Lower Keys Refuges Set Aside to Protect Herons

By the early 1900s, snowy egrets, great white herons and other wading birds 

had been hunted nearly to extinction on their breeding grounds in south 

Florida. Great white herons may have even been reduced to only one hundred 

nesting pairs. Used to decorate fashionable ladies hats, the attractive breeding 

plumes of these and other wading birds garnered high prices when sold to 

hat-makers. In 1901, Florida passed legislation protecting non-game birds and in 

1908 the Key West National Wildlife Refuge (NWR) was established to 

protect native birds and their habitats. In 1938, the Great White Heron National 

Wildlife Refuge was created in the backcountry of the Lower Keys. Managed by 

the U.S. Fish and Wildlife Service, the refuges are havens for over 250 bird species, 

including great white herons. Together, they preserve about 400,000 acres of 

“backcountry” shallow waters, seagrass flats and low-lying mangrove islands. 

Nearly 4,000 acres within the two refuges have been congressionally designated 

as wilderness. Wilderness areas 

are managed to preserve their 

natural condition and are places 

where people are considered 

“visitors”.

Scientists Study Heron Foraging Habits and Habitats

In the early 1940s, the U.S. Fish and Wildlife Service reported that the great white 

heron population was recovering, a trend that continued until the mid-1990s 

when their numbers began to decline for unknown reasons. Recent population 

assessments show that herons have been experiencing a steady decline for 

an extended period of time. Specifically, decreases have been observed in the 

subpopulation that resides in Florida Bay. Another smaller subpopulation resides 

in the backcountry of the refuges and appears more stable, although it experienced 

losses due to hurricanes in 2005. To better understand what may be impacting 

these birds, biologists from Avian Research and Conservation Institute received 

funding from the Florida Fish and Wildlife Conservation Commission and U.S. Fish 

and Wildlife Service to document heron foraging and nesting patterns. The study 

took place in the Great White Heron NWR and western Florida Bay from 2007 

through 2010. 

Great white herons feed on fish they see while wading through shallow waters. 

Toadfish, red porgy, pipefish and needlefish are some of the more common prey. 

Using a striking motion, herons capture fish with their strong beaks. In this project, 

scientists used spotting scopes and binoculars to log over 3,000 observations 

of heron feeding behavior from a distance that did not disturb feeding birds. 

During the observation periods, the number of feeding strikes per minute, catches 

per strike, number of catches per minute and prey size were recorded. Prey fish 

were classified into one of five size categories. Scientists also followed five GPS 

(satellite) tagged adult birds and calculated the amount of time foraging and the 

type of foraging habitat selected. 

 Great White HeronS IN THE BACKCOUNTRY

Florida Keys National Marine Sanctuary

http://floridakeys.noaa.gov/

Great white herons nest on mangrove islands.

Photo: Andy Collins for FKNMS

When first described by 

ornithologists, great white herons 

were considered to be a separate 

species from great blue herons. 

Although some scientists have 

suggested that they have at least 

subspecies status, at present, 

great white herons are considered 

a color variation or “morph” of a 

subspecies of great blue herons 

found in southern Florida and 

the Florida Keys. Blue and white 

birds are known to breed with 

one another and their offspring 

may have a mixture of blue and 

white plumage. Because of their 

small population size and highly 

localized occurrence, great white 

herons qualify for the highest action 

level, Critical Recovery, in the Fish 

and Wildlife Service’s Regional 

Waterbird Conservation Plan 2006. 

This designation resulted, in part, 

from the nesting surveys taken in 

Florida Bay and the refuge 

backcountry. 
Shallow Seagrass Meadows are Critical Feeding Grounds

Field observations made on foraging birds and location data from the tagged adults confirmed that seagrass, specifically turtle 

grass (Thalassia testudinum), habitat was the most commonly selected habitat for foraging. Approximately 60% of the available 

seafloor consisted of dense to moderate continuous seagrass beds and yet 92% of the feeding locations were in this habitat. 

Foraging great white herons were found in only three other habitats, and then only minimally: patchy seagrass, hardbottom with 

seagrass, and bare substrate. Foraging birds were concentrated near the largest grassbeds and clearly favored continuous, not 

patchy, grass beds. Intact expanses of seagrass appear to be important both in and out of the nesting season. Data tracking the 

five tagged herons show that foraging herons exhibited fairly high site fidelity, that is, individual birds tended to use the same area 

repeatedly. In terms of bird foraging decisions and performance, water depth, which is tied to the tidal cycle, appears to be more 

important than time of day. 

Bay and Refuge Herons Show Differences in Foraging and Nesting Success

Observations of foraging great white herons found birds in the refuge generally caught larger fish than birds in Florida Bay and 

had better foraging success as measured by the number of catches per minute (.094 verses .036). The potential to support 

fish-eating birds is clearly different between the two areas and could be significant to breeding birds who are feeding their young. 

Scientists also monitored nests on mangrove islands in Florida Bay and the refuge during the breeding seasons of 2008 and 

2009. They recorded the size and number of eggs and/or chicks, nest height, tree height and type, color of chicks (blue or white) 

and tracked each nest to determine its success. Of the nests for which the final outcome was known, 57% in the refuge were 

successful, having produced at least one young, and only 

40% were successful in the bay. Nesting success in the refuge 

exceeded that of the bay by clear margins and may be related 

to factors that differ between the two areas, including lower 

food supply in grassbeds. In the late 1980s, Florida Bay began 

experiencing significant ecological changes, including the 

widespread mortality of turtle grass beds that had typically 

supported a variety of small prey fish. 

Predation may play a role in mortality of nestlings in Florida Bay. 

An analysis using only nests that were successful in producing 

young revealed that the number of surviving chicks per 

nest was similar in the refuge and bay. This observation 

implicates predation as a cause of mortality because 

predation would likely result in the loss of all young from the 

nest, whereas reduced food supply would probably result in 

some young surviving. Researchers noted the high prevalence 

of crows on or near nesting sites in Florida Bay and suggested 

that nest failures during this study may have resulted from 

predation by crows on unattended nests when birds were 

flushed by some disturbance, including from passing motor 

boats.

Preserving the Wilderness Character is Essential to Survival of Great White Herons

Backcountry seagrass flats and mangrove islands are critical for survival of the small isolated population of great white 

herons. Birds forage on large, intact grassbeds during a limited part of the tidal cycle and nest on nearby mangrove islands. 

Vessel exclusion areas, no-motor, no-entry and buffer zones in the refuge help preserve the quality of these critical habitats by 

protecting flats from boating impacts and keeping people and noise away from feeding, resting and nesting birds. Similar to 

other birds, great white herons often fly when disturbed by people approaching too closely or by loud noises from vessels. 

Restrictions that apply to people and vessels, including non-motorized ones, are in place to prevent such disturbances, thereby 

protecting the vulnerable population of herons and other wildlife that reside in the remote backcountry of the Great White Heron 

NWR. 

Principle Investigator: Dr. Kenneth D. Meyer, Avian Research and Conservation Institute

Written by: Nancy Diersing, Florida Keys National Marine Sanctuary

Publication Date: February 2013

Some areas in the refuge are closed to entry to protect wildllife and habitats.

Photo: Great White Heron National Wildlife Refuge