Western Screech-Owls Research Articles

BODY SIZE VARIATION AND EVOLUTIONARY ECOLOGY OF EASTERN AND WESTERN SCREECH-OWLS

Wing, bill, and mass measurements of eastern (Otus asio) and western (O. kennicottii) screech-owls identify them, their subspecies, and sexes. The eastern screech-owl is the more massive, shorter winged, and longer billed. Females of both species are larger than males. Subspecies follow Marshall (1967), except that O. a. naevius and O. k. macfarlanei are reinstated, and O. k. cardonensis is synonymized with O. k. xantusi. Body size is clinal from large in the north to small in the south in both species. Increasing sexual dimorphism in bill length correlated with decline in mass might reflect increased subdivision of the food niche related to increased competition in more dense populations and among more coexisting insectivorous owls southward. Eastern and western screech-owls coexist in southeastern Colorado, west-central and Trans-Pecos Texas, and northern Chihuahua and Coahuila, where their bill-length differences are 2.3 times greater than among allopatric populations. Interspecific competition is suggested by such character displacement, habitat segregation, and vocal mimicry. The 2 species seem to have mostly separate evolutionary histories, although occasional hybridization indicates continuing speciation.

Bounce and Double Trill Songs of Male and Female Western Screech-Owls: Characterization and Usefulness for Classification of Sex

AbstractBoth sexes of adult Western Screech-Owls (Otus kennicottii) sing in response to playback of conspecific song within their territories. Two primary songs are the bounce and double trill. Using sonographic analysis of tape-recorded vocalizations, our study quantified characteristics of bounce and double trill songs uttered by individuals within a population of Western Screech-Owls inhabiting riparian woodlands in southwestern Idaho. We asked if songs of males and females differed and, if so, how accurately could songs be classified by sex. On average, male bounce songs were ∼30% lower in frequency than bounce songs of females, but song duration, note duration, number of notes per bout, and internote duration did not differ. Similarly, double trill songs of males were significantly lower in frequency than those of females, and double trill songs uttered by females had significantly greater internote distances in the leading portion. A discriminant model based on frequency at maximum amplitude of bounce songs correctly classified all males and females in our sample. Presence of sex-specific patterns of vocalizations in that species would allow owls to assess the sex of potential intruders and respond appropriately. Moreover, sonographic analysis of Western Screech-Owl songs provides a means for noninvasive classification of sex.

Bounce and Double Trill Songs of Male and Female Western Screech-Owls: Characterization and Usefulness for Classification of Sex

Bounce and Double Trill Songs of Male and Female Western Screech-Owls: Characterization and Usefulness for Classification of Sex

Bounce and Double Trill Songs of Male and Female Western Screech-Owls: Characterization and Usefulness for Classification of Sex

Both sexes of adult Western Screech-Owls (Otus kennicottii) sing in response to playback of conspecific song within their territories. Two primary songs are the bounce and double trill. Using sonographic analysis of tape-recorded vocalizations, our study quantified characteristics of bounce and double trill songs uttered by individuals within a population of Western Screech-Owls inhabiting riparian woodlands in southwestern Idaho. We asked if songs of males and females differed and, if so, how accurately could songs be classified by sex. On average, male bounce songs were ∼30% lower in frequency than bounce songs of females, but song duration, note duration, number of notes per bout, and internote duration did not differ. Similarly, double trill songs of males were significantly lower in frequency than those of females, and double trill songs uttered by females had significantly greater internote distances in the leading portion. A discriminant model based on frequency at maximum amplitude of bounce songs correctly classified all males and females in our sample. Presence of sex-specific patterns of vocalizations in that species would allow owls to assess the sex of potential intruders and respond appropriately. Moreover, sonographic analysis of Western Screech-Owl songs provides a means for noninvasive classification of sex.

Virulence of Raptor-Origin Pasteurella multocida in Domestic Chickens

Pasteurella multocida belonging to somatic serotype 1 and capsular type A has been known to cause avian cholera in domestic poultry. Pasteurella multocida serotype 1 has also been isolated from raptorial birds. However, the capsular type for these raptorial isolates remains unknown. Moreover, the virulence of these raptorial isolates for domestic poultry has not been determined. The objectives of this study were to determine the capsular type of raptorial P. multocida serotype 1 isolates and to determine if these isolates were virulent for domestic chickens. Study chickens were inoculated with one of three P. multocida isolates. Isolate WESO-1 was obtained from a western screech owl (Otus kennicottii) and isolates RTHA-2 and RTHA-4 were isolated from two red-tailed hawks (Buteo jamaicensis). These isolates were given by either the oral, intravenous, or intraocular route. Control birds were given brain-heart infusion broth. The capsular serotypes of three isolates were also determined. The RTHA-2 and RTHA-4 isolates belonged to P. multocida capsular type A. The WESO-1 isolate belonged to capsular type F. Results also demonstrated that, for the isolates examined, the intraocular route did not cause mortality in chickens. There was mortality in all groups for the intravenous route. However, various mortality patterns were observed when P. multocida was given orally for the three different isolates. The RTHA-4 isolate (serotype 1:A) was the most virulent for domestic chickens. The WESO-1 isolate (serotype 1:F) was the least virulent for chickens among the raptorial isolates examined.

Effects of social status on the dispersal behaviour of juvenile western screech-owls

Variation in timing and distance of dispersal movements of juvenile birds may result from differences in competitive ability. Dispersal by low-ranking juveniles may be initiated before dominants if the latter force subordinate siblings from natal areas. Conversely, when vacant territories are limited and are acquired on a first-come first-served basis, selection could operate on young to disperse as early as possible. In this case, dominant individuals with priority of access to resources in the natal area will mature more quickly and are expected to disperse first. If costs of dispersal increase with dispersal distance, dominant juveniles are expected to disperse shorter distances. Alternately, if there are advantages to long-distance dispersal, then dominants, which are in better condition, should disperse further than subordinates. We examined effects of social rank on the timing and distance (to wintering area) of dispersal movements by juvenile western screech-owls,Otus kennicottii, in southwestern Idaho. Based on observations of aggressive interactions made using video cameras attached to nestboxes, we assigned dominance ranks to nestlings within nine broods. We radiotracked young throughout the postfledging period to determine order of dispersal, and we located them after leaving their natal areas to determine distances to apparent overwintering areas. In six of seven broods, for which dispersal information could be recorded, the most dominant juvenile dispersed first. Moreover, in five of seven broods, the least dominant individual was the last individual to disperse, and the order of dispersal matched the dominance hierarchy in four of seven broods. In contrast, social status did not affect postfledging dispersal distance. We conclude that social dominance relationships influenced the timing of dispersal in juvenile western screech-owls but not distance travelled to overwinter sites.

Corticosterone and the stress response in young western screech-owls: effects of captivity, gender, and activity period.

We used a standard handling protocol to examine the stress response of captive young western screech-owls during their active (nighttime) and inactive (daytime) periods and to compare the stress responses of captive and free-living owls. Circulating corticosterone levels were significantly higher during the inactive period than in the active period in this nocturnal species. This suggests that the daily pattern of corticosterone secretion is reversed in nocturnal birds and is correlated with activity period rather than with the light/dark cycle. Young (ca. 4-5 mo old) screech-owls of both sexes showed increases in plasma corticosterone up to 30 min after capture, followed by significant decreases at 60 min. This pattern is similar to those of other species of birds examined previously, except that decreases in corticosterone at 60 min rarely have been observed. Such decreases may be the result of physiological differences between adult and young birds, habituation to handling in captive birds, or the effects of body condition. Corticosterone levels and the response to capture and handling were comparable in captive and free-living owls, which suggests that the captive owls were not subjected to chronically high levels of stress.

Locomotor activity levels and the dispersal of western screech-owls,Otus kennicottii

Locomotor activity levels and the dispersal of western screech-owls,Otus kennicottii

Hybridization between Barred and Spotted Owls

Interspecific hybridization has been reported in at least 52 of 516 nonmarine bird species in North America (Johnsgard 1970, Mayr and Short 1970, Short 1965, 1972). It tends to occur infrequently, however, except where allopatric species are brought into contact by range expansions (Cade 1983). Groups in which interspecific hybridization is particularly common include warblers, grouse, and hummingbirds (Mayr and Short 1970, Oliphant 1991). Instances of interspecific hybridization in birds of prey are uncommon except in captivity (Newton 1979, Cade 1983, Oliphant 1991). In owls, interspecific hybridization appears to be rare. Johnsgard (1988) reported instances of hybridization between Eastern Screech-Owls (Otus asio) and Western Screech-Owls (0. kennicottii) in Texas, and Voous (1989) reported instances of hybridization between Whiskered Screech-Owls (0. trichopsis) and Western Screech-Owls in Arizona. Evidence of hybridization in the genus Strix has been limited to Ural Owls (S. uralensis) hybridizing with Tawny Owls (S. aluco) in captivity (Mikkola 1983). We present the first records of hybridization between the Northern Spotted Owl (S. occidentalis caurina) and Northern Barred Owl (S. varia varia), two species that are thought to be closely related and that have recently become sympatric (Grant 1966, Taylor and Forsman 1976, Hamer 1988). Since at least the early 1950s, the Barred Owl has been expanding its range into western North America. In the process, it has invaded much of the historical range of the Spotted Owl (Grant 1966, Taylor and Forsman 1976, Hamer 1988). Barred Owls are now common in forested areas in southwestern British Columbia and northern Washington (Hamer 1988, Dunbar et al. 1991), and they are rapidly increasing in Oregon and northern California. Increasing sympatry between Barred and Spotted owls has led to speculation that the Northern Spotted Owl, which is listed as a threatened species (U.S. Fish and Wildlife Service 1990), may be further threatened by competition with the Barred Owl, as well as by habitat loss (Taylor and Forsman 1976, Dunbar et al. 1991). The possibility that sympatry might also result in hybridization between the two species has not been previously considered. Records of hybrids.-Three adult Spotted Owl/Barred Owl hybrids, two in Washington and one in Oregon, were confirmed during 1989-1992, and one juvenile hybrid was produced by a female Barred Owl paired to a yearling male Spotted Owl in Oregon in 1992. All hybrids were identified by their unique plumage characteristics, vocalizations, and measurements. The first hybrid was an adult male seen on 23 March 1989 at Baker Lake in the Washington Cascades Range, 30 km south of the United States/Canadian border. This bird was fitted with a radio transmitter in 1989 and recorded in the same area in spring 1990, 1991, and 1992. Its nesting status was not determined, but it was always seen with a female Barred Owl. Both birds aggressively defended the territory in response to playback calls. The second hybrid, an adult male located in the Klamath Mountains 29 km northeast of Medford, Oregon, in 1990 was paired with a female Barred Owl. This pair produced two young in 1990 and three young in 1991. In 1992 the Medford hybrid was displaced by a male Barred Owl. It was relocated in June 1992, 1 km from its previous nesting site and did not appear to be paired (James Harper pers. comm.). Another suspected hybrid (or possibly the same individual) was seen and heard in this area in 1987 (James Harper pers. comm.). The third hybrid was an adult female observed several times in May 1991, 2 km south of Ozette Lake, Washington, near the northwestern tip of the Olympic Peninsula. When this bird was captured, we found that it had been banded as a juvenile in 1986, 277 km to the southeast in the Cascade Range of southern Washington. The original bander suspected that the bird may have been a hybrid, but was unsure because it was not fully feathered and because both parents were not observed (H. Allen pers. comm.). When located in 1991, the Ozette Lake female was paired with an adult male Spotted Owl. No young were observed, but the female had a brood patch that was regressing, which suggested that nesting may have been attempted. Both birds were observed together in 1992 when nesting was again attempted but failed for unknown reasons. In addition to the three adult hybrids described above, a fourth hybrid from a pairing between a Barred Owl and Spotted Owl was confirmed in 1992 in Douglas County, Oregon, 8 km southeast of Canyonville. This pair consisted of a yearling male Spotted Owl and an adult female Barred Owl. They produced one young, which was first observed on 1 June 1992. The F, hybrid juvenile was fitted with a tail-mounted radio

Lead in hawks, falcons and owls downstream from a mining site on the Coeur d'Alene River, Idaho

Mining and smelting at Kellogg-Smelterville, Idaho, resulted in high concentrations of lead in Coeur d'Alene (CDA) River sediments and the floodplain downstream, where American Kestrels (Falco sparverius), Northern Harriers (Circus cyaneus), Red-tailed Hawks (Buteo jamaicensis), Great Horned Owls (Bubo virginianus), and Western Screech-owls (Otus kennicotti) nested. Nestling American Kestrels contained significantly higher (P=0.0012) blood lead concentrations along the CDA River (0.24 µg/g, wet wt) than the nearby reference area (0.087 µg/g). A 35% inhibition of blood δ-aminolevulinic acid dehydratase (ALAD) in nestling Northern Harriers (P=0.0001), 55% in nestling American Kestrels (P=0.0001) and 81% in adult American Kestrels (P=0.0004) provided additional evidence of lead exposure in the CDA River population. In nestling American Kestrels and Northern Harriers, ALAD activity was negatively correlated with lead in blood. An earlier report on Ospreys (Pandion haliaetus) showed slightly less inhibition of ALAD than in American Kestrels, but no significant reduction in hemoglobin or hematocrit and no negative influence on production rates. The adult and nestling American Kestrels along the CDA River contained about twice as much blood lead as Ospreys during the same years (adult 0.46 vs. 0.20 µg/g, and nestling 0.24 vs. 0.09 µg/g), but adults showed a 7.5% reduction in hemoglobin (P=0.0356) and nestlings an 8.2% reduction in hemoglobin (P=0.0353) and a 5.8% reduction in hematocrit (P=0.0482). We did not observe raptor deaths related to lead, and although the production rate for American Kestrels was slightly lower along the CDA River, we found no significant negative relation between productivity and lead. Limited data on the other raptors provide evidence of exposure to lead along the CDA River. Several traits of raptors apparently reduce their potential for accumulating critical levels of lead which is primarily stored in bones of prey species.

Community Trophic Structure: The Roles of Diet, Body Size, and Activity Time in Vertebrate Predators

We examined trophic structure in an assemblage of 17 vertebrate predators - two snakes (Pituophis catenifer and Crotalus viridis), six hawks (Circus cyaneus, Buteo jamaicensis, B. regalis, Aquila chrysaetos, Falco sparverius, and F. mexicanus), six owls (Tyto alba, Otus kennicottii, Bubo virginianus, Speotyto cunicularia, Asio otus, and Aegolius acadicus), one passerine (Corvus corax), and two mammals (Canis latrans and Taxidea taxus). These predators fed collectively on prey in nine taxonomic classes. Mammalian prey were numerically most important in 15 diets and contributed the most biomass to all 17 diets. Mean prey mass ranged from 2.2 to 711 g, and correlated with predator mass. No significant differences were apparent in food-niche breadth or mean prey mass between diurnal and nocturnal predators, but dietary overlap among predators with the same diel activity was significantly greater than that among asynchronously-active predators. Mean diet overlap was significantly greater among nocturnal predators than among diurnal predators. Four feeding guilds were defined by dietary similarity. A ground squirrel-eating guild contained one snake, three hawks, and a mammal. A jackrabbit-eating guild consisted of a hawk and a mammal. An arthropod/mammal-eating guild was composed of an owl and the raven. Four owls, one hawk, and one snake formed a small-rodent guild. One owl and one hawk were not members of guilds. Mass ratios of adjacent-sized predators were greatest within guilds.

A cytogenetic comparison of some North American owl species

Karyotypes of three owl species previously not reported, the Burrowing Owl (Athene cunnicularia), the Hawk Owl (Surnia ulula), and the Western Screech Owl (Otus kennicotti), are presented. Comparison with other karyotypes in the literature suggests that the Burrowing Owl should be in a separate genus, as was previously the case. The Hawk Owl karyotype was found to bear considerable similarity to some Athene species, although we are not suggesting they should be placed in one genus. The karyotype of the Western Screech Owl appears to have diverged from the Otus species described previously, but these chromosomal data are not incompatible with these species sharing a common genus. A karyotype of the Long-eared Owl, which differs slightly from a previously published version, is presented. Relationships among the owls karyotyped to date are discussed on the basis of cladistic interpretations of these data.Key words: Strigiformes, phylogeny, karyotype, evolution, chromosome.

Resource partitioning among forest owls in the River of No Return Wilderness, Idaho.

We studied resource partitioning among the forest owls in the River of No Return Wilderness, Idaho, during the winter and spring of 1980 and 1981. The owl assemblage consisted of five abundant species: pygmy (Glaucidium gnoma), saw-whet (Aegolius acadicus), boreal (A. funereus), western screech (Otus kennicottii), and great-horned (Bubo virginianus). Long-eared (Asio otus) and flammulated (O. flammeolus) owls were rarely observed. Information from the literature supplemented our data to describe the pattern of resource partitioning. Stepwise discriminant function analysis and multivariate analysis of variance revealed differences in macrohabitat and microhabitat. The saw-whet, boreal, western screech, and great-horned owls all preferred mammalian prey but exhibited habitat differences. They also differed in activity periods and food habits. The pygmy owl, a food and habitat generalist, foraged diurnally more than the other species and took a higher proportion of brids. The flammulated owl used areas within the territories of other owl species but specialized on forest insects. The observed pattern of resource use was interpreted to result from environmental factors, morphological limitations and interspecific competition. Differences in food and activity time, we suggest, result from environmental factors and differences in owl morphology, while present-day interspecific competition may be important in shaping habitat use. Experiments will be necessary to determine the causal factors responsible for segregation among the forest owls.