Jamaica Bay studies VII: Factors affecting the distribution and abundance of ducks in a New York estuary

Estuarine,

Coastal

and Shelf

Science

(1984) 19,673-689

Jamaica Bay Studies VII: Factors the Distribution and Abundance in a New York Estuary

Joanna Burger”, Glenn S. Ritterd

J. Richard

Troutb,

Affecting of Ducks

Wade

Wanderc,

and

“Department of Biological Sciences, Rutgers University, Piscataway, N3 08854, hDepartment of Statistics, Rutgers University, New Brunswick, NJ 08903, ‘Graduate Program in Ecology, Rutgers Universiry, Piscataway, NJ 08854 and ‘New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Brunswick, NJ 08903, U.S.A. Received

1 September

Keywords: birds; diurnal; New York

1983 and in revised

ducks;

form

bays; estuaries;

15 March

1984

tidal cycles; temperature;

wind;

The abundance and distribution of ducks (Anatini, Cairinini, Aythyini, Mergini, Oxyurini) were examined at Jamaica Bay Wildlife Refuge, a coastal estuary on Long Island, New York. The refuge contains a variety of tidal habitats as well as two freshwater impoundments. The largest concentrations of diving ducks were present in March and April, and from October through December; while the largest concentrations of dabbling ducks were present from June through December. Thus, diving ducks used the refuge on migration whereas dabbling ducks used the refuge during and following the breeding season. Time of year was thus the most significant factor affecting distribution and abundance. Some species were present all year, including Black Duck Anus rubripes, Mallard A. platyrhynchos, Gadwall A. strepera, and Ruddy Duck Oxyura jamaicensis. Although both dabblers and divers used all areas of the bay, dabblers used both ponds while divers used only the East Pond. Dabbling ducks concentrated in the bay at low tide, and on the ponds at high tide. There were more divers on the bay on a falling tide although tide direction did not influence the abundance or distribution of dabblers. Temperature and wind variables influenced the distribution of all ducks: they used the bay at low temperatures, and rafted in large flocks on the bay in intermediate, Northwest winds. Only the numbers and distribution of dabblers were significantly influenced by cloud cover. We conclude that abiotic factors influence the abundance and distribution of ducks on Jamaica Bay Wildlife Refuge, and that tidal factors should be considered when managing migratory or wintering populations of ducks. Introduction

Ducks are common wintering and breeding birds in most of North America. On the northeast coast of North America over twenty duck speciescan regularly be seenduring migration, attesting to the importance of coastal bays and marshes to this resource. Habitat use by waterfowl has been examined for single species(Tamisier, 1974; Taylor, 1977), and for groups of species(Stewart, 1962; White 81James, 1978). However, habitat 673 0272-7714/84/120673+

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J. Burger et al.

use and requirements have not usually been examined for a whole assemblageof migrating waterfowl (but seeWhite &James, 1978), although this approach hasbeen taken by a number of researchersinterested in passerines(Cody, 1968; Whitmore, 1975). Broad scalestudies of all waterfowl in one geographical area are not often undertaken. Further, the effect of physical and environmental factors on waterfowl abundance and distribution has been largely ignored, resulting in a few data on how migrating waterfowl are affected by local environmental factors, or use different habitats seasonally or temporarily (but seeWhite &James, 1978). In this study we examine (I) the abundance and distribution of ducks at a New York estuary (Jamaica Bay Wildlife Refuge-JBWR), (2) the effect of abiotic factors on their abundance and distribution, and (3) the differences between the responsesof dabblers and divers to environmental and habitat differences. Jamaica Bay Wildlife Refuge contains a variety of tidal habitats as well as two large freshwater impoundments. The diversity of habitats allowed us to examine habitat preferences as well as the effect of temporal, tidal and weather-related variables (wind, temperature, precipitation, cloud cover). Since there are differences in feeding niches among dabblers and divers (Martin & Uhler, 1939; Bartonek & Hickey, 1969; Swanson & Sargent, 1972) we predicted that these speciesgroups would respond differently to environmental factors. These results are part of a larger study on waterbird use of Jamaica Bay Wildlife Refuge. Data on other bird groups are presented elsewhere (Burger, 1983a, 6; 1984). Study area and methods Jamaica Bay Wildlife Refuge (National Park Service, 3600 ha) is located on the south shore of western Long Island, New York and is bordered by J. F. Kennedy International Airport, residential communities, several active sanitary landfills, and expressways (Figure 1). During this study human disturbance was minimal in most areasof the bay (seeBurger, 1981). The refuge contains a tidal lagoon with many salt marsh islands; the Bay is shallow (less than 3 m deep at low tide) except for dredged channels, with an average tidal fluctuation of 1.4 m (range 0.9-2.13 m). There are approximately 374 ha of low salt marsh (primarily Spartina alternifloru), submerged at mean high tide, and exposed at mean low tide. High salt marsh (213 ha, primarily S. putens and Distichlis spicata) occurs above the mean high tide limit. Surrounded by the bay are 2 freshwater impoundments that were created by the deposition of spoil in 1953 (West Pond-17 ha, East Pond-39 ha). Both ponds are surrounded by Phrugmites communis. Ducks were censusedduring daylight hours from 1 June 1978 to 31 May 1979. We divided JBWR into 3 study areas: East Pond, West pond, and tidal Bay. The Bay was censused two days every other week; and the ponds were censused two days one week and four days the following week throughout the year. Each study area was mapped, and during each censusthe location of all ducks was plotted on maps. The Bay was censused by following an auto route and around its perimeter which included 17 stops. The ponds were censused by following a walking path around the edges, and noting the locations and numbers of all ducks. Our dependent variable was number of ducks (divers and dabblers) present at each study area (East Pond, West Pond, Bay) during each census. Independent variables were grouped into three categories: temporal, tidal, and weather variables. Temporal variables include date and time of day; tidal variables included tide cycle, tide height, and tide direction; and weather variables included wind velocity, wind direction, cloud

Figure

1. Map

of Jamaica

Bay Wildlife

Refuge

study

SUBWAY

t-l-t

line equals

OWNERSHIP

areas. Dotted

LINES

NON-FEDERAL

m

boundary

of Jamaican

Bay Wildlife

Refuge.

676

J. Burger et al.

TABLE 1. Independent Temporal Date Time Tidal Tide cycle Tide height Tide

direction

Weather Wind direction Wind velocity Cloud cover Precipitation Temperature

variables

used in the study

recorded as the day, month, and year time of the census on a 24-h clock no. of h before (- ) or after ( +) low tide a relative value of the water level of the bay, derived tables rising (+) or falling (-) tide

from

the tide

direction of the wind, including N, NE, E, SE, S, SW, W, NW speed of wind (in mph) recorded at Kennedy Airport estimated at each census location, recorded as ?i, scored from 0 (none) to 8 (heavy rain or snow) temperature recorded by observed

cover, precipitation, and temperature. Each variable was measured and defined asgiven in Table 1. To determine the importance of these variables to the number of ducks we used a stepwise multipl’e regression procedure with maximum R (R,,,) selection criterion to determine the variables and interactions of variables that contributed to the variations in the dependent variables (Barr et al., 1976). This procedure determines the best model, gives RZ values, and levels of significance for each of the contributing variables. Data were transformed for the analysis, and log-transformed data are presented in the tables and graphs. The models procedure eliminates the effect of one variable (for example, date) before examining the effect of another variable. Since wind direction is not ordinal, we created a new variable which compared the dependent measuresat each wind direction against all other wind directions; examining each wind direction in turn. If any wind direction was significant it was entered in the model. Further, we entered data only when they were positive, that is, if no ducks were present the dependent measureswere not entered. The stepwise procedure first selects the variable that contributes the most to the coefficient of determination (R’), and then selects the second variable that gives the greatest increase in R2. This procedure is continued until all variables that have not been included in the model are not statistically significant. Thus, if variables were highly correlated, only one variable (the one giving the highest R*) would be added to the model. The best model is generated by the fewest variables giving the highest R2. In this paper we present the best models for each group of dependent variables and levels of significance for the independent variables. Generally, the relative value of the probability levels are indicative of the contribution made by each variable. Standard statistical procedures, such as x2 tests, were used to distinguish differences among different samples.For most analyseswe grouped the data by location since each area was sampled separately. Since only the Bay is tidal, differences should exist between the Bay and the ponds. Results All ducks

During the study there were 22 speciesof ducks present at JBWR: 11 divers, 9 dabblers,

Jamaica

Bay

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677

VII

TABLE 2. Location by percent to account for lower sampling

Species

Number

Dabblers, Tribes Cairinini and Anatini Wood Duck, Aix sponsa Green-winged Teal, Anas mecca American Black Duck, A. rubripes Mallard, A. platyrhynchos Northern Pintail, A. acuta Blue-winged Teal, A. discors Northern Shoveler, A. clypeata Gadwall, A. strepera American Wigeon, A. americana Divers, Tribes Aythyini, Mergini, Oxyurini Canvasback, Aythya cralisineria Redhead, A. americana Greater Scaup, A. marila Lesser Scaup, A. afinis Oldsquaw, Clangula hyemalis Common Goldeneye, Bucephala Bufflehead, B. albeola Hooded Merganser, Lophodytes

clangula

cucullatus

Totals Percent

perspicillata M. fusca

on Jamaica

Bay Wildlife

Refuge.

Bay (“;,I

East Pond (” ,, )

West Pond (‘1:,)

6 1436 65 618 17 135 2385 59 5448 6675 23 523

0 4 21 9 10 0 0 2 10

0 63 49 41 84 7 79 4 23

100 33 30 50 6 93 21 94 67

27 279 245 113655 59 5 58 4473

3 0 32 15 0 0 81

88 11 57 42 0 24 8

9 89 11 43 100 76 I1

32 2 1049 6491

0 0 43 6

13 100 54 72

87 0 3 22

37 3

100 100

0 0

0 0

263 607 100

47 337 18

147 364 56

68 906 26

Bay adjusted

Percentage of all ducks

24 6 1 2 2 0

and

Common Merganser, Mergus merganser Red-breasted Merganser, M. serrator Ruddy Duck, Oxyura jamaicensis Seaducks, Tribe Mergini Surf Scoter, Melanitta White-winged Scoter,

of ducks schedule

10 43

1

2

100

and 2 sea ducks (Table 2). Because of low numbers sea ducks were excluded from further analysis. Greater Scaup (scientific names in Table 2), Black Duck and Canvasback accounted for 77(x, of all ducks. Over half of the ducks were found on the East Pond (Table 1). For all species combined the number of ducks (as a log to base e) present varied by date and location (Figure 2). Ducks were always present on the West Pond and were almost always present on the East Pond. Ducks were largely absent from the Bay from May through September (Figure 2) as most ducks breed farther north. The greatest number of individuals were present on the East Pond (Figure 2). In general, there were more individuals on the ponds during the fall migration than in spring, although the Bay had similar numbers during the spring and fall migration (Figure 2, Table 3). The lower numbers of migrants in the spring may be due to a reduction in numbers because of hunting and winter mortality. Multiple regression procedures indicated that variations in the number of ducks were explained by weather variables for the ponds and tidal variables for the Bay (Table 4).

678

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J. Burger

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There were more ducks on the Bay at the time of low tide and when the tide heights were particularly low, although ducks were present throughout the tidal cycle (Figure 3). Since we’had initially predicted differences in how dabblers and divers used JBWR, further analyses were performed on these groups.

679

Jamaica Bay studies VII

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contributing

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Multiple regression techniques were used to ascertain the factors contributing to the variability in the numbers of individuals of the two groups of ducks in the three areas. Dabblers: On the Bay, the variation in the number of dabblers was accounted for by tidal and weather-related variables, while on the ponds date, wind and temperature accounted for most of the variability (Table 5).

J. Burger

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Divers: On the Bay, the variance in the number of divers was accounted for by tide height x direction, temperature, and wind velocity, while the variation on the ponds was accounted for by tide height x direction, and temperature. For all census areas, temperature was the only variable that entered all models for divers (Table 5). The models indicate the variance in the number of individuals is explained by date, time, tide, and weather variables for dabbling ducks; and tide and weather variables

Jamaica Bay studies VII

681

Dabbiers

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for the divers. Thus the models confirmed our prediction of differences in how abiotic factors affect the abundance and distribution of ducks. In the following section we will describe the responsesof dabblers and divers to the different abiotic factors. Temporal

variables

Atthough some ducks were always present on JBWR, dabblers and divers did not use all

682

J. Burger

et al.

TABLE 3. Numbers of birds censused by location and month at Jamaica Bay Refuge. Shown in parenthesis are percent of each area by month. (Percent of total = percent of total birds actually censused; corrected percent = percent after correction for unequal sampling regimes among census areas) East Pond

Bay Month Dabbling

number

number

(“,))

number

(“,))

All areas number

(‘I(,)

ducks

Jan Feb Mar Apr May

Jun JUl Aw Sep Ott

933 160 770 100

(14) (2) (11) (1)

69 90

(1) (1)

201 1044 1614 1751

Nov Dee Total Percent of total Corrected percent Diving

((I,,)

West Pond

(3) (16) (24) (26)

6732

4707 2910 1383 1030 454 2460 3302 9665 17603 27 640 25 634 39 281

(3) (2) (1) (0.8) (0.3) (2) (3) (7) (13) (20) (19) (29)

136 069 48 46

3 7

8699 3586 2850 1717 1230 22752 16304 18807 25516 16098 12 827 8459

(6) (3) (2) (1) (0.9) (16) (12) (14) (18) (12) (9) (6)

138 846 49 47

14 339 6656 5003 2857 1684 25 281 19 696 28 472 43 320 44 782 40075 49 482

(5.1) (2.4) (1.8) (1.0) (0.6) (9.0) (7.0) (10.1) (15.3) (15.9) (14.2) (17.6)

281647 100 100

ducks

Jan Feb Mar Apr May

Jun Jul Aug Sep Ott Nov Dee Total Percent of total Corrected percent

1303 1650 3311 30

(8) (10) (21) (0.1)

572 4274 4709

(4) (27) (30)

15 849 12 30

372 774 3686 1412 70

381 23850 39030 22 974 92 549 75 59

(0.4) (0.8) (4) (1.5) (0.08)

(0.4) (26) (42) (25)

128 21 7333 3310 141 13 66 5 55 118 2249 3156 16 595 13 11

(0.8) (0.1) (44) (20) (0.9) (0.08) (0.4) (0.03) (0.3) (0.7) (14) (19)

1830 2445 4752 211 13 66 5 436 24 540 45 553 30 839

(1.4) (1.9) (11.5) (3.8) (0.2) (0.3) (19.6) (36.4) (24.7)

124 993 100 100

areas equally. Some dabblers (Black Duck, Mallard, Gadwall) were present on the West Pond all year, although they peaked in September and October. They were present on the East Pond all year except during June and early July, but on the Bay they were virtually absent from April until September (Figure 2). For all areas, the largest number of dabblers were present in the fall. A few diving ducks (Ruddy Duck) were likewise present on the West Pond all year, although they were absent from the East Pond and Bay from May through August (Figure 2). The East Pond had peak numbers of individuals in the fall, and the Bay and West Pond were used similarly in the spring and fall. Thus, overall, dabblers were present for more of the year than were diving ducks, and all ducks were more common in the fall than the spring.

Jamaica

Bay

studies

TABLE 4. Factors combined. Given factors

683

VII

influencing the variance in the number of individuals of all ducks are the model characteristics and probability levels for contributing

Bay Model R2 F P df

27 5.92 0~0001 4,98

Temporal Date Date x time Tzde Tide cycle Tide height Tide height

East Pond

57 11.98 0.000 1 6,64

West Pond

61 17.57 0~0001 2,85

0~0001

0.04 0~0001 x direction

Weather Wind velocity Wind direction Wind velocity Temperature

x direction

0.0005 0.04

0.02 0.004 0~0001 0.03

0,006

Interactions Date x tide height Cloud cover x wind velocity Tide height x wind direction Date x wind direction

Tidal

variables

Tidal variables entered the models for the number of dabblers and divers (but not sea ducks) on the Bay, but not on the ponds (except divers on the West Pond, seeTable 5). There were more dabblers and divers on the Bay at low tide, but the numbers on the ponds were lessaffected by tide (Figure 3). Peak numbers of dabblers occurred on the West Pond at low tide, but there were no differences on the East Pond. However, there were peak numbers of divers on the West Pond at high tide, and there were no differences on the East Pond (Table 5, Figure 3). Examining the number of flocks (birds not separated by more than 25 m from one another), differences existed in how dabblers, but not divers, used the three censusareas by tide cycle (Table 5, Figure 4). In general there were more observations of dabblers on the Bay and West Pond at low tide, and on the East Pond at high tide. Divers used the East Pond and Bay more on falling tides compared to low tide. Weather

variables

Several weather variables could affect the abundance and distribution of ducks at JBWR. Wind variables entered all models for the number of dabblers and divers on the Bay, but only entered the model for variations in the number of dabblers on the East and West Ponds (Table 6). Generally wind velocity entered the models explaining variations in numbers for the Bay, and wind direction entered the models for the East Pond. There were more ducks on the East Pond during southwest and northwest winds, while the Bay

684

J. Burger

et al.

5. Factors influencing the variability in the number of Dabblers and Divers present in the three census areas at Jamaica Bay Wildlife Refuge. Given are the model characteristics and the probability levels for variables contributing to the variance

TABLE

Dabblers

Model RZ F P df

Bay

East Pond

0.43 10.21 0~000 1 4,55

0.51 1450 0~0001 4,57

Factors Temporal Date Date x time Tidal Tide cycle Tide height

Divers West Pond

Bay

East Pond

0.66 20.10 0.000 1 3,78

0.23 3.55 0.004 4,73

0.63 18.53 0~0001 2,47

West Pond

0.53 8.93 0.000 1 3s 1

0~0001 0~0001

x direction

Weather Wind velocity Wind direction Temperature Cloud cover Interackons Temperature x date Tide height x wind direction Cloud x temperature Temperature x tide height

0~0001

0.001

0.001

0.008

0.01 0.02

0,004 0.001

0~0001 0~0001

0.05

0.01

0.05 0.08

0~0001

0.0002

0.001 0.0002

0.02

had more ducks during west and northwest winds (Figure 5). The West Pond had more dabblers on southwest to northwest winds. For both dabblers and divers there were more individuals on the East Pond during high winds (> 20 mph), and fewer ducks on the Bay in high winds. We conclude that all ducks use the Bay extensively in West and northwest winds (when they are not > 10 mph), and use the ponds irrespective of wind conditions. Temperature entered nearly all models explaining the variance in the number of all ducks on all ducks on all census areas, and there were significant differences in how dabblers and divers used the three census areas (Table 6, Figure 6). Dabblers and divers both used the Ponds at higher temperatures than they used the Bay. Divers also avoided the West Pond at higher temperatures. Cloud cover entered the models as a significant variable for dabblers (Table 4) but not for divers; although both groups used the census areas differently with respect to cloud cover (Table 4). Dabblers used the East Pond more in clear skies than cloudy skies, and they used the Bay more in cloudy compared to clear skies. Divers used the East Pond in a similar way as did the dabblers, but they used the Bay even more during cloudy skies than did the dabblers. Precipitation did not enter any of the models for any group at any census location. Observation confirmed that ducks did not appear to shift locations according to amount of precipitation.

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Dabblers we*, Pond

Divers

Figure 4. Percent of flocks of divers and dabblers present on the three function of tide cycle, where - 2 to + 2 is the period around low tide.

TABLE 6. Comparison of response census areas. Given are x2 values (*=P
4 2 6 4 6 4

of ducks to environmental (3 x 3 contingency table)”

Dabblers 9,81* 0.01 17.57** 0.05 17.27** 9.50* 204

census areas as a

conditions on the three and levels of significance

Divers 4.09 1.79 20.66** 10,01* 38.88*** 1.78 189

“Testing that the response of ducks was the sam’e on all 3 census areas; d.f. refers to the 3 census areas for each duck group. *Divided into 6 to 2 h before low tide, 2 h before to 2 h after low tide, and 2 to 6 h after low tide. ‘Wind direction divided into N, E, S and W. “Velocity divided into 5,6-10 and P 10 km h ’ “Temperature divided into four categories. ‘Cloud over divided into three categories. YNumber of sightings in the three census areas.

686

J. Burger et al.

Dabblers

Divers 60

I-

IO 600 2 2 ‘d

50 40

;

30

2

20

g E ;

IO 0

70 60 50 40 30 20 IO 0 N,NE

E,SE

S,SW Wind

W,NW

N,NE

E,SE

S,SW

W,NW

direction

Figure 5. Number of flocks (observations) and prevailing wind direction.

asa function

of location

on the bay or ponds

Discussion There has been considerable interest in understanding habitat requirements for resident or migratory waterfowl (see Weller, 1975; Fredrickson & Drobney, 1979). Often this attention has focused on habitat requirements dealing with foraging behaviour such as specific vegetation types, o0 emergent vegetation, water depth and other specific requirements (White & James, 1978). White & James (1978) included social factors as well. Nonetheless, other environmental factors can affect the abundance and distribution of migrating and wintering waterfowl. Additionally, differences in foraging behavior could result in different responsesby waterfowl to the sameenvironmental conditions. Temporal

affects

In this study some ducks (mainly Black Ducks, Mallards, and Gadwall) were present throughout the year, although they were most abundant in the spring and fall during migration. The large numbers present in the late fall were often forced out by exceptionally cold temperatures in late December and early January, and by freezing of the ponds. Time of day influenced the number of dabblers on the East Pond and divers on the West Pond. In both casesthere were more birds later in the day when birds generally used these areasfor bathing, preening and loafing. Waterfowl are known to show a die1

687

Jamaica Bay studies VII

hers

Dabblers

Temperature

Figure census

6. Number areas.

of flocks

(OF)

(observations)

as a function

of temperature

for the three

pattern of feeding by day and sleeping at night (Nilsson, 1970; Dunthorne, Campbell, 1978); although Thornburg (1973) found that they will feed at night. Tidal

1971;

effects

Waterfowl frequently inhabit tidal marshes and sloughs (Weller, 1969; 1975; Daiber, 1977), but there are few data indicating the response of waterfowl to tidal changes. Changes in water levels should change the depths at which food items (plants or invertebrates) are located, making it easier to feed on some tides rather than others. Tides, however, determine where eiders feed and how many feed (Pounder, 1971; Milne, 1974; Campbell, 1978). Similarly, Swanson & Meyer (1977) found that feeding behaviour of teal was influenced by changesof water levels in freshwater, non-tidal lakes. In this study tide cycle entered the models as a significant variable in the Bay explaining the variability in numbers of dabblers and divers. Both dabbling and diving ducks concentrated on the Bay at low tide, although there were more divers on a falling tide. At low tide the vegetation the ducks eat would be closer to the surface, requiring lesstime and energy to forage. There was one difference between the groups; dabblers increased in numbers on the East Pond at high tide, and divers did not although both groups increased at high tide on the West Pond. This suggeststhat birds leaving the Bay at high tide go to the West Pond, but only dabblers go to the East Pond. Divers used the East Pond extensively at all tide times, suggesting that it provided optimal foraging and loafing conditions at all times (the food source may have varied between the ponds).

688

J. Burger et al.

Weather-related

effects

Although authors have suggested that temperature affects foraging behaviour and distribution of ducks (for review, seeFredrickson & Drobney, 1979) there are few data describing effects of other weather variables. In this study wind velocity and direction influenced the abundance of divers and dabblers on the Bay. In intermediate winds the ducks either rafted up in large flocks on the Bay (and did not feed), or they moved into the ponds, concentrating along the edges where Phragmites protected them from the winds. This resulted in more ducks where there were more Phragmites on the West Pond, during high winds. The size of the Phragmites patch determined the degree of protection from the wind. Temperature, as suggested by Fredrickson & Drobney (1979), influenced the abundance and distribution of ducks. Both dabblers and divers used the ponds at higher temperatures than they used the Bay. They avoided the ponds at low temperatures becausethe ponds were often frozen, or had few available open areas. Cloud cover was a significant variable in the models for variations in the number of dabblers, but not for divers. Dabblers used the ponds during clear skiesand the Bay in cloudy skies. Cloud cover is usually not examined as an important variable. However, Bovino & Burtt (1979) did find that Great Blue Herons had higher foraging success under cloudy skies. They attributed the higher successto a decrease in glare from reflection off the water. This may also operate for dabblers that feed on the bay, where feeding might otherwise be more difficult (tide and wind factors) when the light conditions are optimal, moving into the ponds when light conditions are lessoptimal. For divers feeding on more submerged food, glare at the surface may be lessimportant. As has been found in other studies, the abundance of ducks at Jamaica Bay Wildlife Refuge is most clearly related to time of the year. Some speciesbreed on the refuge, although most migrate farther north to breeding grounds. However, when time of year is removed, other variables, such as tide, temperature and wind all influenced the abundance and distribution of ducks within the three censusareas. Acknowledgements We wish to thank the personnel at Jamaica Bay Wildlife Refuge for making the work pleasant. Special thanks are due to Paul A. Buckley (Chief Scientist) of the North Atlantic region of the National Park Service for his continued interest, support and advice concerning the project. This project was supported by contract CX 1600-8-007 from the North Atlantic region of the National Park Service.

References Barr, A. J., Goodright, J., Sail, J. H. & Helwing, J. T. 1976 A User’s Guide to SAS. Raleigh, Sparks Press. Bartonek, J. C. & Hickey, J. J. 1969 Food habits of Canvasbacks, Redheads, and Lesser Scaup in Manitoba. Condor 71,280-290. Bovino, R. R. & Burtt, Jr. E. H. 1979 Weather-dependent foraging of Great Blue Herons (Ardea huvdius~. Auk

96,628-630.

Burger, J. 1981 Effect of 231-241. Burger, J. 1983~ Jamaica tion of Ibises, Egrets Burger, J. 19836 Jamaica Estuarine,

Coastal

human

disturbance

on birds

at an East Coast

Estuary.

Biology

Comerewion

21,

Bay Studies. II. Effect of tidal, temporal and weather related variables on disrribuand Herons on a coastal estuary. Acra Oecol. 4, 181-189. Bay Studies: III. Abiotic determinants of abundance and distribution of gulls. and Shelf Science 16, 191-216.

Jamaica

Bay

studies

689

VII

Burger, J. 1984 Abiotic factors affecting migrating shorebirds. In Behavior of Marine Animals Vol. 5 (Burger, J. & Olla, B., eds). Shorebirds Plenum Press, New York City, New York. l-72. Campbell, L. L. 1978 Diurnal and tidal behaviour patterns of Eiders wintering at Leith. Wildfozu[ 29, 47-52. Cody, M. L. 1968 On the methods of resource division in grassland bird communities. A~nerictitr Norural;.
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Management

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A. 1974

Etho-ecological studies of Teal wintering in the Camargue (Rhone Delta, France,. Wildfowl 25, 12>133. Taylor, T. S. 1977 Avian use of moist soil impoundments in southeastern Missouri. Unpublished M.S. thesis, University of Missouri, Colombia, 98 pp. Thornburg, D. D. 1973 Diving duck movements on Keokuk Pool Mississippi River. JouwaI o,f IX’zrdi,t’f~~ Management

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Publications Mexico, 1,89-l

D. H. & James, D. 1978 Differential use of fresh water environments coastal Texas. Wilson Bulletin 90,99-l 11. Whitmore, R. C. 1975 Habitat ordination of passerine birds of the Virgin River Wilson

Bulletin

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in Argentina. Biologicu.~

30. by wintering

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Ey’il~fjorcl20, Imrxuro

dc

waterfowl

of

southwestern

Utah.