DISPERSION OF JUVENILE GREAT BLUE HERONS ALONG SHORELINES OF UPPER
POOL 20, MISSISSIPPI RIVER.

Robert L. Connour II 1 and Thomas C. Dunstan 2.  
1Dept. of Math, Life, and Natural Sciences, Owens Community College, Findlay, OH 45840 
2Dept. of Biological Sciences, Western Illinois University, Macomb, IL 61455.

Great Blue Herons (Ardea herodias) are common residents along the Mississippi River. 
Dispersion of juvenile, or young of the year, herons from the Mud Island heronry in upper Pool
20 near Keokuk, IA, was studied from the first recorded fledging on July 8 through September
31, 1997.  Locations of herons along the shoreline from river mile 360 to 364.5 were plotted
three times each day, three days a week.  Juvenile herons were found to be non-uniformly
dispersed throughout this shoreline habitat, with preferences for certain areas.  Areas exhibiting
the highest numbers of shoreline juvenile sightings within 3 weeks of fledging tended to be close
to the heronry.  These areas are owned and protected from trespass by the Illinois Nature
Conservancy.  Throughout the remainder of the study, most juvenile herons were found near the
dam among the newly exposed rocky areas that appeared as the river lowered late in the season. 
Thus, changes in preferences appear to correspond to fluctuations in environmental conditions as
well as time since fledging.  Juvenile and adult herons were frequently observed in extremely
close proximity to one another, a behavior that appeared to decline in frequency through time. 
Further study of both biotic and abiotic factors influencing this dispersion is needed, but results
appear to be linked to available foraging habitat and human activity.

Key words: Great Blue Heron, Ardea herodias, juvenile, Mississippi River, dispersion