DISPERSION AND  FORAGING  BEHAVIOR  OF  GREAT  BLUE  HERONS  (ARDEA
HERODIAS) DURING FLOODSTAGE IN UPPER POOL 20, MISSISSIPPI RIVER

Jeremy E. Guinn and Thomas C. Dunstan.
Department of Biological Sciences, Western Illinois University, 1 University Circle, 
Macomb, IL 61455.  

Heron dispersion and foraging behavior were studied from 9 May to May 24, 1999 at 
a Great Blue Heron (Ardea herodias) rookery in Upper Pool 20 of the Mississippi River 
near Keokuk, Iowa.  During late spring 1999 heavy rains produced higher than usual 
water levels in this part of the pool that influenced heron foraging activity and 
distribution during the nestling stage of the breeding season.  During this period heron 
dispersal throughout the study area was determined to be non random.  A total of 
497 heron locations were identified during this study, and of these 261 herons were 
determined to be foraging.  The use of specific foraging techniques was related to the 
flood stage environment.  Eighty percent of foraging herons observe showed a 
preference for foraging (search phase) from on top of low snags rather than foraging 
by the more common shoreline wading or open water flight methods used during 
"normal" seasonal pool levels.  Additional implications of high water on heron 
foraging methods will be detailed.

Keywords:  Great Blue Herons, Ardea herodias, wading birds, Mississippi River, 
foraging behavior, flood stage
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