1

REPRODUCTIVE SUCCESS OF GREAT BLUE HERONS 

(ARDIA HERODIAS) NESTING IN SWANSON CREEK, MARYLAND 

DURING THE CHALK POINT OIL SPILL

Final Report

May 16, 2001

Introduction

On April 7, 2000, approximately 126,000 gallons of a mixture of #2 and #6 fuel oil were released from

a break in a pipeline providing fuel to the Chalk Point Generating Station. The pipeline is owned by the

Potomac Electric Power Company (PEPCO) and operated by Support Terminal Services Operating

Partnership, LP (ST Services). The spill initially leaked into a Spartina spp. dominated brackish

wetland located in Swanson Creek, a tidal tributary of the Patuxent River (Figure 1). The marsh covers

approximately 45 acres adjacent to the PEPCO Chalk Point facility. Extensive oiling of the wetlands

within Swanson Creek took place during the first two days of the spill. On April 8, high winds, rain,

and tides resulted in the oil being blown over containment blooms and into the Patuxent River and its

tributaries. The spill spread approximately 17 linear miles downstream in the Patuxent River and into

several downstream tributaries, including Indian Creek, Trent Hall Creek, Washington Creek, and

Cremona Creek, oiling approximately 40 miles of shoreline (Figure 2). In addition, water quality

surveys indicated concentrations of petroleum products remained elevated above background levels in

the Patuxent River for approximately 2-3 weeks following the spill (unpublished data). Shoreline cleanup activities, particularly in the most heavily impacted areas (e.g., Swanson Creek), continued through

the summer and fall.

Primary habitats impacted during the spill were wetlands, sandy beaches and associated open water

areas. Several avian species that are dependent on aquatic habitats were nesting in the area at the time

of the spill, including osprey (Pandion haliaetus), federally threatened bald eagles (Haliaeetus

leucocephalus), and great blue herons (Ardea herodias). The foraging strategy of these species and

willingness to enter water and wetlands make them susceptible to oiling. Several oiled osprey and

great blue herons were observed by wildlife survey teams during the response effort. 

The Natural Resource Trustees for the Chalk Point Oil Spill were concerned about the potential effects

of the oil spill on the reproductive success of these avian species. To this end, the objective of the

present study was to evaluate the potential effects of the oil spill on the nesting success of great blue

herons. Other reports will describe results of monitoring nesting success of bald eagles and osprey.

Study Organism and Susceptibility to Oiling

The great blue heron is the most widely distributed heron species in North America, and is also the

largest wading species in Maryland (Robbins and Blom 1996). Great blue herons are
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3

Figure 2. Extent of shoreline oiling resulting from the Chalk Point Oil spill.
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 identified as a colonial nesting species, a feature commonly associated with members of the heron

family Ardeidae (Robbins and Blom 1996). An active great blue heron heronry was located within

Swanson Creek, approximately 0.75 miles upstream of the spill release. Great blue herons and similar

species spend much of their foraging activity wading in shallow waters often associated with wetlands

(Willard 1997, Robbins and Blom 1996, Kushlan and Hafner 2000), making them highly susceptible to

oiling.

Adults that become oiled may transfer the oil from their plumage to their eggs during incubation. 

Refined oil may be highly toxic to avian embryos depending on the species; stage of embryonic

development; and type, weathering, and dose of oil. Small quantities of oil on eggs can lead to embryo

mortality, or cause deformities, especially during the early incubation phase (Albers 1991, 1995,

Hoopes et al. 1994). Studies have shown that as little as 1-20 uL of some types of oil can have lethal

effects on developing embryos (Parnell et al. 1984, Hoffman 1990). Louisiana heron (Hydranassa

tricolor) eggs treated with 10 uL of weathered crude oil had a reduced hatchability of 17% (Macko

and King 1980). In addition, the oil spill may have reduced or contaminated prey species, such as

crustaceans, fish, amphibians, snakes, and small mammals (Butler 1992). Several studies have

reported reproductive effects on avian species due to dietary exposure to petroleum-derived products

(Coon and Dieter 1981, Ainley, et al. 1981). 

Study Area

Two great blue heron colonies, an exposed heronry and a reference heronry, were selected for

monitoring reproductive success. The exposed heronry was located in Swanson Creek (SCH) and the

reference heronry was located in Black Swamp Creek (BCH).

Swanson Creek Heronry (SCH)

SCH was located (N: 38o 33.093' W: 76o 43.926'; USGS 7.5 min. Quadrangle, Benedict, MD)

approximately 0.75 miles upstream of the release site in Swanson Creek and was composed of

approximately 34 active nests (Figure 1). The nests that were selected for monitoring were all located

in mature sycamore trees (Platanus occidentalis) approximately 75-100 feet in height. The site is

considered a seasonally flooded forested wetland. Extensive emergent wetlands and shallow open

water habitats within Swanson Creek provide excellent foraging habitat for great blue herons nesting

within the heronry.

Black Swamp Creek Heronry (BCH) 

BCH was selected as the reference heronry for comparative purposes and was located (N: 38o 37.737' 

 W: 76o 41.909'; USGS 7.5 min. Quadrangle, Lower Marlboro, MD ) on state game lands identified as

Milltown Natural Resources Management Area near Milltown Road in Prince George’s County,

Maryland, and adjacent to Black Swamp Creek, a tidal tributary of the Patuxent River (Figure 3). 

BCH was located approximately 5.4 miles north of SCH and was comprised of approximately 70 pairs
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of nesting great blue herons, based on a ground census during the study period and a second census in

the fall of 2000. The number of breeding pairs at
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 BCH in 1995 was reported as 90 increasing to 122 pairs in 1999 (Dave Brinker, MDNR; personal

communication). The nesting area encompasses approximately 3 acres of deciduous forested

bottomland/upland. All great blue heron nests were located in sycamore trees (Platanus occidentalis),

approximately the same height as trees located at SCH. The heronry is located adjacent to an

extensive emergent wetland and is associated with shallow open water habitats similar to those found in

Swanson Creek and provides ideal foraging habitat.

Methods

The methodology for this assessment was developed cooperatively by the Wildlife Injury Workgroup

for the Chalk Point Oil Spill Natural Resource Damage Assessment consisting of personnel from

PEPCO, U.S. Fish and Wildlife Service (USFWS), the Maryland Department of Natural Resources,

and the U.S. Geological Survey’s Patuxent Wildlife Research Center and is described in the Wildlife

Injury Assessment Workplan for the Chalk Point Oil Spill (October 2000). The National Oceanic

and Atmospheric Administration and the Maryland Department of the Environment also provided input

to the assessment plan. In general, the approach was to monitor nests on a regular basis until fledging

occurred. Nest inspections consisted of counting and recording live young and eggs by climbing trees

and observing the nests from nearby branches. 

Each great blue heron nest was selected for monitoring based on two criteria established by the

monitoring team, two professional tree climbers experienced with conducting avian nest surveys, during

a pre-monitoring survey. The two criteria used for nest selection were: 1) safety, and 2) accessibility to

nests. Each selected nest was identified by a unique identification number and charted on a pictogram

for future reference (Werschkul et al. 1977, Pratt and Winkler 1985). The nests were initially visited

April 19-20 to determine ease of monitoring, and then visited five times in a period ranging from May

12 to June 14, 2000. During each visit to the heronry, the number of eggs and/or young per nest were

noted and recorded. In addition to the reproductive parameters that were recorded during each site

visit, nests, adults and nestlings were also inspected for oiling.

Ideally, in a reproductive study, the investigator(s) would monitor a nest from the day the first egg was

laid, follow the fate of each successive egg until hatching and then follow each nestling until fledging,

approximately 54-60 days post hatch (Robbins and Blom 1996, Kushlan and Hafner 2000) . In this

study, nesting activity had already been initiated at each heronry, with eggs already laid in a number of

nests at both sites. Therefore, in this study only the young could be reliably monitored. The number of

eggs laid initially could not be positively determined, a result of the time delay between the day of egg

laying and the initiation of monitoring.

Data Analysis 

The total number of live young per nest (the sum of live young in each nest plus live young on branches

nearby) was used for analyses of nest success and productivity. Nest success, defined in this study as

at least one nestling successfully fledging per nest, was analyzed using the Mayfield method (Mayfield
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1961, 1975, Bart and Robson 1982), the results of which were used to compare SCH to BCH, the

reference site. This analysis estimates a daily survival rate for nests at each colony and controls the bias

which may exist due to nests which failed before the collection of data began. Overall survival rate of

young great blue herons from hatching to fledging was calculated by raising the calculated daily survival

rate to the 54th power, the approximate number of days required for great blue heron nestlings to

fledge. The Program Mayfield, written by J.E. Hines (available at:

http://www.mbr-pwrc.usgs.gov:80/software.html), was used to calculate the daily nest survival rates as

described by Bart and Robson (1982). Program Contrast was used to compare the daily nest survival

rates between the two colonies (Hines and Sauer 1989).

Productivity of each nest or the number of young per successful nest (nests with at least one chick) was

analyzed using Fisher’s exact test to compare the productivity of SCH to BCH (Lehmann 1975). 

Each visit was analyzed separately in this analysis because of the unknown fate of chicks once they

were no longer associated with a nest.

The average pre-fledging brood size (i.e., brood size based on 20-40 day old nestlings) as well as the

average number of young observed (per nest) at the age when they are capable of sustained flight, a

surrogate measure for the average number of young fledged, were also calculated as measures of

reproductive success. These statistics can be used to compare the productivity of the heronries to each

other as well as to values reported in the literature (Parsons and McColpin 1995, Kelly et al. 1993). 

Pre-fledging brood size was calculated as the overall average brood size of successful nests at each site

from the beginning of monitoring on May 12 through May 26, 2000. Nestling data for June 7 were

used to estimate the average number of young observed (per nest) at the age when they are capable of

sustained flight. This date was chosen as the best estimate of fledging age based on the reproductive

biology of great blue herons and literature for Maryland. Robbins and Blom (1996) stated that the

earliest hatch date records for Maryland is April 13. Based on this assumption, young in nests

monitored on June 7 would be approximately 53 days old, very close to the age at which birds would

begin to leave the nest. 

Results

The colonies were visited on May 12, 2000 (Swanson Creek only), May 15, 2000 (Black Swamp

Creek only), May 19, 2000, May 26, 2000, June 7, 2000, and June 14, 2000. During the monitoring

period, a number of dead nestlings were observed in nests and on the ground. Table 1 provides a

summary of the dead nestlings found at each heronry.

Nest Success

The daily survival rates, 0.9934 and 0.9947 for the estimates of nest success at SCH and BCH,

respectively, were not statistically different (c1

2

 = 0.0475, P = 0.8275). Overall survival rates (daily

survival rate raised to the 54th power, based on fledging age of 54 days) for nestlings from hatching to

fledging were 70% and 75% for SCH and BCH, respectively.
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Table 1. Summary of dead nestlings observed at each heronry during the great blue heron

reproductive monitoring period of May 12 through June 14, 2000.

Heronry SCH BCH

Date Nest Ground Nest Ground

12 May 2000 1 1

19 May 2000 2

 7 June 2000 3 4

14 June 2000 1 2 2*

Sub-Total 5 3 6

Total 8 6

* Nestlings observed were from the same group of 4 observed dead the previous week.

Table 2. Summary statistics (mean, number of nests (N), and standard deviation (SD)) and p-values

from Fisher’s exact test comparing the number of chicks per nest (nest productivity) of successful nests

(nests with at least one chick) at Swanson and Black Swamp Creeks. 

 Black Swamp Creek (BCH) Swanson Creek (SCH)

Date of visit Mean N SD Mean N SD P-value

12-15 May 2000 2.47 17 0.87 2.08 12 0.51 0.378

19 May 2000 2.18 17 0.95 1.92 12 0.51 0.544

26 May 2000 2.07 15 0.88 1.83 12 0.58 0.556

07 June 2000* 1.50 12 0.67 1.88 8 0.64 0.370

14 June 2000 1.00 5 0 1.25 4 0.50 0.444 

* Estimated fledging date
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Nest Productivity

The results of the analysis of nest productivity found no significant differences (P > 0.05) between the

two colonies during any of the visits to the nests (Table 2). Prefledging brood size based on 20-40 day

old nestlings was calculated as 1.94 and 2.24 at SCH and BCH, respectively. The average number of

young observed (per nest) at the age when they are capable of sustained flight were 1.88 at SCH and

1.50 at BCH (Table 2). 

Discussion

Nest Success

Although extensive oiling of foraging habitats utilized by great blue herons occurred during the Chalk

Point oil spill, the reproductive data showed that there was no statistical difference in daily survival rates

of nestlings to fledging between SCH and BCH. In addition, overall survival rates of SCH nestlings

(70%) were similar or higher than those reported in the literature. For example, Butler (1992) reported

an average survival of 63% for studies of populations ranging from Florida to Alberta.

Nest Productivity

There were no statistical differences between the number of nestlings produced per nest at each

sampling day. However, the average number of young fledged (i.e., mean number of chicks on June 7)

in SCH and BCH, 1.88 and 1.50, respectively, were both lower than the 2.3 fledged per nest average

(S.D. = 0.30, n = 16 studies) reported by Butler (1992). In an extensive reproductive study involving

19 great blue heron colonies in Ohio and Pennsylvania, values for the number of nestlings fledged per

nest ranged from 1.23 - 2.60 (Carlson and McClean 1996). Parsons and McColpin (1995) reported a

value of 1.6 for great blue herons nesting in Delaware Bay; however, they suggest this value may be

low because of difficulties in determining nestling ages. Although the number of fledged young per nest

at BCH and SCH are lower than many literature values reported for the east coast, the numbers

compare favorably with values (1.5 - 1.7) needed to maintain a stable population at approximately 38o

north latitude, the same latitude as the Chalk Point spill area (Henny 1972). However, Henny’s (1972)

fledging values are calculated for west coast populations (California) and should be interpreted with

caution, as they may not be applicable to Mid-Atlantic great blue heron populations. Values similar to

Henny’s have not been calculated for the Chesapeake Bay region (Kathy Parsons, Manomet

Observatory for Conservation Sciences, and Brian Watts, College of William and Mary, personal

communication). 

The mean number of 20 to 40 day old pre-fledged young produced at both SCH and the reference

heronry, 1.94 and 2.24 respectively, were lower than the 2.4 value observed by Parsons and

McColpin (1995) at a great blue heron colony located in Delaware Bay. However, pre-fledging brood

size for SCH was comparable to the 1.93 average reported by Kelly et al. (1993) in a study

conducted in the San Francisco Bay area.

The reason that productivity values (i.e., number of pre-fledged and fledged young) for the BCH and
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SCH populations in 2000 are on the low end of the range reported in the literature may be due to

climatic factors. The spring of 2000 was one of the wettest in past years, particularly April in which the

amount of precipitation was the highest recorded in 30 years (National Weather Service,

http://www.nws.noaa.gov). April is a particularly vulnerable period for great blue heron in the region as

it is the period in which a majority of great blue heron eggs begin to hatch (Robbins and Blom 1996),

making the nestlings vulnerable to the elements. Young nestlings are unable to thermoregulate efficiently

until they are probably at least 18 days old or more (Butler 1992).

Nestling Mortality

The dead nestlings found during the monitoring period, eight at SCH and six at BCH, likely died of

natural causes. Siblicidal aggression is a common occurrence in great blue herons (Mock 1986). 

Great blue heron hatching events are asynchronous, with 4-8 days between hatching of the first and last

egg; therefore, the first nestling born is often the largest in the nest and often outcompetes its sibling

rivals for food resources, resulting in younger nestlings dying of starvation (Butler 1992). At the time of

discovery, the dead nestlings were in various stages decay and/or partially scavenged, making

determination of cause of death impossible.

Uncertainty

One of the drawbacks of the present study is that regular monitoring of the nests did not occur until

mid-May, more than a month after the oil spill occurred. An initial survey was conducted on the

Swanson Creek heronry on April 19 and April 20, 2000. A total of 34 nests were counted. Of the 34

nests, 17 were accessible to climbers. Thirteen out of 17 nests contained chicks and/or eggs for a total

of 18 chicks and 15 eggs. Three adult birds were observed to be oiled and all eggs appeared to be unoiled. However, since nest locations and contents were not accurately recorded, these data could not

be incorporated into the nesting success study. On May 12, 2000, nesting success monitoring was

initiated. Of the 17 nests initially surveyed, 10 contained a total of 20 living chicks and 7 were empty. 

No eggs were found. The decline in the number of young from April to May (33 to 20 chicks and eggs)

is substantial, but may be natural. For example, in a number of great blue heron reproductive studies,

nestling mortality was found to be the greatest within the first four weeks of life (Pratt 1970). 

Reproductive studies conducted by Quinney (1982) found that 73% of great blue heron nestlings died

within the first 10 days of life, and that mortality rates decreased after 4-5 weeks of age. Pratt and

Winkler (1985) reported that the second through fourth week of life were the most hazardous for great

blue heron nestlings, with a 76% nestling mortality rate reported for this period. 

Another area of uncertainty is the appropriateness of BCH as a reference site. Before the 2000

breeding season, the number of breeding pairs had increased annually at BCH; however, a ground

census conducted during the study period and again in the fall of 2000, indicated a decrease in the

number of breeding pairs from 1999 to 2000, from 122 to approximately 70. During the nesting

season, the mean distance flown from the colony to principal feeding sites is reported to be 1.4 to 4.0

miles (Butler 1992), ranging as high as 9.4 miles (Gibbs 1991), which could put the Swanson Creek

spill zone within the foraging area of the great blue herons nesting at BCH. Together, these
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observations raise the issue of whether this population may have been affected by the spill.

As a follow-up to this study, the Trustees will be conducting a nesting pair survey at both colonies in the

Spring 2001.

Summary

Overall survival rates of SCH nestlings were not statistically different from survival rates at BCH. In

addition, both populations exhibited survival rates that were similar to or higher than values reported in

the literature. Productivity of both SCH and BCH appear to be on the low end of the range of

literature values from other areas along the east coast; however, the cause cannot be linked to the oil

spill. Although several great blue herons within Swanson Creek were oiled, none of the eggs in nests

that were monitored were observed to be oiled. Productivity of both great blue heron populations may

have been affected by the weather, with April 2000 having the highest precipitation recorded for the

month in 30 years. There is some uncertainty associated with these results due to the delay in initiating

monitoring and appropriateness of BCH as a reference site. A nesting pair survey will be conducted at

both colonies in the Spring 2001.

Acknowledgments

This report was prepared by the Wildlife Injury Workgroup for the Chalk Point NRDA Council. The

Wildlife Injury Workgroup members are: Beth McGee and Peter McGowan, USFWS Chesapeake

Bay Field Office; Mark Hoffman, Maryland Department of Natural Resources; Jacqueline Michel,

Research Planning, Inc., and Richard Greer, Entrix, Inc., representing PEPCO. We thank Jeff Hatfield

of the Patuxent Wildlife Research Center (USGS/Biological Resources Division) for statistical analysis

of the data. We are extremely indebted to Craig Koppie (USFWS) who conducted the majority of

nest monitoring. Mark Causey, Devin Ray (USFWS) and Dan Murphy (USFWS) also provided help

in field monitoring. 

References

Ainley, D.G., C.R. Grau, T.E. Roudybush, S.H. Morrel and J.M. Utts. 1981. Petroleum ingestion

reduces reproduction in Cassin’s auklets. Marine Pollution Bulletin 12:314-317.

Albers, P.H. 1991. Oil spill and the environment: A review of chemical fate and biological effects of

petroleum. In The effects of oil on wildlife. J. White ed. Sheridan Press:Hanover, PA. 

Albers, P.H. 1995. Petroleum and individual polycyclic aromatic hydrocarbons. In Handbook of

Ecotoxicology. D.J. Hoffman, B.A. Rattner, G.A. Burton, Jr. and J. Cairns, eds. Lewis Publishers: Ann

Arbor, MI.

Bart, J., and D.S. Robson. 1982. Estimating survivorship when the subjects are visited periodically. 
13

Ecology 63(4):1078-1090

Brinker, D., Central Regional Manager, Division of Forest, Wildlife and Heritage, Maryland

Department of Natural Resources, Wye Mills, MD. Personal communication in January 2001. 

Butler, R.G. 1992. The great blue heron. In A. Poole, ed. The Birds of North America, No. 25. The

Academy of Natural Sciences Philadelphia, Pennsylvania and American Ornithologists Union,

Washington D.C.

Carlson, B., and E.B. McClean. 1996. Buffer Zones and disturbance types as predictors of fledging

success in great blue herons, Ardea herodias. Colonial Waterbirds 19(1):124-127

Coon, N.C. and M.P. Dieter. 1981. Responses of adult mallard ducks to ingested South Louisiana

crude oil. Environmental Research 24:309-314.

Gibbs, J.A. 1991. Spatial relationships between nesting colonies and foraging areas of Great Blue

Herons. Auk 108:764-779.

Henny, C.J. 1972. Analysis of the population dynamics of selected avian species. U.S. Fish and

Wildlife Service Research Report 1.

Hines, J.E., and J.R. Sauer. 1989. Program Contrast - A general program for the analysis of several

survival or recovery rate estimates. U.S. Fish & Wildlife Service, Technical Report 24, Washington,

D.C.

Hoffman, D.J. 1990. Embryotoxicity and teratogenicity of environmental contaminants to bird eggs.

Review of Environmental Contamination and Toxicology 15:39 - .

Hoopes, E.M., P.M. Cavanaugh, C.R. Griffin, and J.T Finn. 1994. Synthesis of information on marine

and coastal birds of the Atlantic coast : Abundance, distribution, and potential risks from oil activities. 

University of Massachussetts Cooperative Fish and Wildlife Research Unit Technical Report MMS 93-

001-003, Vol. 1. Amherst, MA.

Kelly, J.P., H.M. Pratt, and P.L. Greene. 1993. The distribution, reproductive success, and habitat

characteristics of heron and egret breeding colonies in the San Francisco Bay Area. Colonial

Waterbirds 16(1):18-27.

Kushlan, J.A., and H. Hafner. 2000. Heron Conservation. Academic Press:New York. 480p.

Lehmann, E.L. 1975. Nonparametrics. Holden-Day, Inc., San Francisco, California.
14

Macko, S.A., and S.M. King. 1980. Weathered oil:effect on hatchability of heron and gull eggs

Bulletin of Environmental Contaminants and Toxicology 25(2):316-320.

Mayfield, H.F. 1961. Nesting success calculated from exposure. The Wilson Bulletin 73(3):255-

261.

Mayfield, H.F. 1975. Suggestions for calculating nest success. The Wilson Bulletin 87(4):456-466.

Mock, D.W. 1986. Siblicide, parent-offspring conflict and unequal parents investment by egrets and

herons. Behavioral Ecology and Sociobiology 20:247-256.

Parnell, J.F., Shields, M.A. and D. Frierson. 1984. Hatching success of brown pelican eggs after

contamination with oil. Colonial Waterbirds 7:22- 

Parsons, K., Senior Scientist, Avian Conservation Division, Manomet Observatory for Conservation

Sciences, Manomet, MA. Personal communication in January 2001. 

Parsons, K.C., and A.C. McColpin. 1995. Great heron reproductive success in upper Delaware Bay.

J. Field Ornithology 66(2):181-191.

Pratt, H.M.., and D.W. Winkler. 1985. Clutch size, timing of laying, and reproductive success in a

colony of great blue herons and great egrets. The Auk 102:49-63

Pratt, H.M. 1970. Breeding biology of great blue herons and common egrets in Central California. 

Condor 74:407-416

Quinney, T.E. 1982. Growth, diet, and mortality of nestling great blue herons. Wilson Bulletin

94(4):571-577

Robbins, C.S., and E.A.T. Blom. 1996. Atlas of Breeding Birds of Maryland and the District of

Columbia. Pittsburgh University Press: Pittsburgh, PA. 479p.

Watts, B., Director, Center for Conservation Biology, College of William and Mary, Williamsburg,

VA. Personal communication in January 2001.

Werschukul, D., E. McMahon, M. Leitschuh, S. English, C. Skibinski, and G. Williamson. 1977.

Observations on the reproductive ecology of the great blue heron (Ardea herodias) in Western

Oregon. The Murrelet 58:7-12

Willard, D.E. 1977. The feeding ecology and behaviour of five species of herons in southeastern New

Jersey. The Condor 79:462-470.
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