alba) ) on the Susquehanna River in Harrisburg, Pennsylvania in Relation to Environmental Characteristics

Transcription

1 Foraging Behavior of Great Egrets (Ardea( alba) ) on the Susquehanna River in Harrisburg, Pennsylvania in Relation to Environmental Characteristics W. Brad Romano, Don L. Detwiler, Dr.Terry L. Master, Dr. Eugenia Skirta ESU Avian Ecology & Behavior Laboratory

2 The 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

3 Great Egrets in Pennsylvania Largest of the Four White Wader Species 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

4 Wade 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

5 Foraging 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

6 Foraging Area

7 Foraging 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

8 Habitat 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)

9 Local Scale: Ideal Foraging Model Year/ Location R R 2 Adjusted R 2 F Value Sig 2006 Riv Wildwood Riv Age Variables Included Variable 2006 Wildwood 2007 Date Elapsed Time (mins) X X Sky Condition X Precipitation Wind Speed Surface Disturbance X Temperature Water Depth X Current Strength X Water Clarity Plant Density X Cobble Index Successful Attempts/min X X X Total Attempts/min X X X X 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.

10 Year/ Location R R 2 R 2 Adjusted Age Date Local Scale: Ideal Foraging Model F Value 2006 Riv Wildwood Riv Variables Included In Variable 2006 Wildwood 2007 Elapsed Time (mins) Sky Condition Precipitation Wind Speed Surface Disturbance Temperature Water Depth Current Strength Water Clarity X X Vegetation Density Cobble Index Capture Efficiency X X X Total Attempts/min X X X X X X X X X Sig 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

11 Effects of experience on capture efficiency Capture Efficiency vs. Age Capture Efficiency Wildwood N=29 N=25 N=11 N=9 N=10 Adult Juvenile Hatch Year 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. Age

12 Seasonal variations in foraging rates Foraging Foraging Metrics Metrics Vs Vs. Date Date in 2007 In 2006 Foraging Metrics Vs Date at Wildwood Lake Capture Efficiency (N=72) Capture Efficiency * (N=50) Capture Efficiency (N=34) Successful Attempts/min (N=71) Successful Attempts/min* Successful Attempts/min* (N=43) Total Attempts/min* (N=70) Total Attempts/min* (N=46) (N=36) Total Attempts/min* (N=34) Rate Rate Rate /12 6/14 0 5/25 6/16 6/4 6/18 5/27 6/20 6/6 5/29 6/22 6/8 5/31 6/24 6/10 6/2 6/26 6/12 6/28 6/30 7/2 6/4 6/14 6/6 6/16 7/4 6/18 7/6 Date (MM/DD) 6/8 6/10 6/20 6/12 7/8 7/10 6/22 7/12 6/24 Date (MM/DD) 6/14 6/16 7/14 6/26 6/18 7/16 6/28 6/20 7/18 7/20 6/30 6/22 7/22 7/2 6/24 7/24 7/4 6/26 6/28 7/6 6/30 7/8 7/10 7/2 7/4 7/6 Date (MM/DD)

13 Sky Condition vs. Capture Efficiency Sky 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 Capture Efficiency * (N=50) 2007(N=72) Lake (N=34) Clear Overcast Broken Hazy Sky Condition

14 Mean Attempt Rates vs. Water Depth On 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 Rate/minute Mean Attempt Rates vs. Depth (m) in 2006 Successful Attempts/minute* Total Attempts/min* Mean Attempt Rates vs. Depth (m) at Wildwood Lake < Depth (M) Mean Attempt Rate/minute Successful Attempts/min* Total Attempts/min* On the lake, however, we observe a decreasing pattern in attempt rates as depth increases < Depth (M)

15 Local Scale: Use 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

16 Landscape Scale Analysis: Aerial Survey Conducted June 13, 2006 Wade Island Counted foraging individuals along a 36 km stretch of river 58% of individuals foraged within 5 km of the colony N Maximum distance of a recorded individual: 28 km

17 Landscape Scale: Departure Vectors

18 Prey Characteristics: Prey Type 2006 River Wildwood Lake 2007 River 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

19 Prey Characteristics: Prey Size 2006 River Wildwood Lake 2007 River A majority of the prey consumed by Great Egrets were between two and six centimeters (1-3 inches)

20 Wrap-Up 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

21 Wrap-Up 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.

22 Acknowledgements Pennsylvania Department of Conservation and Natural Resources Wild Resource Conservation Program Sara Nicholas East Stroudsburg University Department of Biological Sciences Michael Allen Luis Vidal Council of Pennsylvania University Biologists Powdermill Avian Research Center Bob Mulvihill Mike Lanzone Adrienne Leppold Larry Barth Ornithological Technical Committee, PA Biological Survey Daniel Brauning,, Douglass Gross, PA Game Commission Peter Frederick, University of Florida David Norman, RSPB William Davis, Logistical Support Wildwood Lake Sanctuary and Nature Center