMIT 1997a). Most details of Northern Spotted Owl biology are known from studies of populations in the United States. All information presented below is from USA sources, except where noted as from British Columbia.

Life cycle and reproduction

Spotted Owls are typically monogamous, although evidence suggests a low occurrence of separation between pairs, possibly due to displacement by another Spotted Owl or as a result of poor past reproductive success (Forsman et al. 2002b). Owls usually first breed at two to three years of age, but some breed when one year old. In late winter and early spring, Spotted Owls begin roosting together near the nest 4 to 6 weeks prior to egg-laying, with copulation generally occurring 2 to 3 weeks before nesting (Forsman et al. 1984). The average clutch size is 2 eggs but 1 egg is also common for the Northern Spotted Owl. The incubation period is estimated to be approximately 30 +/-2 days (Forsman et al. 1984). Females incubate and brood the juveniles while the males provide food for both females and juveniles (Forsman et al. 1984). Most juveniles leave the nest when they are 34-36 days old.

In British Columbia, there are no data on clutch sizes but most nests (2002-2005) with nestlings observed have contained 2 young (2y-6 nests, 1y-3 nests, unknown-2 nests) (Hobbs 2002, 2004, 2005). Young leave the nest in British Columbia from 9-26 June (Hobbs 2005). Of 14 nests monitored for fledging success in British Columbia in 2002-2006, nine fledged at least one young, two were predated, one nest blew down and success was unknown for two nests (Hobbs 2002, 2004, 2005; Table 1). Fledged young then remain near the nest through August and to late September before dispersing. Dispersal in British Columbia has been documented to occur from 10-21 September (Hobbs 2005) and as late as 30 September (I. Blackburn pers. comm. in Hobbs 2005).

Annual reproductive output of a population is largely influenced by the number of pairs that breed each year, with few owls breeding in “poor” years and most owls breeding in “good” years, rather than the productivity of pairs that do breed (Franklin et al. 2002). The majority of Northern Spotted Owls do not breed every year (Gutierrez et al. 1995). In one study area in Oregon, the percentage of breeding females each year averaged 56% (range:18-82%), over 18 years from 1985-2002 (Forsman et al. 2002b). In the USA, mean annual number of young fledged per territorial adult female from 1985-2003 was 0.372 (Anthony et al. 2006). In British Columbia, most nests (57%; n=14) monitored from 2002-2005 fledged young (Table 1). Of nests that fledged young most (56%; n=9) fledged two young (Table 1).

Table 1. Number of young fledged from nests monitored in British Columbia 2002-2006. Data from Hobbs (2002, 2004, 2005) and J. Hobbs pers. comm.
Year Nest No. No. of fledglings
2002 1 2
2002 2 2
2003 1 1
2003 2 2
2003 3 2
2003 4 failed
2004 1 2
2004 2 predated
2004 3 1
2004 4 1 (+1 still in nest)
2005 1 1
2005 2 predated
2006 1 0
2006 2 ?
Total 14 14

Only a small percentage of one- (1.5-8.4%) and two-year- old Northern Spotted Owls (2.6-33.3%) breed. Higher percentages are paired (20-83% for yearlings and 57-74% for 2-year olds) than breed (Forsman et al. 2002a), and it seems that breeding is delayed for a year or more after initial pair formation. In Oregon, the average age of pair formation was 2.4 years for males and 1.9 years for females, and the average age of first breeding was 3.9 years for males and 3.4 years for females (Forsman et al. 2002b).

Physiology, foraging and nutrition

Spotted Owls have three adaptations for nocturnal predation; acute eyesight, acute hearing and modified feathers for silent flight (USDI 1992). Northern Spotted Owls feed primarily on arboreal and semi-arboreal rodents at night, typically by perching and waiting to detect prey by sight or sound (Gutiérrez et al. 1995). In the northern portion of their range, analysis of regurgitated pellets on a percent frequency basis indicated the main food items are northern flying squirrels (Glaucomys sabrinus –32%), red tree voles (Phenacomys longicaudus –12%), deer mice (Peromyscus spp. –10%), red-backed voles and field voles (Clethrionomys/Microtus spp. –10%), and bushy-tailed woodrats (Neotoma cinerea – 7%) (Gutiérrez et al. 1995).

A recent study of three areas in western Washington confirmed that northern flying squirrels were the most frequently taken prey (29-54% numbers of prey) and comprised the highest percentage of diet by weight (45-59% biomass) in wet habitats. Other primary prey were snowshoe hare (Lepus americanus), bushy-tailed woodrat, red-backed vole, and mice (Forsman et al. 2001). Observed differences between years and areas were thought to be mainly related to variation in abundances of prey species (Forsman et al. 2001).

In British Columbia, northern flying squirrels, bushy-tailed woodrats, and deer mice are the most common prey of Northern Spotted Owls (Horoupian et al. 2000), which is similar to diets in Washington. Squirrels comprise 64.6% of the biomass consumed. Of this biomass, northern flying squirrels contribute 41.2%, other tree squirrels (Tamiasciurus sp.) contribute 0.8% and unidentified squirrel-sized prey a further 22.6%. Deer mice contribute little energetically, due to their small size. Horoupian et al. (2000) found that the percentages of prey species based on numbers of prey consumed do not differ between wet coastal forests and dry interior forests of British Columbia, but woodrats comprise 99% of diet in at least some interior British Columbia (drier sites) nests (J. Hobbs pers. comm.). Birds are rare in Northern Spotted Owl diets but Hobbs (2005) reports one case of predation on a Dark-eyed Junco (Junco hyemalis).

Home range

Home ranges are relatively large for a forest bird. Northern Spotted Owls are non-migratory, and adults typically occupy the same home range year-round (Gutiérrez et al. 1995). Home ranges tend to increase from south to north latitude, from drier to wetter ecosystems, and with increasing fragmentation of old forest (Forsman et al. 1984, 2005; Thomas et al. 1990; Carey et al. 1990; Glenn et al. 2004). Home range size may also be larger in areas where the main Spotted Owl prey species is the northern flying squirrel (Forsman et al. 2005). Home range sizes are smaller in the breeding season than in the non-breeding season as owls concentrate activities closer to their nest (Carey et al. 1990; Forsman et al. 2005).

Home range of adjacent pairs usually overlaps to some degree (Thomas et al. 1990). In Oregon, neighbouring owls occupying adjacent territories shared an average of 12% of their home range. These overlapping areas tended to occur on the periphery of home ranges, in areas where owls spend a small portion of their time (Forsman et al. 1984). On Washington’s Olympic Peninsula, individual home ranges are known to overlap about 70% (Forsman et al. 2005). Other studies in Washington found home ranges from 2100-4000 ha (reviewed by Gutierrez et al. 1995).

In Washington State, median annual home ranges of Spotted Owl pairs were larger in the moister western forests (3300 ha, 67% suitable habitat) than the drier eastern forests (2700 ha, 71% suitable habitat) (Hanson et al. 1993). This pattern likely also occurs in British Columbia. In British Columbia, home range estimates for 3 pairs of owls in dryer ecosystems ranged from about 1,400 ha to 4,600 ha, with suitable habitat compositions ranging from 60% to 66% (Blackburn and Godwin 2003). In British Columbia, one radio-tagged male was observed roosting with the female and juvenile 1.5 km away from the nest suggesting a minimum natal rearing area of 700 ha (Blackburn and Godwin 2003).

CSORT has determined that home range area requirements (minimum area of suitable habitat required per territory and maximum feasible territory size) are reduced as habitat changes from coastal (Maritime) to interior (Continental) subregions. The mean amount of suitable habitat required for a territory is now thought to be: Maritime subregion, 3055 ha, Submaritime, 2211 ha; Continental, 1,912 ha (Chutter et al. 2007).

Behaviour

The Northern Spotted Owl is an easily approached owl and appears relatively “tame” for such a rare and reclusive species. However, the Spotted Owl may actively defend its nest and young from predators and biologists that approach too close (Forsman 1976; Gutiérrez et al. 1995).

Northern Spotted Owls are easily subjected to heat stress and behaviourally thermoregulate through their choice of roosting habitats (Barrows 1981). Owls respond to variation in temperature by moving within the canopy to more favourable microclimates. In summer, when temperatures are warmer, roosts tend to be in cool, shady areas. Their narrow preferred range of ambient temperature has been hypothesized as one of the reasons they strongly select mature and old-growth forest habitats (Gutiérrez et al. 1995).

Radio-telemetry data from three dispersing juveniles in British Columbia showed that owls reacted to snow > few inches depth by moving long distances in short periods of time (Hobbs 2004), presumably related to either thermal stress or lower availability of food. This behaviour was not observed in two radio-tagged owls tracked in 2004/05 as snow accumulations didn’t persist or reach any depth greater than a few inches (Hobbs 2005). Hobbs (2005) observed a remarkable reaction to a large wildfire near Lillooet when a male, female and two fledged young remained roosting calmly approximately 50 m from a burning forest with intense smoke, even when the site was water-bombed by helicopters.

Survival and recruitment

Survival of adults is relatively high (0.750-0.886 annually) and adults are long-lived; some wild individuals are more than 17 years old (Gutierrez et al. 1995). Adults have greater survival rates than one (0.415-0.860) or 2 year olds (0.626-0.886) (Anthony et al. 2006). Survival of juveniles is low and it is typically considered a major limiting factor in population recruitment (Gutiérrez et al. 1995). About 50% (Forsman et al. 2002a) to 70% (Blakesley et al. 2001) of fledglings die before or during dispersal and only about 25% survive their first winter (Gutiérrez et al. 1995).

In British Columbia, 6 of 7 fledglings fitted with radio tags between 2002-2005 have died: four starved, 1 was depredated and 1 died of unknown causes (Hobbs 2005; Hausleitner 2006). The fate of the seventh birds is unknown as the radio transmitter failed (Hobbs 2005).

Recruitment into the British Columbia population has essentially been zero since monitoring began in the 1990s; only one juvenile fledged in 2005 and none fledged in 2006 (Hausleitner 2006).

Predation and competition

Northern Goshawks (Accipiter gentilis), Cooper’s Hawks (Accipiter cooperi), Red-tailed Hawks, Great Horned Owls (Bubo virginianus) (Johnson 1992; Forsman et al. 2002a) and Barred Owls (Leskiw and Gutiérrez 1998) have been implicated as avian predators. Great Horned Owls inhabit forest edge habitats and are suspected to be the primary predator of the Northern Spotted Owl (Forsman et al. 1984; Carey et al. 1992). Predation accounts for a majority of mortality (68.0%) of juveniles, mainly due to avian predators (81%), with Great Horned Owls suspected to be the major cause (Forsman et al. 2002a). Some mammals (e.g., fisher (Martes pennanti) may eat Spotted Owl eggs and young (Gutiérrez et al. 1995) and there is a case of suspected predations of a juvenile Spotted Owl by a black bear (Ursus americanus) (J. Hobbs pers. comm.).

In British Columbia, one of only two active nests in 2005 was destroyed by an unknown predator which killed the adult female and nestling(s) at the nest (Hobbs 2005). In 2004, one nestling Northern Spotted Owl was thought to have been killed by a Northern Goshawk and one was killed by a Great Horned Owl (Hobbs 2005).

It seems logical that predation by Great Horned Owls, Barred Owls, and Red-tailed Hawks will likely occur more frequently as fragmentation increases because these species occur more frequently in more fragmented forests. These relationships, however, remain unproven (Courtney et al. 2004).

Barred Owls (Strix varia) and Great Horned Owls (Bubo virginianus) are known to occur within the range of Northern Spotted Owls and these owls compete for food (Gutiérrez et al. 1995) and space (Kelly et al. 2003). Great Horned Owls and Barred Owls are habitat generalists and occur in a wider variety of forested habitats than the more habitat-specialized Northern Spotted Owl. Northern Spotted Owls within a fragmented habitat landscape have larger home ranges than those within more contiguous habitats (Forsman et al 1984), resulting in more overlap with habitats used by Great Horned Owls and Barred Owls and consequently higher risk of competition.

Competition with Barred Owls is widely accepted as a threat to Northern Spotted Owls through increased competition for space and prey, increased predation and hybridization (Wilcove 1987; Carey et al. 1992; SOMIT 1997a). Other studies show a negative effect on Spotted Owl survival if Barred Owls are present (Anthony et al. 2006). Strong territorial defence has been observed between Barred Owls and Northern Spotted Owls, and Barred Owls are known to displace Northern Spotted Owls in the USA (Hamer 1988; Hamer et al. 2001; Kelly 2002; Kelly et al. 2003; Herter 2004). Kelly’s study showed that Barred Owls sometimes displaced Northern Spotted Owls from their territories when Barred Owls occurred within 0.8 km of a Northern Spotted Owl territory centre. Those Northern Spotted Owls that were not displaced continued to maintain their normal reproductive output, but regional reproductive output was lower because there were fewer Northern Spotted Owls (Kelly 2002). Conversely, one other study in Oregon found that Barred Owls displaced some Northern Spotted Owls but the study population of Northern Spotted Owls has not declined (Forsman et al. 2002b).

Competition for food between Barred Owls and Northern Spotted Owls seems likely as diets greatly overlap (76% in one study in western Washington), food is limiting in some years, and Barred Owls have moved into much of the Northern Spotted Owl’s range (Hamer et al. 2001).  

Competition with Barred Owls is a relatively new phenomenon as previously the two species were geographically separated. In the 1960s, Barred Owls expanded their range westward and southward so that it overlapped that of the Northern Spotted Owl in British Columbia (Campbell et al. 1990; Dunbar et al. 1991) and the USA (Hamer 1988; Gutiérrez et al. 1995). The range expansion is thought to have been a response to fragmentation of coniferous forest habitat and climate change in the boreal forest.

The extent of the potential for competition in British Columbia is illustrated by the discovery in the early 1990s that within the range of Northern Spotted Owl, Barred Owls were four times more abundant than Northern Spotted Owls (Dunbar and Blackburn 1994). This fact was confirmed more recently (2004-2005) as surveys for Spotted Owls in suitable Spotted Owl habitat in British Columbia found responses by Barred Owls were 3-6 times more frequent than responses by Northern Spotted Owls (Hobbs 2004, 2005; Keystone 2004). There is no doubt that Barred Owls are now relatively common throughout British Columbia Northern Spotted Owl habitat.

Dispersal/migration

The nature of dispersal has recently been well-documented in studies in Washington and Oregon. Fledged young disperse in random directions from natal areas in September and October, moving rapidly in a series of movements before settling into temporary home ranges in late October and November. The latest date for juveniles remaining with their parents in British Columbia is September 30 (I. Blackburn pers. comm.). Dispersal may be influenced by barriers such as high elevation terrain, large bodies of water and large open areas of unsuitable habitat (Thomas et al. 1990, Miller et al. 1997, Forsman et al. 2002a). A second dispersal then occurs in February through April with many juveniles moving considerable distances before settling into summer ranges. Thereafter, some one-year- old owls remained in their second-summer range whereas others moved and occupied one or more temporary home ranges until settling on permanent territories when 2-5 years of age.

Dispersal directions, after the first dispersal movement, continued to be random. Final dispersal distances (from natal area to permanent territory) ranged from 0.6 km to 111 km, median distances were about 14 km for males and 24 km for females, and only 8.7% dispersed >50 km (Forsman et al. 2002a). In western Oregon the average dispersal distance was 32 + 14 km (Miller et al. 1997). A small percentage (~6%) of non-juveniles disperse as well. These birds tend to be females, younger owls, owls that did not have a mate the previous year or owls that have lost mates (Forsman et al. 2002a).

The quality of dispersal habitat is likely an important factor in survival of dispersing birds. Dispersing owls used both continuous older forest and a fragmented mosaic of various-aged forests, clearcuts, roads, and non-forested areas (Forsman et al. 2002a). Large non-forested valleys are known to act as barriers to dispersal in western Oregon between the Coastal and Cascade Mountains but dispersal did occur in some broad, forested foothills between the same ranges (Forsman et al. 2002a). Although dispersing juveniles selected equally between less fragmented forest and more fragmented forests, net dispersal distance decreased with the increased use of clearcuts, suggesting that clearcuts may represent a partial barrier to dispersal (Miller et al. 1997).

In British Columbia, dispersal and settlement of juvenile Northern Spotted Owls appears to be not occurring at present as no new owls are showing up in monitored sites, many single adult owls remain unmated, and no radio-tracked juveniles survive beyond their first winter (Hobbs 2002, 2004, 2005). Hobbs (2005) did track one juvenile to cross a large lake and move a substantial distance during the fall dispersal phase.

Diseases and parasites

Thomas et al. (2002) diagnosed acute septicemic spirochetosis in a dead adult male from Washington State. The organism was identified as a member of the Borrelia genus, the most closely related species being B. hermsii, an agent of relapsing fever in humans in the western United States. This was the first report of a relapsing fever-related Borrelia in a wild bird (Thomas et al. 2002). Avian cholera has been reported from one dead owl (Forsman et al. 2002a).

The West Nile virus is a disease of concern, given the small population of Canadian Northern Spotted Owls. Originally the West Nile virus was known only from Africa, West Asia, and the Middle East. This virus was first isolated in the Western Hemisphere in New York in 1999, and has since spread rapidly across North America (Canadian Cooperative Wildlife Health Centre 2003). Although birds infected with West Nile virus can become ill or die, most infected birds survive as carriers (Centers for Disease Control and Prevention 2003). West Nile Virus has not yet been documented in wild Northern Spotted Owls.

Various parasites are ubiquitous in populations of wildlife and their effects on survivorship are likely related to the extent of the infestation. Threats from parasites are much less than those from diseases such as West Nile Virus. Known parasites of Northern Spotted Owls include blood parasites (hematozoans), helminthes (nematodes, cestodes, and acanthocephalans) and louse flies (Hippoboscidae) (Gutiérrez et al. 1995). Of 105 owls sampled from all three subspecies, all were infected with hematozoan parasites and multi-species infections were common (Gutiérrez et al. 1995). A majority of dead owls examined by Forsman et al. (2002a) were infested with blood or intestinal parasites.

Adaptability

The Northern Spotted Owl is adapted for very specific ecological conditions. It seems unable to cope with significant amounts of habitat fragmentation and increased levels of predation and competition from species that are better adapted to fragmented landscapes.

Northern Spotted Owls in British Columbia have adapted to harsher climatic conditions than populations further south; therefore, they may have more genetic capacity to adapt to changing conditions and may play an important role in maintaining the species if large-scale environmental changes occur (SOMIT 1997a).

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