Tyto

Abstract: Owls are important consumers of small vertebrates, and because they regurgitate pellets rich in bone, they may be important potential contributors of the concentrated remains of small vertebrates to the fossil record. Owls of three sizes, the large great horned owl (Bubo virginianus), the medium-sized barn owl (Tyto alba), and the small screech owl (Otus asio), were fed a common diet of mice. The bony contents of the pellets were analyzed to determine the amount of bone loss by digestion, bone completeness, and sites of bone breakage. For all three species, only about half the number of bones ingested were recovered in the pellets. Mandibles and femora were most abundant, and pelves and scapulae were the least abundant. Screech owls broke 80% of the cranial and limb elements, barn owls only 30%. Skulls fared poorly in great horned and screech owl pellets, while barn owls returned 80% of the skulls intact, with only the caudal portion of the cranium damaged; barn owls also returned articulated strings of vertebrae and complete paws. These results provide a baseline for the recognition of owls as agents of accumulation of small bones in the fossil record and suggest that the actions of ancient predators may be revealed by species-specific patterns of bone destruction of an assemblage of fossil prey species.

Abstract: Four species of owls, Great Horned (Bubo virginianus), Long-eared ( Asio otus), Burrowing (Speotyto cunicularia), and Barn (Tyto alba), which occur together on the short-grass prairie and farmland of north-central Colorado, were selected for a study of feeding ecology. The purpose of this study was to assess the overlap in foods of the four owls and to examine feeding mechanisms which allow them to coexist. Primary objectives were (1) to analyze and compare the food habits of each owl species considering frequency of occurrence of prey species and biomass contributed by each prey species, and (2) to describe and evaluate the primary factors involved in capture of prey by the four owl species. Few studies have been attempted in relation to the feeding of owls in this geographic area and none was a long-term study. Reed (1957) examined Barn Owl pellets from Larimer County, and Kelso (1938) and Hamilton (1941) noted Burrowing Owl foods in the Denver area. Catlett et al. (1958) studied foods of Long-eared Owls near Boulder, and Long and Kerfoot (1963) listed Great Horned Owl foods from east-central Wyoming. Most abundant of these owls is the Great Horned. This species, a permanent resident of the area, is extremely versatile in both its nesting and feeding, allowing it to utilize a wide variety of habitats. The Burrowing Owl is abundant during summer. Burrowing Owls are restricted to open lands with available nesting and roosting sites in abandoned mammal burrows, chiefly those of black-tailed prairie dogs (Cynomys ludovicianus). Arriving by mid-April, they spend about 6 months in the area before returning south in midto late October. Barn Owls are less common. Northcentral Colorado is close to the northern limits of this species' range and some may move south in winter. However, some remain through the year. Rock cliffs or ditch banks are sought for nesting and roosting by Barn Owls. Long-eared Owls are also uncommon. They appear to be nomadic-there one year and gone the next. Low, dense coniferous or deciduous trees attract them for nesting and roosting. A number of other vertebrate predators are present in this area whose diets may overlap with those of the four owl species. Other avian predators are the Golden Eagle (Aquila chrysaetos), Marsh Hawk (Circus cyaneus), Red-tailed Hawk (Buteo jamaicensis), Swainson's Hawk (B. swainsoni), Rough-legged Hawk (B. lagopus), Ferruginous Hawk (B. regalis), American Kestrel (Falco sparverius), Prairie Falcon (F. mexicanus), and Loggerhead Shrike (Lanius ludovicianus). Mammalian predators include red fox (Vulpes fulva), coyote (Canis latrans), raccoon (Procyon lotor), American badger (Taxidea taxus), striped skunk (Mephitis mephitis), and longtailed weasel (Mustela frenata). The more abundant reptilian predators are the gopher snake (Pituophus melanoleucus) and prairie rattlesnake (Crotalus viridis). The research was carried out from Decem-

Abstract: The good genes hypothesis of sexual selection postulates that ornamentation signals superior genetic quality to potential mates. Support for this hypothesis comes from studies on male ornamentation only, while it remains to be shown that female ornamentation may signal genetic quality as well. Female barn owls ( Tyto alba ) display more black spots on their plumage than males. The expression of this plumage trait has a genetic basis and it has been suggested that males prefer to mate with females displaying more black spots. Given the role of parasites in the evolution of sexually selected traits and of the immune system in parasite resistance, we hypothesize that the extent of female plumage ‘spottiness’ reflects immunological defence. We assessed the genetic variation in specific antibody production against a non–pathogenic antigen among cross–fostered nestlings and studied its covariation with the plumage spottiness of genetic parents. The magnitude of the antibody response was positively correlated with the plumage spottiness of the genetic mother but not of the genetic father. Our study thereby provides the first experimental support, to our knowledge, for the hypothesis that female ornamentation signals genetic quality.