Graduation Year

1983

Document Type

Thesis

Degree

M.A.

Degree Name

Master of Arts (M.A.)

Department

Biology

Degree Granting Department

Biology

Major Professor

Andrew J. Meyerriecks

Committee Member

John M. Lawrence

Committee Member

Bruce C. Cowell

Abstract

The foraging ecology and behavior of Little Bule Herons (Egretta caerulea), Snowy Egrets (E. thula), and Tricolored Herons (E. tricolor) was studied from October 1981 to August 1982 in Old Tampa Bay and Safety Harbor, Hills borough and Pinellas Counties, Florida. Little Blue Herons foraged exclusively by walk-slow and employed head sways or tilted during 75% of the observations. Snowy Egrets used stand, walk-slow, disturb-and-chase, and foot-stirring. Tricolored Herons used stand, walk-slow, disturb-and-chase, ana stand-and-chase. Tricolored Herons using stand-and-chase stood near shore facing open water and when prey was spotted they ran or hopped in the direction of the prey and attempted capture. After an attempt the bird returned to its original location and posture. Open-wing and neck-tilt behaviors may be mechanisms for reducing glare.

Interspecific aggression was infrequent, but intraspecific aggression by Snowy Egrets was more frequent. Snowy Egrets occasionally initiated foraging associations with others species of birds but these associations did not appear to be immediately profitable for the Snowy Egrets.

Microhabitat use was similar for all three species. All three species spent more time along the shoreline than would be expected from a random use of the habitat. Snowy Egrets and Little Blue Herons spent more time foraging in pools than would be expected from random use of the habitat. Tricolored Herons did not forage on shore. The overlap index indicated that all three species foraged at the same depth. Snowy Egrets used only walk-slow on shore, while pools were usually foot-stirred, and disburb-and-chase was most prevalent in open water. Tricolored Herons used walk-slow most often along the shoreline and in canals, and disburb-and-chase in open water.

Little Blue Herons and Snowy Egrets had the most diverse diets and showed significant overlap (Cz = 0.82) for prey type. Polychaetes (30.3%) and fish (33.6%) were the most frequent prey items of Little Blue Herons; prawns (37.6%) and fish (42.2%) of Snowy Egrets. There was significant dietary overlap (Cl = 0.80) between Snowy Egrets and Tricolored Herons. Fish (83.0%) was the most frequent items in the diet of Tricolored Herons. Fish made up the greatest percentage of weight of all prety species for all three heron species. All three ehron species had high overlap (>= Cl = 0.97) for size of prawns eaten. Fish-size overlap was low (Cl = 0.31) between Little Blue Herons and Snowy Egrets and significant between Little Blue Herons and Tricolored Herons (Cl = 0.79) and between Snowy Egrets and Tricolored Herons (Cl = 0.79). When the results of the three species are combined, prawns and polychaetes were caught with the walk-slow behavior and fish with stand and walk-slow behaviors.

Little Blue Herons were more efficient than Snowy Egrets and Tricolored Herons, both for percentage successful strikes and grass per minute of food. Snowy Egrets were most successful using stand behavior, while Tricolored Herons were equally successful using all behaviors. Little Blue Serons were most successful on shore and Tricolored Herons along the shoreline. Snowy Egrets were equally successful in all microhabitats. Little Blue Herons and Tricolored Herons differed in major food items and Snowy Egrets and Little Blue Herons differed by fish size. No obvious food differences were detected between Snowy Egrets and Tricolored Herons. Foraging behavior use differed among all three heron species while habitat use was very similar. Little Blue Herons used one behavior in all microhabitats to catch all prey items. Snowy Egrets and Tricolored Herons adjusted behaviors which sometimes were used to capture different prey items.

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