INTERACTION BETWEEN GREAT BLUE HERONS AND OSPREYS ON UPPER POOL 20, MISSISSIPPI 
RIVER

Robert L. Connour II1 and Thomas C. Dunstan2
1Dept. of Math, Life, and Natural Sciences, Owens Community College, Findlay, OH 
45840 2Dept. of Biological Sciences, Western Illinois University, Macomb, IL 
45840

Great Blue Herons (Ardea herodias) are common residents along the Mississippi 
River and are year round residents in many portions of their range.  Ospreys 
(Pandion haliaetus), although less common, are seen in greater numbers as they 
begin their fall migration in August and September.  For this study, data on 
great blue herons were collected from 29 May to 29 September 1997.  Also in 1997 
ospreys (N=30) first appeared in the study area on 27 August and were counted, 
along with herons, three times per day three days per week until 27 September.  
Numbers of herons seen remained consistent with numbers prior to the ospreys' 
appearance, and the two species did not appear to displace one another, often 
being seen in close proximity.  This is probably due to differences in foraging 
strategies.  Although the two species often perched in trees along the same 
shorelines, it is noteworthy that they were never observed together in the same 
tree.  Herons were often spotted along the water's edge directly below ospreys 
that were perched in trees above.  Although herons were common along the Iowa 
shoreline where much area is exposed and open for wading, no ospreys were seen 
along this edge.  This is probably due to the complete lack of suitable perch 
trees along this area compared to the more wooded areas along the Illinois and 
Missouri shorelines.  Both species were common near Lock and Dam 19 and along 
the Illinois shorelines.

Key words: Great Blue Heron, Osprey, Mississippi River, migration, shoreline