NEST INITIATION AND CLUTCH SIZE OF GREAT BLUE HERONS ON THE MISSISSIPPI RIVER IN RELATION TO THE 1993 FLOOD

Great Blue Heron (Ardea herodias) eggs were collected from ten colonie s between Clinton, Iowa and Royalton, Minnesota on the Mississippi Riv er in 1993, a year of record floods in the midwestern United States. I n the live southernmost colonies where record flooding occurred, Great Blue Herons initiated nesting two weeks later than herons nesting in the five northernmost colonies that were less affected by flooding. Th e southern nests had a-smaller average clutch size than the northern n ests, but egg size was similar between south and north. Weather patter ns in 1993 were similar between northern and southern colonies. We sus pect that flooding of the available feeding habitat influenced nest in itiation and clutch size. Data from 1995, a year without record floodi ng on the Mississippi River, support this hypothesis. In 1995, timing of nesting and number of eggs per clutch were similar between sites th at had record flooding and sites that were less affected by flooding i n 1993.