Threat Impact CommentsHabitat Loss and Degradation: Population declines in the upper midwestern U.S. have been attributed mainly to commercial development of farmland, reduction in dairy and sheep industry, conversion to intensive row-crop farming, and decline in the number of farms and old farm structures, resulting in a loss of nest sites and important high-quality foraging habitat (Colvin et al. 1984, Colvin 1985, Rosenburg 1986, Ehresman et al. 1989, Gubanyi 1989). Loss of farmland to development and the intensification of agricultural practices on remaining farmlands have substantially reduced the quantity and quality of dense grass habitats in agricultural areas (Honer 1963, Shrub 1970, Colvin 1985, Rosenburg 1986). Colvin (1985) reported an approximately 53% decline in hayfield acreage in Ohio between 1921-1980 and identified a significant correlation between a population decline in Ohio and the replacement of grass-associated agriculture by row crops. In Virginia, the acreage of pasture, grass hayfield, and idle areas was reduced 55% between 1945-78 and the quality of pasture declined because of increased grazing pressure (Rosenburg 1986). In many intensively-farmed areas, dense grass habitats are present only in small fields that are patchily distributed; such fields would apparently provide a limited foraging resource (Rosenburg 1986). The reduced quality and quantity of dense grass habitats has substantially reduced prey availability in some areas. Prey availability has been shown to be closely associated with owl productivity (Ault 1971, Otteni et al. 1972, Colvin 1984, Gubanyi 1989). This association is so close that one year of poor meadow vole abundance can result in a rapid population decline while one year of substantial meadow vole abundance can result in rapid population recovery (B. Colvin, pers. comm.).
Nest site availability: The availability of secure nest sites is an important limiting factor in some areas (Marti et al. 1979, Schulz and Yasuda 1985, Byrd and Rosenburg 1986, Gubanyi 1989). Tree cavity sites may be limited in availability (Rosenburg 1986), they are ephemeral (Colvin et al. 1984, Byrd and Rosenburg 1986), and relatively insecure (Colvin et al. 1984). Competition for this resource may also be a factor; Colvin (1984) documented usurpation of nest cavities by wood ducks (AIX SPONSA) and raccoons (PROCYON LOTOR). Secure nest sites within human-made structures are limited in availability (Schulz 1986; C. Rosenburg, unpubl. data). In addition, the gradual deterioration and disappearance of old-style barns, silos, and water tanks plus the screening of entrances to prevent rock dove (COLUMBA LIVIA) access has eliminated many previously productive nest sites (Honer 1963, Heintzelman 1966, Schulz and Yasuda 1985, Byrd and Rosenburg 1986, Parker and Castrale 1990). Areas that support abundant foraging habitat may lack an adequate supply of secure and stable nest sites in close proximity to foraging habitat. Kirkpatrick and Colvin (1986) suggested that salmonellosis may limit survival and reproduction at times when stresses, such as severe weather and poor prey availability, are acting. This stress dependent impact may be true for other diseases and parasites as well (Honer 1963, Kirkpatrick and Colvin 1989). Weather is the most important factor influencing annual productivity in southwestern New Jersey (B. Colvin, pers. comm.). Moist weather conditions enhance dense grass habitats and thereby enhance vole populations, which results in higher productivity. Exceedingly dry conditions have a negative impact on vole populations and result in poor productivity (Colvin and Hegdal 1986, 1987, 1988, 1989). Starvation of chicks, the single most important mortality factor (B. Colvin, pers. comm.), is widespread during exceedingly dry conditions. Throughout the Northeast, the owl is susceptible to starvation and exposure during extended periods of extreme cold and deep snow cover. Winter weather mortality has been documented in Wisconsin (Errington 1931), Illinois (Speirs 1940), Ohio (Stewart 1952), Massachusetts (Keith 1964), Utah (Marti and Wagner 1985), and Virginia (C. Rosenburg, unpubl. data). Marti and Wagner (1985) reported that some winter weather mortality appears to occur every winter in northern Utah and that such mortality is widespread and consequential in some years. They found 77 dead owls after severe weather during the winter of 1981-82, and a 40% decline in breeding attempts occurred the following summer. Keith (1964) documented winter weather mortality of more than ten owls during the winter of 1960-61 and the subsequent lack of nesting during the following three breeding seasons at Martha's Vineyard, Massachusetts. Barn owls did not nest at Martha's Vineyard again until 1973 (Brett 1987).
Pesticides: Secondary poisoning from rodenticides has been considered to be a potential hazard because of the importance of rodents in the diet and the fairly widespread use of rodenticides in agricultural areas. Laboratory studies have demonstrated that the consumption of rats or mice poisoned with bromadiolone or brodifacoum rodenticides can cause lethal hemorrhaging and that consumption of rats poisoned with difencoum rodenticide can cause sublethal hemorrhaging (Mendenhall and Pank 1980; Newton et al., in press). These studies demonstrated that the owl is especially sensitive to the anticoagulant brodifacoum. Secondary poisoning from rodenticides has been documented in the U.S. (Schulz 1986; L. Soucy, pers. comm.) and Great Britain (Newton et al., in press). The potential for poisoning appears to be greatest in marginal habitat areas such as intensively farmed sites (Rosenburg 1986). However, there appears to be no appreciable impact to populations from rodenticide poisoning (Colvin 1984; Hegdal and Blaskiewicz 1984; Newton et al., in press). Organophosphate insecticides have been shown to be potentially hazardous. Laboratory experiments conducted by Hill and Mendenhall (1980) demonstrated that owls which consumed famphur-poisoned prey exhibit secondary poisoning in the form of significant cholinesterase inhibition. Mass mortality of wild raptors, including 22 barn owls, occurred after azodrin was improperly used to kill voles in Israel (Mendelssohn and Paz 1977). Rodents contaminated with organophosphate and carbamate insecticides are present in agricultural fields (Montz 1988). These rodents are potentially hazardous to raptors because birds in general are extremely sensitive to anti-cholinesterase compounds (Brealey et al. 1980). It is unlikely that organophosphate or carbamate insecticides have impacted populations since these pesticides are not targeted for foraging habitats or prey species (B. Colvin, pers. comm.). However, no field studies have examined cholinesterase levels of owls in agricultural areas where these pesticides are widely used. Further investigation may be warranted (L. Brewer, pers. comm.). The owl is sensitive to contamination from organochlorine insecticides. Laboratory studies by Mendenhall et al. (1983) found that it is very sensitive to eggshell thinning by DDE and that dieldrin can cause adult mortality. These insecticides were found in concentrations that may have been detrimental to reproduction in 15% of the barn owls in the lower Potomac River, Maryland in the early 1970s (Klaas et al. 1978). Extensive feeding on passerine birds by this portion of the population is believed to have caused the elevated organochlorine levels; the majority of the population preyed chiefly on mammals and remained relatively uncontaminated. In Great Britain, poisoning from organochlorine pesticides was an important cause of mortality during the years 1963-77 when these chemicals were used extensively (Newton and Wyllie, in press). Organochlorine insecticide residues have been found in barn owls and their eggs collected in Florida (Johnston 1978), Oregon (Henny et al. 1984), and Virginia (Gwynn 1987). Acute effects from organochlorine insecticides are unlikely, though, since raptors which feed chiefly upon small mammals are not highly susceptible to organochlorine insecticide poisoning (Henny 1972) and since few potentially harmful organochlorine insecticides are in use in the U.S. today (Newton 1979; S. Wiemeyer, pers. comm.).
OTHER FACTORS: A number of other mortality factors have been identified. Collision with vehicles has been reported as an important mortality factor (Glue 1971; Smith and Marti 1976; Keran 1981; Schulz 1986; Newton and Wyllie, in press). Drowning of young as they attempt to fledge from offshore duck blinds appears to be acting as a population sink in coastal Maryland and possibly other areas where substantial numbers nest in these structures (G. Therres, pers. comm.). Other mortality factors include electrocution, entrapment in buildings, shootings, and entanglement in farm or industrial machinery (Glue 1971; Smith et al. 1974; Smith and Marti 1976; Keran 1981; Colvin 1984; Hegdal and Blaskiewicz 1984; Lerg 1984; Schulz and Yasuda 1985; Schulz 1986; Ehresman et al. 1989; Newton and Wyllie, in press; C. Rosenburg, unpubl. data). The degree to which these latter four factors limit numbers appears to be low. PREDATION: May limit numbers in some areas. Raccoons and black rat snakes (ELAPHE OBSOLETA) prey on eggs and nestlings (Ehresman 1984; B. Colvin, pers. comm.; C. Rosenburg, unpubl. data), and great horned owls (BUBO VIRGINIANUS) prey on juveniles and adults (Rudolph 1978, Knight and Jackman 1984, Lerg 1984, Rosenburg 1986, Millsap and Millsap 1987, Ehresman et al. 1989) and may inhibit barn owl activity because of their dominance (Rudolph 1978). Information concerning predation rates on eggs, nestlings, juveniles, and adults is limited and further investigation is warranted (Hands et al. 1989).