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<br />n <br />Environmental Assessment -Mexican Spotted Owl Critical Habitat Designation <br />January, 2001 <br />use patterns, and some migrate considerable distances (20-50 kilometers (km)) (12-31 miles <br />(mi)) during the winter, generally migrating to more open habitat at lower elevations (Ganey and <br />Balda 19896, Willey 1993, Ganey et a1.1998). The home-range size of Mexican spotted owls <br />appears to vary considerably among habitats and/or geographic areas (USDI 1995), ranging in <br />size from 261-1,487 ha (647-3,688 ac) for individuals birds, and 381-1,551 ha (945-3,846 ac) for <br />pairs (Ganey and Balda 19896, Ganey et al. 1999). Little is known about habitat use by juveniles <br />dispersing soon after fledging. Ganey et al. (1998) found dispersing juveniles in a variety of <br />habitats ranging from high-elevation forests to pinon-juniper woodlands and riparian areas <br />surrounded by desert grasslands. <br />Mexican spotted owls do not nest every year. The owl's reproductive pattern varies somewhat <br />across its range. In Arizona, courtship usually begins in March with pairs roosting together <br />during the day and calling to each other at dusk (Ganey 1988). Eggs are typically laid in late <br />March or early April. Incubation begins shortly after the first egg is laid, and is performed <br />entirely by the female (Ganey 1988). The incubation period is about 30 days (Ganey 1988). <br />During incubation and the first halfof the brooding period, the female leaves the nest only to <br />defecate, regurgitate pellets, or receive prey from the male, who does all or most of the hunting <br />(Forsman et al. 1984, Ganey 1988). Eggs usually hatch in early May, with nestling owls <br />fledging 4 to 5 weeks later, and then dispersing inmid-September to early October (Ganey <br />1988). <br />Little is known about the reproductive output for the spotted owl. It varies both spatially and <br />temporally (White et al. 1995), but the subspecies demonstrates an average annual rate of about <br />one young per pair. Based on short-term population and radio [racking studies, and longer-term <br />monitoring studies, the probability of an adult owl surviving from 1 year to the next is 80 to 90 <br />percent. Average annual juvenile survival is considaably lower, at 6 to 29 percent, although it is <br />believed these estimates may be artificially low due to the high likelihood of permanent dispersal <br />from the study area, and the lag of several years before marked juveniles reappear as territory <br />holders and are detected as survivors through recapture efforts (White et al. 1995). Little <br />research has been conducted on the causes of mortality, but predation by great horned owls <br />(Bubo vireinianusl, northern goshawks Acci ter eg n<ifis), red-tailed hawks uteo jamaicensis), <br />and golden eagles A uila c saetos), as well as starvation, and collisions (e.g., with cars, <br />powerlines), may all be contributing factors. <br />Mexican spotted owls consume a variety of prey throughout their range, but commonly eat <br />small- and medium-sized rodents such as woodrats eotoma spp.), peromyscid mice <br />(Peromyscus spp.), and microtme voles icrotus spp.). Owls also may consume bats, birds, <br />reptiles, and arthropods (Ward and Block 1995). Each prey species uses a unique habitat, so that <br />the differences in the owl's die[ across its range likely reflect geographic variation in population <br />densities and habitats of both the prey and the owl (Ward and Block 1995). Deer mice LP. <br />maniculatus) are widespread in distribution in comparison to brush mice LP. bovlei), which are <br />