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Oil spills have long been recognized as a significant threat to wildlife. Historically, mortality estimates have served as the basis for assessing impact to natural resources. However, these mortality estimates alone neglect the more wide-spread impact of oil spills on wildlife including birds, many of which may not immediately succumb to exposure, but instead suffer sublethal injury that may negatively affect physiological homeostasis, reproduction, and long-term survival. Therefore, there is a need to improve our understanding of the risk of exposure and effect of sublethal oiling during damage assessments. In this dissertation I evaluated the extent of sublethal oil exposure in the immediate aftermath of the Deepwater Horizon spill on American oystercatchers (Haematopus palliatus), black skimmers (Rynchops niger), brown pelicans (Pelecanus occidentalis), clapper rails (Rallus crepitans), and seaside sparrows (Ammodramus maritimus) through both visual evaluation of and under the application of ultraviolet light to individual birds potentially exposed to oil. I found that there were many individual birds with modest oil exposure, demonstrating that more birds are exposed to oil than are accounted for by mortality estimates. Additionally, I developed a field-adapted technique using an in vitro method in brown pelicans that was effective in determining oxidative hematologic injury as measured by a suite of parameters including a reduction in circulating erythrocytes and hemoglobin, formation of Heinz bodies, and an increase in reticulocytes, in birds exposed to oil. I then applied this suite of parameters to individual birds affected in the aftermath of the Deepwater Horizon spill, and found that birds with modest visible or UV-detectible oil exposure suffer hematologic injury, a quantifiable adverse sublethal effect of modest oil exposure. Finally, I used an experimental approach to evaluate the pathologic effects of crude oil exposure in zebra finches (Taeniopygia guttata), evaluating the same suite of hematologic parameters as well as gross pathology, histopathology, and electron microscopy. This controlled study provided evidence that there may be significant variability in the response of birds to oil exposure that may be attributable to species-specific sensitivity and/or other factors such as the use of dispersants after oil spills. Collectively, this body of work demonstrated that many more birds are exposed to oil during spill events than are accounted for by mortality estimates alone, and that these birds can suffer quantifiable sublethal hematologic injury. The ability to accurately assess the extent of exposure and hematologic damage caused by oil spills is critical to determine the appropriate approach to management needed to offset impacts to fisheries, wildlife, habitats, and economic resources impacted by oil spills. / Doctor of Philosophy / Fossil fuels are the world's primary energy source and are an important part of everyday life. Our reliance on petroleum requires extraction, transportation, storage, and refinement of millions of gallons of crude oil each day. As an unintended consequence, some of this oil is inadvertently spilled into the environment, and these oil spills have long been recognized as a threat to wildlife. Assessing the impact of oil spills on wildlife is a major concern to industries, government, and the general public. Historically, mortality estimates have served as the basis for assessing impact to natural resources. However, these mortality estimates alone neglect the more wide-spread impact of oil spills on wildlife including birds, many of which may not immediately succumb to exposure, but instead suffer sublethal physiologic injury that negatively affects physiology, reproduction, and long-term survival. Therefore, there is a need to improve our understanding of the risk of exposure and effects of sublethal oiling during damage assessments. In this dissertation, I evaluated the extent of sublethal exposure to oil from The Deepwater Horizon spill for several species of birds through both visual evaluation of and under the application of ultraviolet light. This demonstrated that many more birds are affected by oil exposure than are accounted for by mortality estimates. Additionally, I developed a field-adapted technique in a controlled setting that is effective in determining oxidative injury to red blood cells in birds exposed to oil, and applied this approach to several species in the field during the aftermath of the Deepwater Horizon spill. Finally, I used an experimental approach to evaluate the extent of pathologic effects of Deepwater Horizon crude oil exposure in individuals under controlled dosages. The ability to accurately assess the extent of damage caused by oil spills is critical to determine the appropriate approach to management needed to offset impacts to fisheries, wildlife, habitats, and economic resources impacted by oil spills.