Cactus Ferruginous Pygmy-owl (Glaucidium brasilianum cactorum)

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Cactus Ferruginous Pygmy-owl (Glaucidium brasilianum cactorum)

Background

The cactus ferruginous pygmy-owl (Glaucidium brasilianum cactorum) (pygmy-owl) is in the order Strigiformes and the family Strigidae. It is a small bird, approximately 17 centimeters (cm) (6.75 inches (in)) long. Males average 62 grams (g) (2.2 ounces (oz)), and females average 75 g (2.6 oz). The pygmy-owl is reddish brown overall, with a cream- colored belly streaked with reddish brown. Color may vary, with some individuals being more grayish brown. The crown is lightly streaked, and a pair of black/dark brown spots outlined in white occur on the nape suggesting ``eyes.'' This species lacks ear tufts, and the eyes are yellow. The tail is relatively long for an owl and is colored reddish brown with darker brown bars. The pygmy-owl is primarily diurnal (active during daylight) with crepuscular (active at dawn and dusk) tendencies. They can be heard making a long, monotonous series of short, repetitive notes, mostly during the breeding season.

The pygmy-owl is one of four subspecies of the ferruginous pygmy- owl. It occurs from lowland central Arizona south through western Mexico to the States of Colima and Michoacan, and from southern Texas south through the Mexican States of Tamaulipas and Nuevo Leon. Only the Arizona population of the pygmy-owl is listed as an endangered species (62 FR 10730; March 10, 1997).

The total number of pygmy-owls and their distribution in Arizona are unknown. Survey and monitoring work in Arizona resulted in documenting 41 adult pygmy-owls in 1999, 34 in 2000, 36 in 2001, and, most recently, 18 in 2002. A cumulative total of 85 occupied sites (includes both single or paired birds) were recorded during these 4 years (Abbate et al. 1999, 2000, AGFD unpubl. data). Most of these pygmy-owls were distributed in four general areas: northwest Tucson, southern Pinal County, Organ Pipe Cactus National Monument, and the Altar Valley. We believe that more pygmy-owls exist in Arizona, but systematic surveys have not been conducted in all areas of potential habitat.

In addition, recent survey information has shown pygmy-owls to be more numerous adjacent to and near the Arizona border in Mexico (Flesch and Steidl 2000). There also exists considerable unsurveyed habitat on the Tohono O'odham Nation, and, although we have no means of quantifying this habitat, the distribution of recent sightings on non- Tribal areas east, west, and south of the U.S. portion of the Tohono O'odham Nation lead us to reasonably conclude that these Tribal lands may support meaningful numbers of pygmy-owls. Consequently, we believe that it is highly likely that the overall pygmy-owl population in Arizona is maintained by the movement and dispersal of owls among groups of pygmy-owls in southern Arizona and northern Mexico resulting from the connectivity of suitable habitat. The extent to which pygmy- owls disperse across the U.S./Mexico border is unknown. Therefore, addressing habitat connectivity and the movements of pygmy-owls within Arizona is the primary consideration of this proposal due to the importance of maintaining dispersal and movement among pygmy-owl groups.

Given recent data, it is probable that conservation of the pygmy- owl in Arizona requires both sufficient numbers and productivity of pygmy-owls north of the border and immigration of pygmy-owls from Mexico into Arizona, although we do not know at this time to what extent immigration does or needs to occur.

The patchy, dispersed nature of the pygmy-owl population in Arizona suggests that the overall population may function as a metapopulation. A metapopulation is a set of [[Page 71033]] subpopulations within an area, where movement and exchange of individuals among population segments is possible, but not routine. A metapopulation's persistence depends on the combined dynamics of the productivity of subpopulations, the maintenance of genetic diversity, the availability of suitable habitat for maintenance and expansion of subpopulations, and the ``rescue'' of subpopulations that have experienced local extinctions by the subsequent recolonization of these areas by dispersal from adjacent population segments (Hanski 1999, Hanski and Gilpin 1991, 1997). The local groups of pygmy-owls within Arizona may function as subpopulations within the context of metapopulation theory. However, more information is needed regarding the population dynamics of pygmy-owls in Arizona.

Historically, pygmy-owls were recorded in association with riparian woodlands in central and southern Arizona (Bendire 1892, Gilman 1909, Johnson et al. 1987). Plants present in these riparian communities included cottonwood (Populus fremontii), willow (Salix spp.), ash (Fraxinus velutina), and hackberry (Celtis spp.). However, recent records have documented pygmy-owls in a variety of vegetation communities such as riparian woodlands, mesquite (Prosopis velutina, and P. glandulosa) bosques (Spanish for woodlands), Sonoran desertscrub, semidesert grassland, and Sonoran savanna grassland communities (see Brown 1994 for a description of these vegetation communities). While native and nonnative plant species composition differs among these communities, there are certain unifying characteristics such as the presence of vegetation in fairly dense thickets or woodlands, the presence of trees, saguaros (Carnegiea giganteus), or organ pipe cactus (Stenocereus thurberi) large enough to support cavities for nesting, and elevations below 1,200 meters (m) (4,000 feet (ft)) (Swarth 1914, Karalus and Eckert 1974, Monson and Phillips 1981, Johnsgard 1988, Enriquez-Rocha et al. 1993, Proudfoot and Johnson 2000). Large trees provide canopy cover and cavities used for nesting, while the density of mid- and lower-story vegetation provides foraging habitat and protection from predators, and it contributes to the occurrence of prey items (Wilcox et al. 2000).

The density of trees and the amount of canopy cover preferred by pygmy-owls in Arizona has not been fully defined. However, preliminary results from a habitat selection study indicate that nest sites tend to have a higher degree of canopy cover and higher vegetation diversity than random sites (Wilcox et al. 2000). Overall vegetation density may not be as important as patches of dense vegetation with a developed canopy layer interspersed with open areas. Vegetation structure may be more important than species composition (Wilcox et al. 1999, Cartron et al. 2000). This is related to the fact that canopy cover and layers of vegetation provide hunting perches, thermal cover, and promote predator avoidance regardless of species. Larger trees with greater canopy also have a greater potential to support cavities needed for nesting. Flesch (1999) indicated that areas with large trees and canopy coverage are likely important areas for pygmy-owls in the Altar Valley. Riparian and xeroriparian (dry washes) areas, which are often used by pygmy-owls, are generally characterized by increased vegetation layers, higher plant diversity and larger tree sizes because of increased moisture availability. Food

Pygmy-owls typically hunt from perches in trees with dense foliage using a perch-and-wait strategy; therefore, sufficient cover must be present within their home range for them to successfully hunt and survive. Pygmy-owls also hunt by inspecting tree and saguaro cavities for other nesting birds, and possibly bats. Their diverse diet includes birds, lizards, insects, and small mammals (Bendire 1888, Sutton 1951, Sprunt 1955, Earhart and Johnson 1970, Oberholser 1974, Proudfoot 1996, Abbate et al. 1996,1999). Observations in Arizona from 1996 through 1998 indicate that reptiles, birds, mammals, and insects were 44, 23, 6, and 3 percent, respectively, of pygmy-owl prey deliveries recorded; 24 percent were unidentified (Abbate et al. 1999). It is likely that use of insects was underestimated in these observations because of the speed at which they are consumed and the difficulty in observing such small prey items. The density of annual plants and grasses, as well as shrubs, may be important to enhancing the pygmy-owl's prey base.

Vegetation communities which provide a diversity of structural layers [[Page 71038]] and plant species likely contribute to the availability of prey for pygmy-owls (Wilcox et al. 2000). Pygmy-owls also utilize different groups of prey species on a seasonal basis. For example, lizards, small mammals, and insects are utilized as available during the spring and summer during periods of warm temperatures (Abbate et al. 1999). However, during winter months, when low temperatures reduce the activity by these prey groups, pygmy-owls likely turn to birds as their primary source of food and appear to expand their use area in response to reduced prey availability (Proudfoot 1996). Therefore, conservation of the pygmy-owl should include consideration of the habitat needs of prey species, including structural and species diversity and seasonal availability. Pygmy-owl habitat must provide sufficient prey base and cover from which to hunt in an appropriate configuration and proximity to nest and roost sites. Water

Free-standing water does not appear to be necessary for the survival of pygmy-owls. During many hours of research monitoring, pygmy-owls have never been observed directly drinking water (Abbate et al. 1999, AGFD unpubl. data). It is likely that pygmy-owls meet much of their biological water requirements through the prey they consume. However, the presence of water may provide related benefits to pygmy- owls. The availability of water may contribute to improved vegetation structure and diversity which improves cover availability. The presence of water also likely attracts potential prey species improving prey availability. Reproduction and Rearing of Offspring

Male pygmy-owls establish territories using territorial- advertisement calls to repel neighboring males and attract females. Usually, pygmy-owls nest as yearlings (Abbate et al. 1999, Gryimek 1972), and both sexes breed annually thereafter. Territories normally contain several potential nest-roost cavities from which responding females select a nest. Hence, cavities/acre may be a fundamental criteria for habitat selection. Historically, pygmy-owls in Arizona used cavities in cottonwood, mesquite, ash trees, and saguaro cacti for nest sites (Millsap and Johnson 1988). Recent information from Arizona indicates nests were located in cavities in saguaro cacti for all but two of the known nests documented from 1996 to 2002 (Abbate et al. 1996, 1999, 2000, AGFD unpubl. data). One nest in an ash tree and one in a eucalyptus tree were the only non-saguaro nest sites (Abbate et al. 2000).

Pygmy-owls exhibit a high degree of site fidelity once territories (the area defended) and home ranges (the area used throughout the year) have been established (AGFD unpubl. data). Therefore, it is important that habitat characteristics within territories and home ranges be maintained over time in order for them to remain suitable. This is important for established owl sites, as well as new sites established by dispersing pygmy-owls.

Shrubs and large trees also provide protection against predators for juvenile and adult pygmy-owls and cover from which they may capture prey (Wilcox et al. 2000). Little is known about the rate or causes of mortality in pygmy-owls; however, they are susceptible to predation from a wide variety of species. Documented and suspected pygmy-owl predators include great horned owls (Bubo virginianus), Harris' hawks (Parabuteo unicinctus), Cooper's hawks (Accipiter cooperii), screech- owls (Otus kennicottii), and domestic cats (Felis catus) (Abbate et al. 2000, AGFD unpubl. data). Pygmy-owls may be particularly vulnerable to predation and other threats during and shortly after fledging (Abbate et al. 1999). Arizona Game and Fish Department (AGFD) telemetry monitoring in 2002 indicated at least three of the nine young were killed by predators prior to dispersal during a year when tree species failed to leaf out due to drought conditions (AGFD unpubl. data). Therefore, cover near nest sites may be important for young to fledge successfully (Wilcox et al. 1999, Wilcox et al. 2000). A number of fledgling pygmy-owls have perished after being impaled on cholla cactus, probably due to undeveloped flight skills (Abbate et al. 1999). Conditions which promote the proliferation of cholla (overgrazing, vegetation disturbance, etc.) may contribute to this mortality factor. Habitat that provides for successful reproduction and rearing of young provides trees and cacti that are of adequate size to provide cavities in proximity to foraging, roosting, sheltering and dispersal habitats, in addition to adequate cover for protection from climatic elements and predators in an appropriate configuration in relation to the nest site.

The primary constituent elements determined necessary for the conservation of the pygmy-owl include: (1) Elevations below 1,200 m (4,000 ft) within the biotic communities of Sonoran riparian deciduous woodlands; Sonoran riparian scrubland; mesquite bosques; xeroriparian communities; tree-lined drainages in semidesert, Sonoran savanna, and mesquite grasslands; and the Arizona Upland and Lower Colorado River subdivisions of Sonoran desertscrub (see Brown 1994 for a description of vegetation communities); (2) nesting cavities located in trees including, but not limited to cottonwood, willow, ash, mesquite, palo verde, ironwood, and hackberry with a trunk diameter of 15 cm (6 in) or greater measured 1.4 m (4.5 ft) from the ground, or large columnar cactus such as saguaro or organ pipe greater than 2.4 m (8 ft); (3) multilayered vegetation (presence of canopy, mid-story, and ground cover) provided by trees and cacti in association with shrubs such as acacia, prickly pear, desert hackberry, graythorn, etc., and ground cover such as triangle-leaf bursage, burro weed, grasses, or annual plants. By way of description, preliminary data gathered by AGFD indicates 35 percent ground cover at perch sites and 48 percent ground cover at nest sites; mid-story cover of 65 percent at perch sites and 65 percent at nest sites; and 73 percent canopy cover at perch sites and 87 percent canopy cover at nest sites (Wilcox et al. 1999) (This AGFD information is based on a limited study area, a small sample size, and methods used to describe microhabitat characteristics and may have only limited applicability in project evaluation); (4) vegetation providing mid-story and canopy level cover (this is provided primarily by trees greater than 2 m (6 ft) in height) in a configuration and density compatible with pygmy-owl flight and dispersal behaviors. Within 15-m radius plots centered on nests and perch sites, AGFD has documented the mean number of trees and average height of trees for Sonoran desertscrub and semidesert grassland areas. The mean number of trees per plot in Sonoran desertscrub plots was 12.5 with a mean height of 3.95 m. The mean number of trees in semidesert grassland was 28.5 with a mean height of 8.1 m (Wilcox et al. 2000) (This AGFD information is based on a small sample size using a method designed to describe microhabitat characteristics. These numbers may have only limited applicability in project evaluations); and (5) habitat elements configured and human activity levels minimized so that unimpeded use, based on pygmy-owl behavioral patterns (typical flight distances, activity level tolerance, etc.), can occur during dispersal and within home ranges (the total area used on an annual basis).

We determined that these proposed primary constituent elements of critical [[Page 71039]] habitat provide for the physiological, behavioral, and ecological requirements of the pygmy-owl. The first primary constituent element provides the general biotic communities which are known to support pygmy-owl habitat in Arizona. We conclude that this element is essential to the conservation of the pygmy-owl because the species is not known to occur outside of these biotic communities.

The second primary constituent element provides the components necessary for nesting, such as cavity availability and cover. The third primary constituent element describes the structural makeup of habitat necessary to meet the biological needs of the pygmy-owl such as breeding, nesting, roosting, perching, foraging, predator avoidance, and thermal cover, and also promotes prey diversity and availability.

The fourth primary constituent element describes the structural makeup of vegetation necessary to meet the biological needs of the pygmy-owl related to movements and dispersal. This includes small-scale movements for foraging, defense, predator avoidance, pair formation, nest site selection, etc., as well as landscape level movements needed to promote genetic diversity and expansion of the population.

The fifth constituent element describes landscape conditions which may affect pygmy-owl behavioral patterns and relates to the need to protect habitats from various disturbances. Pygmy-owl behavior is not typically affected by low levels of human activity or activities which are predictable (Abbate et al. 1999, 2000, AGFD unpubl. data). Low- density (< 3 houses per acre) residential areas and roads with low traffic volumes are examples of this type of activity. However, high levels of human activities, high-intensity activities, or activities which cannot be predicted may affect the areas pygmy-owls will use for nesting, foraging and dispersal (AGFD unpubl. data). High-density ( 3 houses per acre) residential, commercial areas with lights and constant high levels of activity or unpredictable activities of any level, ball fields, and roads with high traffic volumes are some examples of activity levels that could potentially affect pygmy-owl behavior and habitat use. Habitat elements should be configured, and human activities should be minimized, so dispersal and pygmy-owl activities within its home range are not impeded.

We did not map critical habitat in sufficient detail to exclude all developed areas and other lands unlikely to contain primary constituent elements essential for pygmy-owl conservation. Within the proposed critical habitat boundaries, only lands containing some or all of the primary constituent elements (defined above) are proposed as critical habitat. Existing features and structures within proposed critical habitat, such as buildings; roads; residential landscaping (e.g., mowed nonnative ornamental grasses); residential, commercial, and industrial developments; and lands above 1,200 m (4,000 ft) do not contain some or all of the primary constituent elements. Therefore, these areas are not considered critical habitat and are specifically excluded by definition.

Facilitating the movement of juvenile pygmy-owls to establish breeding sites, as well as movements among currently known local populations of pygmy-owls, is important for dispersal and gene flow, and providing such connectivity is a widely accepted principle of conservation biology. Thus, portions of CHUs may function primarily to provide such connectivity within and among CHUs and may contain only the primary constituent elements required for dispersal, but we recognize the essential nature of such connectivity to the persistence of pygmy-owls in Arizona.