Mexican Spotted Owl General Biology and Ecological Relationships
The Mexican spotted owl (S. o. lucida) is one of three subspecies of spotted owl recognized by the American
Ornithologists' Union (AOU) in its last checklist that included subspecies (AOU 1957:285). The other two subspecies are the northern
(S. o. caurina) and the California spotted owl (S. o. occidentalis). The Mexican
subspecies is geographically isolated from both the California and northern subspecies.
The spotted owl is mottled in appearance with irregular white and brown spots on its abdomen, back and head. The spots of the Mexican
spotted owl are larger and more numerous than in the other two subspecies, giving it a lighter appearance.
Strixoccidentalis translates as "owl of the west"; lucida means "light" or "bright.
" Unlike most owls, spotted owls have dark eyes. Several thin white bands mark an otherwise brown tail.
Based on plumage characteristics, adult male and female spotted owls similar (i.e., monochromatic). However, the sexes can be readily
identified by voice (see below). Juveniles, subadults, and adults can be distinguished by plumage characteristics, however (Forsman 1981,
Moen et al. 1991). Juvenile spotted owls (hatchling to approximately five months) have a downy appearance. Subadults (5 to 26 months)
possess adult plumage but have pointed rectrices with white tips (Forsman 1981, Moen et al. 1991). The rectrices of adults (>27 months)
have rounded and mottled tips.
Although the spotted owl is often referred to as a medium-sized owl, it ranks among the largest owls in North America. Of the 19 species
of owls that occur in North America, only 4 are larger than the spotted owl (Johnsgard 1988). As a full species, the spotted owl averages
41-48 cm (16-19 inches) long (Earhart and Johnson 1970), 107-114 cm (42-45 inches) across the spread wings (Walker 1974), and weighs 547-647
grams (19.5-23 ounces). These measures are expressed as ranges because, similar to other owl species, spotted owls exhibit reversed sexual
dimorphism (i.e., females are larger than males). Adult female Mexican spotted owls in Arizona and New Mexico averaged 647 grams (23 oz)
(SE = 9.6, n = 11), whereas males averaged 547 grams (19.5 oz) (SE = 18.7, n = 12; Ganey and Ward, unpublished data).
The Mexican spotted owl occurs from southern Utah and Colorado south through the mountains of Arizona, New Mexico, and west Texas into the
mountains of central Mexico (McDonald et al. 1991). Gaps remain in our knowledge of the distributional pattern of the Mexican spotted owl
within this range, however. This is especially true in Mexico, where very little of the geographic range of the owl has been surveyed.
Consequently, although the owl appears to be widely distributed in Mexico, we do not know whether its' range is fairly continuous throughout
the Sierra Madre and associated highlands, or whether it is restricted to scattered mountain ranges.
Information gaps also exist in the United States. For example, several mountain ranges in west-central Arizona remain unsurveyed, and numerous
canyon systems that may contain spotted owl habitat in southern Utah have not been surveyed for owls.
Despite these gaps, it is apparent that the Mexican spotted owl is widely but patchily distributed throughout its' range in the United
States, with distribution reflecting the availability of forested mountains and canyons, and in some cases rocky canyonlands. Consequently,
the owl's habitat within the Southwest is naturally fragmented.
Mexican spotted owls nest, roost, forage, and disperse in a diverse assemblage of biotic communities. Mixed-conifer forests are commonly used
throughout most of the range which may include Douglas-fir and/or white fir, with codominant species including southwestern white pine, limber
pine, and ponderosa pine. The understory often contains the above coniferous species as well as broadleaved species such as Gambel oak, maples,
box elder, and/or New Mexico locust. In southern Arizona and Mexico, Madrean pine-oak forests are also commonly used. These forests are
tpically dominated by an overstory of Chihuahua and Apache pines (and probably other species in Mexico) in conjunction with species such as
Douglas-fir, ponderosa pine, and Arizona cypress. Evergreen oaks are typically prominent in the understory (Brown et al. 1980). In the northern
part of the range, including southern Utah, southern Colorado, and far northern Arizona and New Mexico, owls occur primarily in rocky canyons
(Kertell 1977, Reynolds 1990, Rinkevich 1991, Willey 1993).
Spotted owls nest and roost primarily in closed-canopy forests or rocky canyons. They nest in these areas on cliff ledges, in stick nests built
by other birds, on debris platforms in trees, and in tree cavities. In southern Utah, Colorado, and some portions of northern New Mexico, most
nests are in caves or on cliff ledges in rocky canyons. Elsewhere, they also use caves and cliffs, but the majority of nests appear to be in
trees. Forests used for roosting and nesting often contain mature or old-growth stands with complex structure, are typically uneven-aged,
multistoried, and have high canopy closure. A wider variety of trees are used for roosting, but again Douglas-fir is the most commonly used
Several hypotheses have been proposed to explain why spotted owls typically nest in closed-canopy forests. One hypothesis suggests that spotted
owls are relatively intolerant of high temperatures, and roost and nest in shady forests because they provide favorable microclimatic conditions
(Barrows 1981). This could explain why owls typically nest in either closed-canopy forests or deep shady canyons, as both habitat types provide
cool microclimates. Ganey et al. (1993) provided support for this hypothesis. They observed that metabolic rates were higher and rates of
evaporative water loss lower in Mexican spotted owls than in sympatric great horned owls, a habitat generalist. Thus, spotted owls appeared
to produce more metabolic heat than great horned owls, and were less able to dissipate that heat. This may lead them to seek out cool
microclimates during the breeding season (Ganey et al. 1993).
Little is known about patterns of habitat use by foraging owls. The only available data describe habitat use by eight owls occupying five home
ranges on three study areas in northern Arizona (Ganey and Balda 1994). In general, owls clearly foraged in a wider variety of forest conditions
than they used for roosting.
Forsman (1976) described spotted owls as "perch and pounce" predators. They typically locate prey from an elevated perch by sight
or sound, then pounce on the prey and capture it with their talons. Most information on the food habits of Mexican spotted owls comes from
analyses of regurgitated pellets. Specific prey groups identified from spotted owl pellets included woodrats, mice, voles, rabbits, gophers,
bats, birds, reptiles, and arthropods. The diet in most areas was heavily dominated by small, terrestrial, nocturnal mammals, but species
composition varied among areas. Woodrats were generally more abundant in pellet samples collected from northern latitudes, and peromyscid
mice and birds were generally more abundant in southern regions of the owl's range. Voles appear more common in the diet of owls dwelling
at higher elevations.
Courtship begins in March and eggs are laid in late March or, more typically, early April. Incubation begins shortly after the first egg is
laid, and is performed entirely by the female. Female spotted owls generally incubate for approximately 30 days. During incubation, the
female leaves the nest only to defecate, regurgitate pellets, or receive prey delivered by the male, who does most or all of the foraging.
The eggs usually hatch in early May (Ganey 1988). Females brood their young almost constantly, leaving their nests for only brief periods
during the night. Nestling owls fledge from four to five weeks after hatching, from early to mid-June in most cases (Ganey 1988). Owlets
often leave the nest before they can fly, simply jumping from the nest onto surrounding tree branches or the ground. Within a week after
leaving the nest, most owlets can make short, clumsy flights between trees. Three weeks after leaving the nest owlets can hold and tear up
prey on their own, and by late July most have become proficient at pouncing on crawling insects (Forsman et al. 1984). The young depend on
their parents for food during the summer and will eventually disperse out of the natal area in the fall.
In many ways, the Mexican spotted owl appears to be quite similar to both the northern and California spotted owls with respect to general
behavioral patterns and ecology. For example, all three subspecies are most common in forests of complex structure, prey mainly on
nocturnally-active small mammals, and share similar vocalizations and reproductive chronologies.
There are also important differences between the Mexican spotted owl and the other subspecies, however. The distributional pattern of the
Mexican spotted owl is more disjunct than that of the other subspecies, with the possible exception of the California spotted owl population
in the mountain ranges of southern California (Noon and McKelvey 1992). The Mexican subspecies also appears to use a wider range of habitat
types than the other subspecies. These unique aspects of the ecology of the Mexican spotted owl require unique approaches to management for
this owl. For example, both threats to owl habitat and the management recommendations designed to address those threats may well differ among
the diverse habitats occupied by Mexican spotted owls. In addition, because of its' disjunct distributional pattern, dispersal among
subpopulations of Mexican spotted owls is an important consideration. Thus, habitat management plans may need to consider not only areas
occupied by owls but also intervening areas, even where such areas are very different in habitat structure from habitats typically occupied
by spotted owls.
We have learned a great deal about the Mexican spotted owl in the last decade, but significant information gaps still remain. Most studies
of the owl to date have been descriptive rather than experimental. Thus, although we have identified patterns with respect to aspects of the
owls' ecology such as habitat use, cause and effect relationships have not been documented. Further, many aspects of spotted owl demography
and population structure remain unclear. These considerations suggest that much additional research is needed, and that management recommendations
in the near term must deal with significant levels of uncertainty.