Foraging Behavior of Great Egrets (Foraging Behavior of Great Egrets (ArdeaArdea albaalba) on the ) on the Susquehanna River in Harrisburg, Pennsylvania in Susquehanna River in Harrisburg, Pennsylvania in

Relation to Environmental CharacteristicsRelation to Environmental Characteristics

W. Brad Romano, Don L. Detwiler, Dr.Terry L. Master, Dr. Eugenia Skirta

ESUAvianEcology & BehaviorLaboratory

The ProjectThe Project• Goals:

– Habitat: Landscape Scale• Determine extent of habitat use up and down-river• Determine critical water levels for egret use

– Habitat: Local Scale• Discover how egrets use microhabitat• Discern which variables are important to egret foraging

– Dietary Components• Which prey items are important in the diet of individuals

from the Wade Island population?

• Ultimate Goal:– Compile data useful to a future management

plan for the colony and its surrounding area

Great Egrets in PennsylvaniaGreat Egrets in Pennsylvania• Largest of the Four White Wader Species

– 94-104 cm– Black Legs– Yellow to green lores– Plumes during breeding season

• Seasonal Migrant– Abundance decreases in winter

• Habitat generalist– Dynamic use of habitats– Diverse diet recorded

• State Endangered– Recovery from millinery trade

Wade Island RookeryWade Island Rookery• Primary Study Site

• Wade Island Area• 3 Km radius of the Island• Rookery home to:

• Great Egret (Ardea alba)

• Double-Crested Cormorant (Phalacrocorax auritus)

• Black-Crowned Night Heron (Nycticorax nycticorax)

• Secondary Study Site• Wildwood Lake Nature

Sanctuary• 110 Acre Sanctuary• Spring-fed lake

surrounded by wetland habitat

N

Foraging ConstraintsForaging Constraints

• Environmental Constraints• River Level• Habitat Availability• Predator

Avoidance/deterrence• Physical Constraints

• Experience• Nutritional Requirements• Situational Plasticity

• Prey Constraints• Availability• Replenishment Rates

In the face of many constraints, the goal of a forager is to maximize fitness through optimal exploitation of its environment. In this study, we attempt to gain insight into how the Great Egrets of Wade Island accomplish such a seemingly insurmountable task……

Foraging AreaForaging Area

Foraging ObservationsForaging Observations• Foraging Observations were conducted

over two field seasons (2006 and 2007)• Foraging Observation Protocol

– Direct observation of focal individual• 10-20x Bushnell® binoculars• 20-60x Kowa® spotting scope• Stratified random 4-hour time blocks

– Five-minute minimum observation, 10 minute maximum

– Recorded environmental conditions, time spent foraging, prey data, and individual behaviors

Habitat SurveyHabitat Survey• Once a foraging observation was completed, a

habitat survey was performed.• Use and Non-use sites were marked with a GPS

unit– Use site was determined as center of foraging area used by

focal individual– Non-use site was randomly chosen 50 meters either up or

downstream from the extremities of the use area• During the habitat survey, data were collected on

plant densities (PD), bottom substrate composition( Cobble Index, CI), water depth (D), water clarity (WC), current strength (CS), and surface disturbance (SD)

Local Scale: Local Scale: Ideal Foraging ModelIdeal Foraging Model

Year/ Location R R2 Adjusted R2 F Value Sig2006 Riv. 0.867 0.751 0.717 21.750 0.000Wildwood 0.838 0.702 0.631 9.881 0.0002007 Riv. 0.764 0.584 0.565 30.421 0.000

Variable 2006 Wildwood 2007Age XDateElapsed Time (mins) X XSky Condition XPrecipitationWind SpeedSurface Disturbance XTemperatureWater Depth XCurrent Strength XWater ClarityPlant Density XCobble IndexSuccessful Attempts/min X X XTotal Attempts/min X X X

Variables Included A Multiple Linear Regression was performed to discern which variables, if any, affected Capture Efficiency in foraging individuals. Included variables gave the best explanation for the variance found within the Capture Efficiency data.

Local Scale: Local Scale: Ideal Foraging ModelIdeal Foraging Model

• Successful attempts/minute models revealed interesting results

• In 2006, temperature, depth, and water clarity were included

• In 2007, it was date, current strength, and water clarity

• In the Wildwood set, it was elapsed time and wind speed

Year/ Location R R2Adjusted

R2F

Value Sig

2006 Riv. 0.875 0.765 0.730 22.123 0.000

Wildwood 0.983 0.967 0.962 205.263 0.000

2007 Riv. 0.970 0.941 0.936 197.586 0.000

Variables Included In

Variable 2006 Wildwood 2007Age

Date X

Elapsed Time (mins) X

Sky Condition

Precipitation

Wind Speed X

Surface Disturbance

Temperature X

Water Depth X

Current Strength X

Water Clarity X X

Vegetation Density

Cobble Index

Capture Efficiency X X X

Total Attempts/min X X X

Effects of experience on capture Effects of experience on capture efficiencyefficiency

Capture Efficiency vs. Age

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Adult Juvenile Hatch Year

Age

Cap

ture

Eff

icie

ncy

2006Wildwood

N=29 N=11 N=10N=25 N=9

Increases in capture efficiency due to experience gained with age are well documented (Kushlan, 1981; Groves, 1978)Marchetti and Price (1989) also suggest that differences in developmental stage may play a role in foraging success, although this idea was not tested here.

Seasonal variations in foraging ratesSeasonal variations in foraging ratesForaging Metrics Vs. Date In 2006

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

6/12

6/14

6/16

6/18

6/20

6/22

6/24

6/26

6/28

6/30 7/2

7/4

7/6

7/8

7/10

7/12

7/14

7/16

7/18

7/20

7/22

7/24

Date (MM/DD)

Rat

e

Capture Efficiency * (N=50)

Successful Attempts/min*(N=43)Total Attempts/min* (N=46)

Foraging Metrics Vs Date in 2007

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

6/4 6/6 6/8 6/10

6/12

6/14

6/16

6/18

6/20

6/22

6/24

6/26

6/28

6/30 7/2 7/4 7/6 7/8 7/10

Date (MM/DD)

Rat

e

Capture Efficiency (N=72)

Successful Attempts/min (N=71)

Total Attempts/min* (N=70)

Foraging Metrics Vs Date at Wildwood Lake

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

5/25

5/27

5/29

5/31 6/2 6/4 6/6 6/8 6/10

6/12

6/14

6/16

6/18

6/20

6/22

6/24

6/26

6/28

6/30 7/2 7/4 7/6

Date (MM/DD)

Rat

e

Capture Efficiency (N=34)

Successful Attempts/min*(N=36)Total Attempts/min* (N=34)

Sky Condition vs. Capture EfficiencySky Condition vs. Capture EfficiencySky condition may be an important factor in surface glare and the ability of

prey to view predator and vice versa.Sih (1997) hypothesized that prey must make a decision as to when to leave refuge in search of food, and a predator’s presence influences that decision. Pirene and Crombie (1944) studied the effects of sky on visibility of different

colored objects. They determined that a background which concealed movement may be more beneficial than one that matched the color of a

predator.

Capture Efficiency Vs. Sky Condition

0

0.2

0.4

0.6

0.8

1

1.2

Clear Overcast Broken Hazy

Sky Condition

Cap

ture

Eff

icie

ncy

2006* (N=50)

2007(N=72)

Lake (N=34)

Mean Attempt Rates vs. Water DepthMean Attempt Rates vs. Water DepthOn the river, relatively minute

changes in water depth may not be as important as river level

changes. Many researchers have argued the effects of depth on

Great Egret foraging, coming to variable conclusions.

Mean Attempt Rates vs. Depth (m) in 2006

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

< 0.1 0.15 0.2 0.25 0.3 0.35

Depth (M)

Mea

n A

ttem

pt R

ate/

min

ute

Successful Attempts/minute*Total Attempts/min*

Mean Attempt Rates vs. Depth (m) at Wildwood Lake

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

< 0.1 0.15 0.2 0.25

Depth (M)

Mea

n A

ttem

pt R

ate/

min

ute Successful Attempts/min*

Total Attempts/min*

On the lake, however, we observe a decreasing pattern in attempt rates as depth increases.

Local Scale:Local Scale: Use within the Study SiteUse within the Study Site

• Foraging individuals forage in a variety of microhabitats– Island edges, river banks,

emergent vegetation beds, gravel bars, and surrounding water bodies

• Water Depth a physically limiting factor

• Environment as a whole influences an individual’s decision of where to forage

• An individual’s preference or avoidance of certain areas of its environment may improve foraging success

Landscape Scale Analysis:Landscape Scale Analysis: Aerial SurveyAerial Survey

Wade Island

Conducted June 13, 2006

Counted foraging individuals along a 36 km stretch of river

58% of individuals foraged within 5 km of the colony

Maximum distance of a recorded individual: 28 kmN

Landscape Scale: Landscape Scale: Departure VectorsDeparture Vectors

Prey Characteristics: Prey TypePrey Characteristics: Prey Type

• Primary prey consisted of fish in all data sets

• Secondary prey items were mainly insects tadpoles, and crayfish, with variable composition of the diet in each data set

Wildwood LakeWildwood Lake2006 River2006 River

2007 River2007 River

Prey Characteristics: Prey SizePrey Characteristics: Prey Size

• A majority of the prey consumed by Great Egrets were between two and six centimeters (1-3 inches)

Wildwood LakeWildwood Lake2006 River2006 River

2007 River2007 River

WrapWrap--UpUp• In all wetland environments, hydrological regimes

are important to the survival of many species.• Typically, riverine environments have relatively unpredictable

hydrological patterns that affect plant densities, prey distribution, turbidity, topography, and distance to available foraging locations (Custer et al, 2004).

• Wade Island’s location may be the key to its survival. • In estuarine environments, hydrological regimes are much

more predictable, and thus provide a more stable platform for nesting wading birds.

• The ability to survive in many different environments is made possible by plasticity in behavior, greatly exhibited among many wading birds, especially the Great Egret.• Aggregations vs solitary individuals• Dietary composition

WrapWrap--Up ContinuedUp Continued• Management of the colony must include

management of a wide area surrounding it, including off-river foraging locations.– Water Levels– Water Quality

• Great Egrets are dynamic creatures of opportunity, sometimes flexing the assumptions of optimal foraging, perhaps to maximize their overall fitness.

• Further research into the prey distribution must be completed in order to fully grasp the effects of variability in the environment on the future success of the Wade Island colony of Great Egrets.

AcknowledgementsPennsylvania Department of Conservation and Natural ResourcesPennsylvania Department of Conservation and Natural Resources

Wild Resource Conservation ProgramWild Resource Conservation ProgramSara NicholasSara Nicholas

East Stroudsburg UniversityEast Stroudsburg UniversityDepartment of Biological SciencesDepartment of Biological SciencesMichael AllenMichael AllenLuis VidalLuis Vidal

Council of Pennsylvania University BiologistsCouncil of Pennsylvania University BiologistsPowdermillPowdermill Avian Research CenterAvian Research Center

Bob Bob MulvihillMulvihillMike Mike LanzoneLanzoneAdrienne Adrienne LeppoldLeppoldLarry BarthLarry Barth

Ornithological Technical Committee, PA Biological SurveyOrnithological Technical Committee, PA Biological SurveyDaniel Daniel BrauningBrauning, Douglass Gross, PA Game Commission, Douglass Gross, PA Game CommissionPeter Frederick, University of FloridaPeter Frederick, University of FloridaDavid Norman, RSPBDavid Norman, RSPBWilliam Davis, Logistical SupportWilliam Davis, Logistical SupportWildwood Lake Sanctuary and Nature CenterWildwood Lake Sanctuary and Nature Center