Boves, Than J. – The effects of roadway mortality on Barn Owls in southern Idaho and a study of ornamentation in North American Barn Owls. 2007.
This thesis consists of two chapters describing my investigations of roadway mortality and ornamentation of barn owls (Tyto alba) in southern Idaho. My research objectives were to (1) quantify roadway mortality of barn owls along a 248-kilometer stretch of Interstate 84, (2) examine the factors that influence roadway mortality of barn owls in this area, (3) refine my estimates of roadway mortality by performing bias experiments, and (4) investigate the potential relationships between ornamentation (in the form of ventral spottiness) and quality in North American barn owls (T. a. pratincola) and compare my results to similar studies of European barn owls (T. a. guttata and T. a. alba). I performed my research from July 2004 to December 2007. Information contained in this thesis should be of interest to individuals investigating human-induced mortality of wildlife, roadway mortality of birds, or sexual selection and ornamentation in avian species, as well as to biologists involved with general research and conservation of barn owls.
Chapter One: The effects of roadway mortality on Barn Owls in southern Idaho
Collisions between moving vehicles and animals are the most common direct cause of wildlife mortality by humans. The barn owl (Tyto alba) is a species that is greatly affected by this type of mortality. I quantified roadway mortality of barn owls in southern Idaho by performing systematic road surveys along a 248-km stretch of Interstate 84 from July 2004 to June 2006. I detected 812 dead barn owls during these surveys. In addition to performing these surveys, I adjusted my estimates of mortality by performing several bias experiments, which when combined with the survey data resulted in the highest roadway mortality rates ever recorded for barn owls. I determined that over 1400 barn owls may be dying yearly from vehicular collisions along this stretch of highway. I calculated a mortality rate range of 287.7 599.4 owls/100 km/yr over the entire study area and 311.2 741.4 owls/100 km/yr in agricultural areas in the winter. Females were affected significantly more than males and juveniles were killed three times more often than adults. In the winter, owls were killed significantly more often near agricultural lands, but in the summer, neither habitat harbored more barn owl deaths. In relation to landscape features, I found that owls were killed significantly more often in areas of the survey route near the Snake River canyon. I recorded several areas of extremely high mortality (hot spots) including one area where over 80 dead owls were found in a 3.5 km stretch of road. I also found a large variation in mortality rates between the first and second year of the surveys which could have been related to variation in weather and barn owl productivity between the years. The effect of this roadway mortality on the overall barn owl population of southern Idaho is not known and depends on the ability of barn owls to find nesting sites, barn owl productivity levels, and immigration and dispersal of young barn owls. Future efforts at mitigation will need to focus on keeping owls away from the roads and/or keeping vehicles from colliding with the owls.
Chapter Two: A study of ornamentation in North American Barn Owls
Ornamental displays can evolve via sexual selection. One explanation for the evolution of ornaments is that the individual displaying the ornament is honestly advertising its genetic makeup. In this case, selecting a mate that displays the most elaborate ornamentation would be adaptive. In most avian species, males display more ornamentation. However, in barn owls (Tyto alba), this pattern is reversed such that females display more ornamentation in the form of ventral spottiness. Previous studies suggest that female European barn owls (T. a. gutatta and T. a. alba) indicate their quality to male barn owls in the form of ventral plumage spottiness. I examined possible relationships between ventral spottiness and quality (in the form of wing length, fluctuating asymmetry, tail length, heart fat deposits, and stomach contents) in North American barn owls (T. a. pratincola) that were killed by vehicular collisions in southern Idaho. I found relationships between North American barn owl ornamentation and quality that differed from their European counterparts. Females were significantly more ventrally spotted than males, and there were no relationships between spottiness and quality measurements in females.
However, in males spottiness was significantly correlated with both heart fat and wing length. These results suggest that mutual honest ornamentation may be occurring in barn owls, with males signaling different characteristics than females. Secondly, there appears to be differences in the signaling dynamics between European and North American barn owl subspecies.