Wild common loons (Gavia immer) were captured across much of their southern North American range. Hematologic and physiologic data were collected to establish reference ranges for adults and chicks from four geographic areas: New England, Canadian Maritimes, south-central Quebec, and the Upper Great Lakes. Mean body mass and blood-glucose levels of adult loons varied among geographic regions. Chicks had lower PCV and total solids than adult loons, and also had faster heart rates and higher blood glucose levels. Although mean body mass in males was consistently greater than in females, hematologic and physiologic reference ranges between sexes were not significantly different. Geographic and age-related differences exist in common loon hematologic and physiologic reference ranges and should be considered when assessing such parameters.