Nest Initiation and Clutch Size of Great Blue Herons on the Mississippi River in Relation to the 1993 Flood

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Great Blue Heron (Ardea herodias) eggs were collected from ten colonies between Clinton, Iowa and Royalton, Minnesota on the Mississippi River in 1993, a year of record floods in the midwestern United States. In the five 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. The southern nests had a smaller average clutch size than the northern nests, but egg size was similar between south and north. Weather patterns in 1993 were similar between northern and southern colonies. We suspect that flooding of the available feeding habitat influenced nest initiation and clutch size. Data from 1995, a year without record flooding on the Mississippi River, support this hypothesis. In 1995, timing of nesting and number of eggs per clutch were similar between sites that had record flooding and sites that were less affected by flooding in 1993.