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NORTHERN SPOTTED OWL (Strix occidentalis caurina)

4. Biologically Limiting Factors

4.1 Recruitment

Spotted Owl populations naturally have low fecundity and low juvenile survivorship that are offset by high survivorship of adults. Inter-annual variation in these processes are less critical in large populations and across large spatial scales. However, effects of low recruitment on population growth rate may be exaggerated because of the small population and its increasingly fragmented distribution found currently in British Columbia. These effects may hinder chances of the population’s recovery. Despite banding and monitoring efforts, there is no evidence of successful recruitment in recent years and the adult population is aging. Without recruitment, the population will continue to decline as adult birds age and die. In British Columbia, causes of low recruitment are unknown but likely include nesting and dispersal habitat issues, competition and predation, and availability of prey. Recruitment must be increased to achieve a self-sustaining Spotted Owl population.

4.1.1 Reproductive Rate

Annual reproductive output of a population is largely influenced by the number of pairs that breed each year. There is substantial inter-annual variation in the number of pairs that breed, with few owls breeding in “poor” years and most owls breeding in “good” years (Franklin et al. 2002). Most Spotted Owls do not breed every year (Gutiérrez et al. 1995). In one study area in Oregon, the percentage of breeding females each year averaged 56% (range, 18 to 82%) over 18 years from 1985 to 2002 (Forsman et al. 2002b). Clutch sizes typically range from 1 to 3 eggs. Of 2113 broods examined from 1984 to1994, 42%, 56%, and 2% contained 1, 2, and 3 fledged young, respectively (Gutiérrez et al. 1995). Reproductive rates may be influenced by prey abundance, environmental conditions, and predator effects.

4.1.2 Age of First Breeding

Although some Spotted Owls breed as 1- or 2-year-olds, most breed beginning as 3-year-olds. Between 1.5 and 8.4% of 1-year-old birds and 2.6% to 33.3% of 2-year-old birds will breed (Forsman et al. 2002a). Despite this infrequent incidence of sub-adult breeding, 20 to 83% of first-year and 57 to 74% of second-year birds may be paired (Forsman et al. 2002b). 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).

4.1.3 Dispersal and Survival

Juvenile Spotted Owls disperse from the natal area at the end of the breeding period. Dispersal generally refers to the movement of juveniles away from the natal territory and includes all subsequent movements until territory acquisition or death (Miller et al. 1997). This movement begins in September and October. The latest date for juveniles observed with their parents in British Columbia is 28 September (Blackburn and Godwin 2003). Some owls do not find permanent territories until 2 to 5 years of age. About 6% of non-juveniles disperse as well. These birds tend to be females, or owls that did not breed the previous year or had lost their mate (Forsman et al. 2002a).

Juveniles exhibit two distinct phases of dispersal behaviour. The first is characterized by rapid movement away from the natal area and is referred to as transience (Miller et al. 1997). This phase of dispersal is typically followed by the colonization phase wherein the owl settles in a general area (e.g., an area of 2.4 km2 or less) for more than a few days (Miller et al. 1997). The overall dispersal period may consist of multiple occurrences of transience and colonization and, in the United States, the dispersal direction is generally random (Miller et al. 1997; Forsman et al. 2002a).

Dispersal involves substantial movements across the forested landscape. Dispersal of juveniles tends to occur in random directions and juveniles travel variable distances before settling on territories (Forsman et al. 2002a). Final dispersal distances ranged from 0.6 to 111 km, median distances were about 14 km for males and 24 km for females, and only 8.7% of the owls dispersed more than 50 km (n = 940; Forsman et al. 2002a). Breeding-age owls also occasionally disperse, especially those who have lost their mates. These movements may be to new territories, or back and forth movements between alternate territories (Forsman et al. 2002a). Most studies did not follow the subject owls frequently enough to measure all of the movements that constituted the dispersal phase. Consequently, the measure of movement reported in the literature is the straight-line distance between natal site and the site of death or territory establishment; the published values do not represent the total distance traveled.

Spotted Owl dispersal behaviour may be influenced by the presence of potential 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). Miller et al. (1997) found that although dispersing juveniles selected equally between less fragmented forest and more fragmented forest, net dispersal distance decreased with the increased area of clearcuts, suggesting that clearcuts may represent a partial barrier to dispersal (Miller et al. 1997). The quality of dispersal habitat is likely an important factor in survival of dispersing birds. Dispersing owls are found in both continuous older forest and fragmented landscapes containing mosaics of various-aged forests, clearcuts, roads, and non-forested areas (Forsman et al. 2002a). Large non-forested valleys are known barriers to dispersal in western Oregon between the Coastal and Cascade ranges, but owls did disperse through broad, forested foothills between those areas (Forsman et al. 2002a).

Survival of juveniles is low and is typically considered a major limiting factor in population recruitment (Gutiérrez et al. 1995). About 50% of fledglings die before or during dispersal (Forsman et al. 2002a), whereas older owls (2+ years old) are longer lived, averaging an 84% annual survival (Gutiérrez et al. 1995; Franklin et al. 1999). Some banded individuals are known to have survived to at least 16 to 17 years in the wild (Gutiérrez et al. 1995).


4.2 Habitat Requirements

Throughout its range, the Spotted Owl is strongly associated with mature and old, late-successional coniferous and mixed-coniferous forests. These forests are typically characterized by an uneven-aged cohort of trees; a multi-layered, relatively closed canopy; numerous large trees with broken tops, deformed limbs, and large cavities; and numerous large snags and accumulations of logs and downed woody debris (Thomas et al. 1990; USDI 1992). In moist parts of the range, these habitat characteristics are found naturally in late seral and old forests (see Appendix 5). In drier parts of their range (i.e., east of the Cascade Range), owls have been observed in younger forest stands where similar structural complexity was created by fire, wind events, selective logging, or disease factors such as root rot or mistletoe infections (Dunbar and Blackburn 1994; Buchanan et al. 1995).

4.2.1 Nest Structures

Territory establishment may be determined by the availability of nesting structures. Spotted Owls do not build their own nests, but depend on naturally occurring or previously constructed nest sites that typically possess nest platforms that are at least 50 cm in diameter. Nest sites include broken treetops, tree cavities, abandoned raptor nests, mistletoe brooms, and debris accumulations captured in clusters of branches (Forsman et al. 1984; Dawson et al. 1986). In general, cavities are used in moist climates, platforms are used in drier climates, particularly where cavities larger than 50 cm in diameter are not available. A variety of different trees are used for nesting throughout North America, suggesting, to a certain extent, that the presence of structures (i.e., cavities or platforms) is more important than the species of tree in nest site selection (Forsman and Giese 1997). Pairs may re-use the same nest each year or use alternate nest sites (Gutiérrez et al. 1995).

4.2.2 Home Ranges

Spotted Owls are non-migratory, and their home ranges, particularly in the northern parts of their distribution, are very large (Gutiérrez et al. 1995). Home ranges tend to increase in size from south to north, from drier to wetter ecosystems, and with increasing fragmentation (Forsman et al. 1984; Carey et al. 1990; Thomas et al. 1990). Home range sizes are smaller in the breeding season, when owls concentrate activities closer to their nests, than in the non-breeding season (Carey et al. 1990). Although Spotted Owls are site-faithful in most parts of their range, shifting among sites is not uncommon in the eastern Cascade Range in Washington (Herter and Hicks 1995).

In Washington, median annual home ranges of Spotted Owl pairs were larger in the moister western forests (3321 ha, 67% suitable habitat) than the drier eastern forests (2675 ha, 71% suitable habitat; Hanson et al. 1993). Other studies in Washington found home ranges varied from 2100 to 4000 ha (reviewed by Gutiérrez et al. 1995). Preliminary home range size estimates from telemetry studies in British Columbia appear to be consistent with estimates from Washington (Blackburn and Godwin 2003). The large home ranges used by Spotted Owls may be a response to the low density of the owl’s principal prey species and the amount of habitat needed to find sufficient food (Carey et al. 1992).

Territory establishment by Spotted Owls appears to be limited by the amount, quality, and distribution of habitat found within the landscape. Higher fecundity and adult survivorship is associated with larger amounts of suitable habitat in areas surrounding the nest area (Bart 1995; see Meyer et al. 1998).

4.2.3 Thermal Requirements

Spotted Owls are easily subjected to heat stress and behaviourally thermoregulate through their choice of roosting habitats (Barrows 1981). Owls respond to variations 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 may be one of the reasons they strongly select mature and old forest habitats (Gutiérrez et al. 1995).


4.3 Prey Availability

Spotted Owls feed primarily on arboreal and semi-arboreal small mammals. They forage primarily at night, typically by perching and waiting to detect prey by sight or sound (Gutiérrez et al. 1995). A recent study of three areas in western Washington confirmed that Northern Flying Squirrels (Glaucomys sabrinus), which are also nocturnal, were the most frequently taken prey (29 to 54% numbers of prey) and made up the highest percentage of diet by weight (45 to 59% biomass). Other primary prey were Snowshoe Hares (Lepus americanus), Bushy-tailed Woodrats (Neotoma cinerea), Red-backed Voles (Clethrionomys sp.), and mice (Forsman et al. 2001). Observed differences between years and areas were thought to be mainly related to variation in abundance of prey species (Forsman et al. 2001).

In British Columbia, Northern Flying Squirrels, Bushy-tailed Woodrats, and deer mice (Peromyscus sp.) are the most common prey of Spotted Owls (Horoupian et al. in prep), similar to diets in Washington. Tree squirrels comprise 64.6% of the biomass consumed (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, because of their small size. The percentages of prey species based on numbers of prey consumed do not differ between the wet coastal forests and dry interior forests of British Columbia (Horoupian et al. in prep.), although woodrats were more important food at one nest in the Cascade Forest District (Hilton and Hilton 2002).

Numbers of Spotted Owls may fluctuate in response to prey availability. Prey may become limited through natural fluctuations in climatic conditions, suppression by other predators (e.g., Barred Owls), or by the amount and quality of habitat types found in the landscape. Carey et al. (1992) reported that Northern Flying Squirrels tend to be more abundant in old forests than in young forests; but their density in old forests is also low. In British Columbia, Ransome and Sullivan (2003) found the density of Northern Flying Squirrels in old forest in the wet coastal ecosystem to be 1.5 +/-1.8 squirrels/ha (range, 0.3 to 2.9), and in 80-year-old second-growth stands to be 1.0 +/-1.4 squirrels/ha (range, 0.06 to 1.8). They concluded that although quality of habitat provided by old forests can be better than that in second-growth forests, Northern Flying Squirrels are not consistently more abundant in old forests than in 80-year-old forests. However, their results support a pattern of increasing numbers of Northern Flying Squirrels in old forests. As well, the 0.5-squirrel difference between habitat types may have biological importance to the survival of the Spotted Owl. Because of the low density of this principal prey species, Carey et al. (1992) hypothesized that the Spotted Owl’s use of large home ranges may be in response to the amount of habitat needed to search and find enough food to sustain itself.


4.4 Causes of Mortality

Although many Spotted Owls have been known to succumb to starvation due to low prey availability, there are other natural causes of mortality on juveniles and adult Spotted Owls.

4.4.1 Predation

Although predation has not been identified as a limiting factor causing declines in Spotted Owl populations (Gutiérrez et al. 1995), predation accounts for most (68.0%) juvenile deaths, mainly attributed to avian predators (81%). Great Horned Owls (Bubo virginianus) are suspected to be the major cause (Forsman et al. 2002a). Other avian predators include Northern Goshawks (Accipiter gentilis), Red-tailed Hawks (Buteo jamaicensis), Common Ravens (Corvus corax), and possibly Barred Owls (Strix varia) (Forsman et al. 2002a). The remaining juvenile mortality is due to mammalian predators. Adults also fall victim to predators. The effects of predation may be more significant for small populations of Spotted Owls, such as those found in British Columbia, because the loss of individuals may have a relatively higher impact than in the larger populations in the United States.

4.4.2 Disease and Parasites

Little is known about diseases and parasites that affect Northern Spotted Owls (Gutiérrez et al. 1995). It is thought that disease or parasites may contribute to the cause of death because 99% of the juveniles that starved to death were infected with either parasites or disease (Forsman et al. 2002a). It has been well documented that high parasite loads may reduce individual fitness in birds (Moller 1993; Loye and Carroll 1998), but because parasites are ubiquitous in wildlife populations, it is likely that mortality resulting from parasites is related to the extent of infestation.

Several diseases or parasites have been documented in Spotted Owls. Avian cholera (Forsman et al. 2002a) and acute septicemic spirochetosis (Thomas et al. 2002) have been documented from the Pacific Northwest. Other parasites include blood parasites (hematozoans), helminths (nematodes, cestodes, and acanthocephalans), and louse flies (Hippoboscidae) (Gutiérrez et al. 1995). Of 105 Spotted Owls sampled from all three subspecies, all were infected with hematozoan parasites and multi-species infections were common (Gutiérrez et al. 1995). Most dead owls examined by Forsman et al. (2002a) were infested with blood or intestinal parasites. In addition to the importance of parasites to individual fitness, parasite presence may influence nest site selection in some birds (Loye and Carroll 1998), although this has not been demonstrated in Spotted Owls. Although West Nile virus has not been reported in the Spotted Owl this virus has the potential to impact Spotted Owls at the population level in British Columbia.