The Condor 99:299-3 13
D The Cooper OrnithologicalSociety

1997

EFFECTS OF THE EXXON VALDEZ OIL SPILL ON BIRDS:COMPARISONSOF PRE- AND POST-SPILL SURVEYS IN PRINCE
WILLIAMSOUND, ALASKA’
STEPHENM. MUFZHY AND ROBERT H. DAY
ABR, Inc., P.O. Box 80410, Fairbanks, AK 99708, e-mail: smurphy@abrinc.com
JOHN A. WIENS
Departmentof Biology, Colorado State University,Fort Collins, CO 80523
KEITH R. PARKER
Data AnalysisGroup, 5100 Chern, Creek Road, Cloverdale, CA 95425
Abstract. We used data from pre- and post-spill surveys to assessthe effects of the
Exxon Valdez oil spill on the abundanceand distributionof birds in Prince William Sound,
Alaska. We conductedpost-spill surveys during mid-summer (1989-1991) in 10 bays that
had been surveyed prior to the spill (1984-1985) and that had experienceddifferent levels
of initial oiling from the spill (unoiled to heavily oiled). We evaluatedwhether there werechangesin overall abundanceacrossall bays between the pre-spill and post-spill sampling
periods, and changesin abundancein unoiled/lightly oiled bays versusmoderately/heavily
oiled bays that would suggestoiling impacts. Of 12 taxa examined for changesin overall
abundance,7 showed no significant change, 2 (Bald Eagle and Glaucous-winged Gull)
increasedin abundance,and 3 (Red-necked Grebe, Pelagic Cormorant, and Pigeon Guillemot) decreasedin abundanceduring all three post-spill years. Of the 11 taxa examined for
differences in use of oiled versus unoiled habitats, 7 showed no significant response, 1(Black-legged Kittiwake) exhibited a positive responseto oiling, and 3 (Pelagic Cormorant,
Black Oystercatcher,and Pigeon Guillemot) exhibited negative responsesto initial oiling.
We conclude that the impacts of this oil spill on abundanceand distribution of birds weremost evident in 1989, the year of the spill, and were most pronouncedfor Pigeon Guillemots.
By 1991, signs of recovery were evident for all taxa that showed initial oiling impacts.
Key words: Prince William Sound,Alaska, Exxon Valdez, oiling impacts,oil spill, marine birds, before-after comparisons.
INTRODUCTION
When the Exxon Valdez went aground in Prince

William Sound (PWS), Alaska, on 24 March
1989, the ensuing spill of approximately
41,000,OOOL of Alaska North Slope crude oilbecame the largest marine oil spill in U. S. history. Marine birds, which are among the mostconspicuousvictims of oil spills at sea, experienced substantial mortality: nearly 30,000 carcasses were collected throughout the spill-affected area (PWS, Kenai Peninsula, Kodiak Island archipelago) during the spring and summerof 1989 (Piatt et al. 1990). Simulation models
that used functions relating carcass recovery
rates to total mortality produced a “best estimate” of total mortality of 375,000-435,000
birds (Ecological Consulting, Inc., 1991). Although not all scientistsagree that mortality was’ Received 1 August 1996. Accepted 22 January
1997.

this high (e.g., Erikson 1995), there is little
doubt that many thousandsof birds were killed
as a direct consequenceof the spill (Fry 1993,
Parrish and Boersma 1995, Wiens 1995), mostly
near large seabird colonies in the Gulf of Alaska. Perhaps as many as 42,000 birds died inPWS (Piatt et al. 1990, Ecological Consulting,
Inc. 1991), with the greatest mortality documented for sea ducks, cormorants, murres,grebes, and murrelets.
We initiated a researchprogram in June 1989
to examine the effects of the oil spill on marine
birds. One aspectof this researchevaluated both
impacts to and recovery of use of oil-affected
habitats by birds in PWS and along the Kenai
Peninsula during a 3-year period (1989-1991)
after the spill (Day et al. 1995, in press; Wienset al. 1996). During that 3-year period, 15 sampling cruises were conducted during different
seasons,and analyses of impacts and recovery
reported by these studieswere based exclusively
on post-spill data. Another way to gauge the ef-

12991

300

STEPHEN

M. MURPHYET AL.

fects of an oil spill on birds is to compare abun- with the post-spill data for that site, thereby aldances of birds before and after the spill. If mor- lowing a clear assessmentof effects of oiling on
tality or displacement from contaminated habi- bird densities. This analysis has been describedtats was great, we would expect reduced densi- as a “pre/post pairs” design (Wiens and Parker
ties of birds, particularly in the most heavily
1995) and approximates a BACI (before-after/
oiled sites. Here, we compare data collected in control-impact) design (Skalski and McKenzie
mid-summer during 3 years following the Exxon 1982, Stewart-Oaten et al. 1986, 1992, Schroeter
Vuldez oil spill (1989-1991) with historical data et al. 1993), which was developed for evaluatingcollected before the oil spill.
effects of planned perturbations(e.g., large-scaleStudies in PWS conducted in 1972 and 1973 industrial developments).
(Dwyer et al. 1976) and during the summers of1984 and 1985 (Irons et al. 1988) provide the METHODS
only systematic pre-spill information on avian
abundance and distribution. The data collected STUDY AREA
by Dwyer et al. had poor geographic correspon- Prince William Sound (PWS) is a semi-enclosed
dence with our study area and were not useful body of salt water located in the northern Gulf
for making historical comparisons.The data col- of Alaska (Fig. 1). It is surroundedby the Chulected by Irons et al (1988) had complete geo- gach Mountains and is characterized by tidegraphic overlap with our study area and good water glaciers, fjords with steep, forested slopes
temporal overlap with surveys we conducted and rocky shorelines, and numerous islands, isduring mid-summer (July-August). We acquired lets, and intertidal rocks. The climate of the rethe original survey data from the U. S. Fish and gion is cool-maritime with frequent and intense
Wildlife Service and determined that 320 km of storms;precipitation is highest in fall and wintershoreline in the 10 bays that we were sampling (Isleib and Kessel 1973, Royer et al. 1990).
also had been surveyed by Irons et al. (1988).
Approximately 16% of the 4,800 km of shoreIn this paper, we use mid-summer data from line in PWS was oiled by the Exxon Valdez oil
Irons et al. (1988) and from surveys that we conspill (Neff et al. 1995). Oiling of shorelineswas
ducted in 1989-1991 to evaluate two hypothediscontinuous and patchy, and only some of theses:bays in the general path of the spill were inunH,l: Densities of birds in the 10 study bays did dated with oil. We selected 10 bays for study
not differ between 1984/1985 (pre-spill) and that were typical of western PWS: they were
fjords with rocky shorelines, although they dif1989-1991 (post-spill).
fered in size, shape, exposure to larger water
H,2: Changesin densities of birds from pre-spill masses,and other habitat features. Nine of theselevels did not differ between oiled and unoiled 10 bays were on or near the Knight Island groupbays during 1989-1991.
(Fig. 1) and were in the general path of the spill(Galt et al. 1991). The study bays ranged in area
The first hypothesis was a simple “beforefrom 1.5 to 24.3 km*; four were unoiled or lightafter” baseline comparison (Wiens and Parker
1995) that evaluated whether there were signif- ly oiled, and six were moderately to heavily
icant changesin overall densities of birds in the oiled. Of the 10 bays that we surveyed, 9 werestudy area. We made no distinction between surveyed by Irons et al. (1988) in 1984 and thesampling sites (bays) that were oiled and those 10th (Galena Bay) was surveyed in 1985; hence,that were unoiled and did not separateoiling ef- the single survey of the 10 study bays is referredfects from natural factors that might have con- to here as 1984/1985.
We used bays as our basic sampling unit betributed to overall population changes between
cause they are discrete areas that can be dethe two time periods.
The secondhypothesisaddresseda fundamen- scribed quantitatively in terms of initial expotally different question, one about impacts sure to oil. Bays were not chosen randomly, butcausedby oiling. Among-bay differences in hab- instead were selectedto be representativeof theitat, natural temporal variation, and interobserv- types of bays found in western PWS and to eser differences were accounted for by comparing tablish a gradient of oiling conditions (Day et al.
Irons et al’s (1988) data from each sampling site 1995).

EXXONPWS STUDY

VALDEZOIL SPILL AND BIRDS

301

SITES

1 - Shelter Bay
2 - Hogan Bay
3 - Snug Harbor
4 - Bay of Isles
5 - Drier Bay
6 - Lower Herring Bay
7 - Eshamy Bay
6 - Hening Bay
9 - Northwest Bay
IO - Galena Bay-LEVELo - None-Light
0 - Moderate-Heavy

FIGURE 1. Locations of study bays in Prince William Sound, Alaska.

DATA COLLECTION

Our survey proceduresgenerally were the sameas those usedby Irons et al. (1988). In both studies, nearshore areas were surveyed during midsummer (-20 July to 10 August) by slowly driving a small boat along the shoreline of a bay andidentifying and counting all birds seen on thewater within 200 m of shore and in the air overthis zone. Although including flying birds intransect survey counts may bias density estimates (Tasker et al. 1981), we did not have a
choice as to whether we would include flying
birds becausethey were included in the pre-spill
data set. We do not, however, consider inclusionof flying birds to have been a major source ofbias in this instance because (1) we were notconducting our surveys at sea, where this problem is most acute, (2) accurate population estimates were not a goal of this researchprogram,and (3) we accounted for natural variability byusing replicate surveys. We also carefully
searchedfor and counted birds in the intertidal
and supratidal (5 100 m from high-tide line)
zones; thesezones appearto have been surveyedthoroughly by Irons et al. (1988) only for Bald
Eagles and Black Oystercatchers(see Appendix
1 for scientific names of bird species). We typ-ically drove our boats -50 m from shore, although the actual distance from the shoreline often was greater because of the need to avoid
intertidal and subtidal rocks. Irons et al. (1988)
reported driving their boats - 100 m from shore;
overall, we think that similar survey tracks were
followed by both survey teams. Because bothteams used experienced observers and traveledslowly during surveys, we do not think differences in distanceswould affect survey results.
We developed an oiling index to provide acontinuous measure of the initial exposure ofeach bay to oil (Day et al. 1995). Data used to
calculate the oiling index had been mapped in
the field by Shoreline Cleanup Assessment
Teams in 1989, and thus representeda mutuallyagreed upon (by government and Exxon scientists) and consistent system for measuring theamount of oil on the shorelines.Given the fjordlike structure of the bays and substantial tidal
ranges that occur in PWS, we believe that thesequantitative measures of oil on the shorelines
provided a reasonable indication of the amountof oil to which avian habitats in a bay initiallywere exposed. Oiling on shorelines was categorized at five levels, ranging from “no oil” to“heavy oil” (Neff et al. 1995). For each bay, we

302

STEPHEN M. MURPHYET AL.

NORTHWEST
SNUGHARBOR
HOGAN
HERRINGSHELTERBAY OF ISLESESHAMY

I

DRIER

m

-

UNOILED AND
LIGHTLYOILED

0

I
50

LOWER HERRINGGALENA

”

:
,
I
I
100

MODERATELY ANDHEAVILY OILED
I ”
”
I
150
200
OILING INDEX
”

”

I ’
250

”

300

FIGURE 2. Oiling index values for the study bays in Prince William Sound, Alaska (modified from Day etal. 1995). Oiling indices reflect the initial oiling exposurefor each bay. Dashed line separatesunoiled and lightly
oiled bays fro& moderately and heavily oiledIbay&

calculated an oiling index value (see Day et al.
1995) that could range between 0 (100% no oil)and 400 (100% heavy oil). Actual index values
calculated for the bays surveyed in PWS ranged
between 0 and 288.2 (Fig. 2). For analyses, we
combined the four unoiled and lightly oiled bays
(index values < 100) to form a group of “unoiled” bays and the six moderately and heavily
oiled bays (index values 2 200) to form a groupof “oiled” bays.
DATA ANALYSIS AND INTERPRETATION

Although the length of shoreline surveyed insome bays differed slightly between 19840985
and 1989-1991, we standardized all counts ofbirds from nearshore surveys to linear densities(birds/km of shoreline surveyed) to facilitatecomparisons. If Irons et al. (1988) recorded< cu. 20 individuals of a taxon in the 10 study
bays, that taxon was omitted from our analyses.
Consequently, data that we collected on shorebirds other than oystercatchers, wading birds,
and corvids could not be compared with historical data.
Irons et al. (1988) visited each bay only once.
We conducted multiple (3-5) visits to a bay oneach cruise, so we averaged density estimatesto
calculate mean densities for each year. We expected the effects of oiling to be multiplicative(i.e., effects would be proportional to populationlevels in the various bays) and not additive (i.e.,
effects would be constant regardless of population levels in the various bays). Because statistical tests assume additivity, it thus was neces-sary to make the relationships additive by logtransforming the density estimates before conducting the statistical tests (Stewart-Oaten et al.
1986). We added a constant of 0.167 to all density estimates to avoid calculating a log of zero
(Mosteller and Tukey 1977).
For each taxon, we calculated the amount ofchange (A) in densities that occurred in each bay(i) during each post-spill year (after) relative tothe pre-spill baseline (before):
A, = In density (after) - In density (before).
(1)
These log-transformed changesin densities thenwere used to calculate the percent change between pre-spill and post-spill years across allbays and the percent difference in densities between oiled and unoiled bays in each post-spillyear relative to the pre-spill baseline.
The overall change in densities between prespill and post-spill years across all bays for agiven taxon was calculated by taking a mean of
the changes for all 10 bays:
overall change (& = 2

n ’

where IZ = 10.
(2)

The percent change across all bays and the percent difference between oiled and unoiled bays
then were calculated as:
percent change = (e” - 1) X 100.

(3)

We used a one-sample t-test (Zar 1984) forEXXON VALLIEZ OIL SPILL AND BIRDS

303

COMPARISONBEFORE-AFTER

3

I
-‘AC’

I

PIWPOST

PAIRS

Bay
1

B
II
-A.

4

1.
-Ai--+’AI

FIGURE 3. Approaches
usedto examinetheeffectsof theZ&on Vuldezoil spill on birdsusinghistoricaldata(Irons et al. 1988) and data collected during 3 years after the spill (1989-1991) in 10 bays in Prince WilliamSound, Alaska. The before-after comparisons(H,,l) evaluatedwhether the mean differences (D) between prespill (B) bird densities and post-spill (A) densities differed from zero. The pre/post pairs comparisons(H,2)
evaluated whether the mean difference between pre-spill (B) bird densities and post-spill (A) densitiesin oiled(0) bays differed from mean differences in unoiled bays (u).
The combination of high variance and small
sample size could reduce power to detect an oil= 0; Fig. 3). We considered the overall abun- ing effect and increase the likelihood of making
dance to have decreased (increased) across all Type II errors (i.e., failing to detect a spill imbays if the mean change in densities was signif- pact that actually did occur). To counter this
icantly lower (higher) than zero.
problem, we established a priori that statistical
The difference in densities between oiled and significance for all comparisonswould be tested
unoiled bays for a given taxon was calculated as with a two-tailed a = 0.20. Thus, we sought tothe difference between the mean difference for increase the likelihood of detecting oiling effects
oiled (0) bays and the mean difference for un- in these statistical analyses, realizing that by dooiled (u) bays:
ing so we also increased the probability of detecting some oiling effects that did not actuallydifference in density = &, - A,,,
where n = 6 and 4, respectively.
(4) occur (Type I errors) (Shrader-Frechetteand McCoy 1993, Mapstone 1995, Wiens and Parker
The percent difference between oiled and un1995). To aid in the interpretation of test results,
oiled bays then was calculated as:
we also tallied results at a = 0.05 and 0.10.
percent difference = (ecn,t- &) X 100. (5)Recovery was evaluated for all taxa thatshowedinitial (1989) negative impacts by exWe used a two-sample t-test (Zar 1984) for
each taxon to evaluate whether changesin mean amining both trends in densities and statisticsin
densities differed_ between oiled and unoiled 1990 and 1991. That is, we evaluated the percent
bays (H,2: A, - A, = 0; Fig. 3). We conducted change in densities between pre- and post-spill
tests only on those taxa that occurred in at least years, the results of statistical comparisons,andtwo bays in each of the two oiling categories the power analyses to determine if a detectedduring the post-spill years. We considereda neg- impact had diminished in subsequentyears.
Finally, we conducted power analyses to asative (positive) oiling impact to have occurred if
sessesthe probability of Type II errors and,the mean change in densities between one ormore of the post-spill years and the pre-spill hence, our ability to detect impacts. Power inyear was significantly lower (higher) in the oiled creaseswith increasing (Y level and sample size
and with decreasing variance (Osenberg et al.
bays than in the unoiled bays.
each taxon to evaluate whether there was a significant change in density over all bays (I-&,1: A

304

STEPHEN M. MURPHYET AL.

TABLE 1. Pre-spill counts, percent changes,results of t-tests, and overall responsesof birds based on comparisonsof densitiesduring mid-summer before (1984/198.5;Irons et al. 1988) and after (1989-1991) the Exxon
Vuldez oil spill in 10 bays in Prince William Sound, Alaska. A negative (positive) percent changeindicatesthat
densitiesdecreased(increased)in the post-spill year. Percent change was calculatedfrom log-transformeddata.
Significancecodes for t-tests: * = P 5 0.20, ** = P 5 0.10, *** = P % 0.05.
Percent change
Speciedtaxon

Red-necked Grebe
Pelagic CormorantHarlequin Duck
Common Merganser
Bald Eagle
Black Oystercatcher
Bonaparte’s Gull
Mew GullGlaucous-wingedGull
Black-legged Kittiwake
Pigeon Guillemot“Total murrelets”

Pre-spill
cO”“t

19
28
103
101
81
24
131
825
579
920
179
798

1989

- 17.7**-32.0***
-13.5
9.5
-3.9
-7.3
-2.4
-8.1
75.9*
52.4
-55.7***
1.7

1990

1991

OVeIdlresponse-17.7**-22.0***-6.4-7.6
55.8***
-9.4
11.6
20.8
248.2***-4.4-56.5***
15.3- 17.4**-l&4***-11.9
13.6
50.9***
5.3
-4.3
-33.2
197.4***
38.8-37.5***
69.1**

Decrease
Decrease
None
None
Increase
None
None
None
Increase
None
DecreaseNone1994). It also is a function of the amount of ate changes in overall abundance between prechange (A) to be detected, becauseit is easier to and post-spill years. During a single survey ofthe 10 study bays in 19840985, Irons et al.
detect a large change than a small one. Our interest in conducting power analyses was in eval- (1988) recorded 3,788 individuals of these 12
uating the strength of, and therefore our confi- taxa, with numbers for individual taxa ranging
dence in, the statisticalresults. To facilitate com- from 920 for Black-legged Kittiwakes to 19 forparing our tests among different taxa, we con- Red-necked Grebes (Table 1). During our surducted the power analyses using a standardized veys of these same bays, we recorded averages
amount of change. Because of the large amount of 3,756, 5,175, and 3,592 individuals of theseof oil spilled, there were expectations of dra- 12 taxa in 1989, 1990, and 1991, respectively.
matic impacts on many taxa (e.g., Piatt et al. Because we surveyed approximately 11% more
1990, Fry 1993); therefore, the expected effect shoreline than did Irons et al. (1988), we adsize would be large. Moreover, the degree of justed the abundance totals to reflect that differnatural variability in marine bird abundance in ence. The adjusted totals indicate that numbersthese environments may be great (Zwiefelhofer of birds were lower in 1989 and 1991 (-12 andand Forsell 1989). Therefore, we computed -16%, respectively) and were higher in 1990
power for a hypothetical change of either a (+22%) than in 1984/1985.
100% increase or a 50% decreasein abundance
Percent changes in overall abundance in the(i.e., a two-fold increase or decrease).
10 study bays between 1984/1985 and the 3Power was calculated by the procedures de- post-spill years for individual taxa ranged fromscribed in Winer et al. (1991). Sample standard a 57% decrease by Pigeon Guillemots in 1990
deviations were used in all before-after analy- to a 248% increase by Glaucous-winged Gulls
ses, and pooled variance estimates were used in in that same year (Table 1). In 1989, 8 of 12
the pre/post pairs analyses.Power was computed (67%) taxa had decreasedin overall abundancewith cx = 0.20. Results were classified as high from the pre-spill baseline, whereas4 (33%) had
power (2 90% chance of detecting a two-fold increased.In 1990,7 of these 12 taxa (58%) hadchange), moderate power (60-89% chance), and decreased in overall abundance from the prelow power (< 60% chance).
spill baseline and 5 (42%) had increased. By
1991, six (50%) taxa had decreased and six
RESULTS(50%) had increasedin abundance from the preOVERALL CHANGES IN ABUNDANCE
spill baseline.
Before-afier comparisons. In the 198411985
General patterns. Twelve taxa occurred in sufficient abundance in 198411985 for us to evalu- vs. 1989-1991 comparisons, 6 (50%) of the 12

EXXON VALDEZ OIL SPILL AND BIRDS

305

TABLE 2. Percent differences, results of r-tests, and overall responsesof birds to oiling based on relativechangesin densitiesduring mid-summer in oiled (n = 6) and unoiled (n = 4) bays after (1989-1991) the Exxon
Vuldezoil spill in Prince William Sound,Alaska. A negative (positive) percentdifference indicatesthat densities
in oiled bays were lower (higher) than in unoiled bays during a particularpost-spill year. Percentdifference wascalculatedfrom log-transformeddata. Significancecodes for z-tests:* = p 5 0.20, ** = P 5 0.10, *** = P 5
0.05.
Percent difference
Specieskaon

Red-necked Grebe
Pelagic CormorantHarlequin Duck
Common Merganser
Bald Eagle
Black Oystercatcher
Mew GullGlaucous-wingedGull
Black-legged Kittiwake
Pigeon Guillemot“Total murrelets”

1989

1990

1991

-7.6
naa
13.9
-49.2
-3.9
-27.5*
21.2
52.8
215.0*-51.6**-47.9-7.6-26.7*
4.6
36.8

-8.4
na
1.7
-7.4
27.2
-11.5
63.9
193.4*
132.2
-21.8
11.7

-8.8
-13.8
122.8
13.1
305.7*-51.2*-30.1

Overall response

None
Negative
None
None
None
Negative
None
None
Positive
NegativeNone

1 na = no analysis

taxa examined showed no significant changesin
abundance during any of the 3 post-spill years(Table 1). During all 3 post-spill years, Rednecked Grebes, Pelagic Cormorants, and Pigeon
Guillemots decreased significantly, whereas
Glaucous-winged Gulls increased significantly.
Two additional taxa (Bald Eagle and “total murrelets”) did not differ significantly in 1989 but
did in 1990 and/or 1991 (Table 1). We used “total murrelets” for these comparisons because
Irons et al. (1988) recorded44% of the murrelets
in our 10 study bays as “unidentified murrelets.” Our observations during 1989-1991 wereof > 99% Marbled Murrelets and < 1% Kittlitz’s Murrelets. Bald Eagles were classified as
exhibiting an overall increase becausethey weresignificantly more abundant in both 1990 and1991.
OILING EFFECTS

General patterns. Differences in densities between oiled and unoiled bays relative to the prespill baseline ranged from a 52% decrease by
Pigeon Guillemots in 1989 to a 306% increase
by Black-legged Kittiwakes in 1990 (Table 2).
In 1989, 6 (60%) of 10 taxa examined had decreased in abundance in oiled bays relative to
unoiled bays, whereas 4 (40%) of 10 taxa had
increased (Table 2). In 1990, 6 (55%) of 11 taxa
decreasedin oiled bays relative to unoiled bays,
whereas 5 (45%) taxa increased. In 1991, 4(40%) of 10 taxa analyzed had decreased in

oiled bays relative to unoiled bays, whereas 6(60%) had increased.
Pre/post pairs comparisons.In 1989, 3 (30%)
of 10 taxa examined decreased significantly in
oiled versus unoiled bays relative to pre-spillbaseline, indicating oiling effects (Table 2).
Black Oystercatchersand Pigeon Guillemots exhibited negative responses to oiling, whereas
Black-legged Kittiwakes exhibited a positive response. In 1990, 3 (27%) of 11 taxa examinedexhibited significant oiling effects, with Pelagic
Cormorants and Pigeon Guillemots responding
negatively and Black-legged Kittiwakes responding positively. In 1991, only 1 (10%) of
10 taxa (Glaucous-winged Gull) exhibited a significant oiling effect, and it respondedpositively(Table 2). Thus, significant positive responsesto
oiling were detected in all 3 post-spill years,
whereas negative responseswere found only in
1989 and 1990. Three species responded negatively to oiling in at least 1 year, but only Pigeon
Guillemots showed significant negative responsesin more than 1 year.
NATURAL VARIATION AND STATISTICALPOWER

Natural variation. One of the most challenging
aspectsof using historical data for impact analyses is sorting out the effects of a perturbationfrom natural temporal variability that occurs independently within the system (Osenberg et al.
1994, Wiens, in press). Natural variability can

306

STEPHEN M. MURPHYET AL.

of densities of birds during mid-summer
1984/1985and 1989-1991 and for pre/postpairscomparisons(H,2) of densitiesduringmid-summerin oiled (n= 6) and unoiled(n = 4) baysafter (1989-1991) the ExxonValdez oil spill in Prince William Sound, Alaska.

TABLE 3. Power analyses for before-after comparisons (H,l)

Power was calculatedfor a hypothetical two-fold decreaseor increasein density between years for the beforeafter comparisonand for a hypothetical two-fold difference in density between oiled and unoiled bays for the
pre/post pairs comparisons.Pre-spill (19840985) data are from Irons et al. (1988).
Before-afterSpeciealtaxon

Red-neckedGrebe
PelagicCormorantHarlequinDuck
CommonMerganser
Bald Eagle
Black Oystercatcher
Bonanarte’s Gull

Mew&GullGlaucous-wingedGull
Black-leaped Kittiwake
Pigeon C%illemot“Total murrelets”

compansons

Prdpost pairs comparisons

1989

1990

1991

1989

1990

1991

1.oo
1.oo
1.00
0.85
1.oo
1.oo
1.00
0.55
0.80
0.70
1.oo
0.40

1.00
1.00
0.95
0.85
1.00
1.00
1.oo
0.70
0.95
0.60
0.95
0.70

1.oo
1.oo
0.85
0.95
1.oo
1.oo
1.00
0.60
0.90
0.70
1.oo
0.70

1.00

1.oo

1.oo

0.30
0.70
0.95

1.oo
0.55
0.45
0.85
0.90

0:5”0

0:3”0
0.40
0.40
0.75
0.30

o::o
0.55
0.35
0.65
0.40

0:30
0.45
0.35
0.70
0.50

0%

0.55
0.80
1.oo

1 na = no analysis.

be measured, however, if there is an adequate
time series of data prior to the perturbation.
Unfortunately, such a multi-year data set ofbird abundance collected prior to the spill does
not exist for PWS. Becauseof the importance of
this issue, however, we examined the natural annual variability in abundance (densities) of seabirds around Kodiak Island (located southwestof PWS in the Gulf of Alaska), using data collected during fall and winter in 1979-1983(Zwiefelhofer and Forsell 1989). For both fall
and winter data sets, we calculated the percentage of annual change in abundance over successive years for 19 taxa for which a complete
5-year data series was available (primarily from
Uyak and Uganik bays; the procedure for calculating percent change followed that describedin the Methods section for before-after comparisons in PWS). The degree of annual variability
differed among taxa and between seasons,but
annual changes in both seasons combined exceeded a two-fold increase or decreasein overall
abundance in -21% of the comparisons (n =152 for all taxa, seasons,and years). This threshold of a two-fold change in abundance was exceeded in at least one annual comparison by 9(47%) of 19 taxa in fall and by 8 (42%) of 19
taxa in winter.
From these results, we conclude that naturalannual variability often is high for marine birds
during fall and winter in southcoastalAlaska.
Consequently, one’s ability to detect statisticallysignificant effects on overall population levels
from environmental perturbations is confoundedby this high variability. These results also indicate that the abundanceof birds in bays can fluctuate substantially between years and that anevaluation of overall changes in abundance between years is likely to produce statistically significant results as frequently as 20% of the time,even in the absenceof a perturbation. Undoubtedly, there are differences between the variability measured during fall and winter on Kodiak
Island and that which occurs in PWS during
summer. The Kodiak data are useful nonethelessbecause they involve the same general suite oftaxa in habitats that are fairly similar to those in
PWS.
Statistical power. Power analyses for the before-after analyses indicated that our ability todetect significant changes in densities between
1984/1985 and the 3 post-spill years was moderate (60-89%) to high (2 90%) for most taxa:
mean power for the 36 comparisons was 88%(Table 3). Overall, power was high for 22 of the
36 comparisons, moderate for 12, and low (<60%) only for 2 comparisons (Mew Gulls andtotal murrelets in 19840985 vs. 1989). Mew
Gulls showed nonsignificant declines in 2 of 3
post-spill years, but there was no consistentpattern from year to year that would suggest an
impact. Total murrelets increasedsignificantly in1991 but had nonsignificant increases(with low
power) in 1989 and 1990.

EXXONVALDEZOIL SPILL

AND

BIRDS

301

TABLE 4. Responses of marine birds to oiling based on pre/post pairs comparisons, power analyses, and general
patterns of abundance in 10 bays in Prince William Sound,Alaska,aftertheExxonValdez oil spill, mid-summer,

1989-1991.
Response to oiling

No significant response and moderate-high

TZXOllpower

No significantresponse,
but low powerand negativetrends”
No significantresponse,
but low powerand positivetrend+
Significantnegativeresponse
Significant positive response

Grebe
HarlequinDuck
Bald Eagle
CommonMerganser“total murrelets”
Mew GullGlaucous-winged
Gull
PelagicCormorantBlack Oystercatcher
PigeonGuillemot
Red-neckedBl‘&k-legged

Kittiwake

ANegative trend defined as Z - 10% relative difference between oded and unoiled bays (see Table 2)h Positive trend defined as > 10% relative difference between oiled and unoded bays (see Table 2).

oiling, but power was low and relative densities
in oiled bays were > 10% different from thosein unoiled bays. Densities in oiled bays werehigher than in the unoiled bays relative to thepre-spill baseline for Mew Gulls, so this species
was classified as “no significant response but
low power and positive trends.” Glaucouswinged Gulls also were placed in this response
category because they had low power in all 3
years and nonsignificant positive trends in 1989
and 1990. Although a significant positive responsewas detected in 1991, the available evidence did not justify classifying this species as
positively impacted. Black-legged Kittiwakes,
CLASSIFICATION OF OILING RESPONSES
on the other hand, did show significant positive
We classified the responsesof each taxon to oil- responsesto oiling in 1989 and 1990 and were
ing based on (in decreasingorder of importance) the only species classified as “significant posithe results of the pre/post pairs analyses (includ- tive response” (Table 4).
Densities of Common Mergansers and total
ing the temporal pattern of significant results),
the power analyses, and the general patterns of murrelets were lower in oiled than in unoiled
abundance in oiled and unoiled bays. Based on bays relative to the pre-spill baseline; thus, these
these criteria, we concluded that the taxa we ex- two taxa were classified as “no significant reamined fell into five fairly distinct responsecat- sponsebut low power and negative trends” (Taegories that ranged from no significant response ble 4). Pelagic Cormorants, Black Oystercatchand moderate-high power to significant negative ers, and Pigeon Guillemots respondednegatively
to oiling during at least 1 of 3 post-spill years
response (Table 4).
For 7 (64%) of 11 taxa, no significant overall and were classified as “significant negative reeffects could be attributed to oiling. For 4 of sponse.” Pelagic Cormorants showed a negativethese 7 taxa, however, statistical power was low response only in 1990 but were not present inand there was > 10% relative difference be- enough bays in 1989 and 1991 to conduct analtween oiled and unoiled bays (Tables 2 and 4). yses. Consequently, there were insufficient data
to evaluate whether recovery was occurring.
Thus, only Red-necked Grebes, HarlequinBlack Oystercatchers and Pigeon Guillemots
Ducks, and Bald Eagles were categorized as “nosignificant responseand moderate-high power.” were negatively impacted in 1989 (and 1990 for
Common Mergansers, Mew Gulls, and total Pigeon Guillemots) but showed no significant
mm-reletsalso showed no significant responseto negative response in 1991. We suspect, there-

Power analyses for the pre/post pairs comparisons indicated that our ability to detect significant changes in densities between oiled and unoiled bays was low to moderate for most taxa:
mean power for 31 comparisonswas 61% (Table3). Power was high for 7 (23%) of 31 comparisons, moderate for 7 (23%) comparisons, andlow for 17 (55%) comparisons. Power was low
in at least 1 year for 6 (55%) of the 11 taxa
analyzed. Four of these six taxa showed positive
responsesto oiling, whereastwo (Common Merganser and total murrelets) appearedto have had
at least initial negative responsesto oiling.

308

STEPHEN

M. MURPHYET AL.

sonal context, however. Red-necked Grebes arepresent in PWS primarily during winter and areuncommon visitors during summer (Isleib andKessel 1973). Indeed, Irons et al. (1988) saw
only 19 Red-necked Grebes in their pre-spill
surveys of the 10 study bays (Table l), and 12of those grebes occurred in 1 bay. Although werecorded only one Red-necked Grebe during our
1989-1991 mid-summer surveys, we question
the biological significance of this apparent decline because this species generally is uncommon in PWS during summer and our statisticalevaluations of annual changesare based on scantdata. Pelagic Cormorants also are much morecommon in PWS during winter (Isleib and Kesse1 1973), and Irons et al. (1988) counted only
28 in the 10 study bays during summer
1984/1985. Pigeon Guillemots, on the other
hand, are more common in PWS during summer(Isleib and Kessel 1973), so both the pre-spill
and post-spill data setswere adequateto support
DISCUSSION
the conclusion of significant and persistentoverOil spills at sea can affect marine birds along all declines in abundance during each of the 3
three primary pathways, namely, by altering post-spill years. Pigeon Guillemots, however,
population size and structure, reproduction, and were thought to be declining in PWS prior tohabitat occupancy and use (Wiens 1995). The the spill (Oakley and Kuletz 1993). The fact thatanalyses presented here addressedeffects of the the magnitude of the difference in Pigeon Guiloil spill on overall bird abundance and distri- lemot abundance decreased in each successive
bution, and therefore provide insight into effects post-spill year, however, suggeststhat there wason the first and third of these pathways. The be- an initial decline in overall abundance immedifore-after analyses (HJ) indicated that half of ately after the spill but that numbers of birds
the 12 taxa that we examined exhibited no sig- were increasing l-2 years later.
nificant changes in overall abundance during
In the pre/post pairs analysis (H,2), the conmid-summer between 1984/1985 and 1989founding effects of natural variation were re1991. Fewer taxa showed negative trends in duced because the historical data were used toabundance compared to the pre-spill baseline estimate the differences between oiled and unduring each successive year after the spill (8 oiled bays that would exist in the absenceof the
taxa declined in 1989,7 in 1990, and 6 in 1991), perturbation (Stewart-Oaten et al. 1992). Our fisuggestingthat declines in abundance were most nal classification of overall impacts for the 11
pronounced in the year of the spill and that re- taxa examined indicated that 7 taxa showed nocovery was occurring in 1990 and 1991. Over- significant response to oiling, 3 (Pelagic Corall, we documented few dramatic declines or inmorant, Black Oystercatcher and Pigeon Guilcreases in the abundance of most bird species lemot) showed significant negative responsesto
during the three summers after the Exxon Vuldez oiling, and 1 (Black-legged Kittiwake) had a sigoil spill. In addition, power analyses indicated nificant positive response to oiling. Statisticalthat we should have good confidence in these power was a greater consideration in these analresults.
yses than in the before-after comparisons, beSpecies that were significantly less abundant cause there were fewer degreesof freedom after
during all 3 post-spill years than they were prior partitioning the data into oiled and unoiled catto the spill included Red-necked Grebes, Pelagic egories. The consequence of low power would
Cormorants, and Pigeon Guillemots. The de- be to increase the likelihood of failing to detectcreases in abundances for Red-necked Grebes significant differences (Type II errors). Four ofand Pelagic Cormorants must be placed in a sea- the 7 taxa that showed no significant response

fore, that recovery was underway by that year,
although the relative difference between oiled
and unoiled bays for both of these species stillwas > 10% in 1991 (Table 2).
It is noteworthy that all seven pre/post pairs
comparisons that were significant had P values> 0.075 and 5 0.20 (Table 2). Thus, with thepossible exception of the 1989 pre/post pairs
comparison for Pigeon Guillemots (P = 0.075),
if we had partitioned (Yin any way or used more
restrictive decision-making criteria (e.g., (Y =0.05), none of the pre/post pairs comparisons
would have been significant. Our use of a large
IX, however, representsan emerging trend in impact assessmentsthat considers Type II errors(i.e., failing to detect an impact that did occur)
to be equally or more problematic than Type I
errors (i.e., falsely identifying an impact that did
not occur) (Eberhardt and Thomas 1991, Shrader-Frechette and McCoy 1993, Mapstone 1995).

EXXON VALDEZ OIL SPILL AND BIRDS

to oiling had low power or trends that led us toreconsider those statistical results (Table 3). Of
these questionable taxa, Common Mergansers
and total mm-relets exhibited negative trends in
1989, suggesting that these taxa initially responded negatively to oiling. Thus, an inclusive
accounting of the taxa that were affected negatively by the oil spill includes Pelagic Cormorants, Common Mergansers, Black Oystercatchers, Pigeon Guillemots, and total murrelets (Table 4). Most of these impacts were evident only
in 1989, however, and signs of recovery were
evident for all taxa for which we had data insubsequentyears. Pigeon Guillemots showedthe
most persistent negative effects, with significant
decreasesin oiled bays in 1989 and 1990 and anonsignificant but substantial decrease(22%) in
1991. Considering that the decreases in 1989
and 1990 were significant and > 50%, the 1991
results suggestthat recovery was beginning.
Black-legged Kittiwakes, Mew Gulls, andGlaucous-winged Gulls appearedto be attracted
to spill cleanup activities. Their apparent positive responseto oiling probably was a secondary
consequence of the spill, as some of the mostheavily oiled bays provided anchorage for thehundreds of vessels and their crews that participated in the shoreline cleanup.
The results of the pre/post pairs analyses(I-&,2) can be compared directly with the resultsof two other analyses that addressedthe samegeneral question of oiling impacts to birds in
PWS. Klosiewski and Laing (1994) conductednearshoresurveys in PWS during summer 19891991 in a subset of the areas that had been surveyed by Irons et al. (1988) in 1984/1985. Although their study had only partial geographic
overlap with our study area, their survey andanalytical techniques were similar to ours andalso can be described as pre/post pairs analyses(Wiens and Parker 1995). Based on comparisonswith Irons et al. (1988), Klosiewski and Laing(1994) found that six taxa (“loons,” Harlequin
Duck, “scoters,” Black Oystercatcher, Mew
Gull, and Arctic Tern) exhibited negative oiling
impacts (Table 5). The number of impacted taxa
ranged from zero in 1989 to five in 1991; however, if the same two-tailed (Y that was used inthis study (cx = 0.20) is applied to their results,
those numbers change to one taxon (Black Oystercatcher) in 1989 and six taxa (including
murres) in 1991.
Day et al. (1995, in press) also used the same309

post-spill data set that we used but did not use
historical data. Instead, Day et al. assessedoiling
impacts with two analytical techniquesdescribedby Wiens and Parker (1995) as “single-time gradient analysis” and “multiple-time trend-bytime analysis.” Although they assessedimpacts
and recovery based on six sampling periods
spacedthroughout the entire year, only analysesof nearshoresurvey data from mid-summer sampling periods in 1989-1991 were directly comparable to our analyses. During mid-summer,
Day et al. recorded negative oiling impacts forfive species: Pelagic Cormorants, Harlequin
Ducks, Common Mergansers, Black Oystercatchers,and Mew Gulls (Table 5).
Thus, in addition to the three significant andtwo probable taxa that we identified as negatively impacted during mid-summer, four othertaxa (“loons,” Harlequin Ducks, and “scoters”[Klosiewski and Laing 19941; Harlequin Ducks
and Mew Gulls [Day et al. 1995, in press]) wereidentified as negatively impacted by the two other studiesevaluating spill effects on birds (Table5). When these studies are viewed collectively,
they suggestthat up to 10 (38%) of the 26 taxa
studied were negatively impacted by the spill insummer. By mid-summer 1991, however, there
was evidence of recovery from at least one study
for all of these taxa except “loons” and “scoters.” However, neither of these taxa was abundant in our study bays during summer either before or after the spill and neither was evaluatedin this study or in mid-summer by Day et al.
(Table 5).
Based on year-round sampling, Day et al.
(1995) found evidence of initial impacts to 19(54%) of the 35 species evaluated in 1989,
whereas the percentage of impacted speciesdeclined to 40% in 1990 and 10% in 1991. In comparison, this study showed that 20-40% of thespecieswere impacted in mid-summer 1989 andthat no species showed significant negative impacts by 1991, although several species still
showed some (nonsignificant) evidence of persistent effects. Thus, the overall results of these
two studies were quite similar (Table 5).
Although there was broad concurrence among
all three studies in the classification of taxa thatwere not impacted and in the percentageof taxa
that were impacted, there was a lack of concurrence in the classification of negatively impacted
taxa. This lack of concurrencemay be related to
how the three studies classified oiled areas in

310

STEPHEN M. MURPHYET AL.

TABLE 5. Comparison of the results of three studiesthat assessedimpacts of the Exxon Vuldez oil spill on
marine-oriented birds during mid-summer in Prince William Sound, Alaska, 1989-l 99 1. Negatively impacted
taxa that showed evidence of recovery are depicted in bold-faced type.
Historical comparisonsSoeciesltaxon

No Evidence of Impact
Red-necked Grebe“Goldeneyes”
Bald Eagle
Wandering Tattler
Spotted Sandpiper
Red-necked Phalarope“Shorebirds”
Glaucous-wingedGull“Murres”“Total murrelets”eHomed Puffin
Belted Kingfisher
Steller’s Jay
Black-billed Magpie
NorthwesternCrow
Common Raven
Evidence of Impact“Loons”
Pelagic Cormorantf
Harlequin Duck
“Scoters”
Common Merganser8
Black Oystercatcher
Mew GullBlack-legged Kittiwake
Arctic Tern
Pigeon Guillemot

This study

USFWS

study”

No impactna
No impact
na
na
na
na
No impact?“na
No impact?
na
na
na
na
na
na

na’
No impact
No impactnana
No impact
No impact
No impact
No impact
No impact
No impact
na
na
na
na
na

na
Negative impact
No impact

Negative impact
No impact
Negative impact
Negative impact
No impact
Negative impact
Negative impact
No impact
Negative impact
No impactf;“, impact?
Negative impact
No impact?
Positive impactna
Negative impact

Habitat-use study”nana
No impact
No impact
No impact
No impactna
No impactna
No impactna
No impact
No impact
No impact
No impact
No impact

Eegative impact
Negative impact
Egative impact
Negative impact
Negative impact
Positive impact
No impact
Positive impact

d U.S. Rsb and Wddlife Service comparisons with 1984/1985 baseline (Klosiewski and Laing 1994).
DDay et al. (1995, in press): mcludes nearshore data from mid-summer (July/August) cruises in 1989-1991(na = no analysis.
* ? = taxon that had nonsignificant differences but low power (see Table 3).
*Analyzed
as Marbled Murrelets by Day et al. (1995, in press).
‘Analyzed
as “cormorants” by Klosiewski and Laing (1994).
c Analyzed as “mergansers” by Klosiewski and Laing (1994).

Prince William Sound and how those classifications affected data analyses. Day et al. (1995)
used an oiling index based on the amount of oilthat initially contaminated the shorelines of thestudy bays to describe the relative amount of oil
in each bay as a quantitative, continuous variable. This strategy was highly accurate and facilitated the use of regression statistics in hypothesis testing, which were more powerful for
detecting impacts than were the proceduresusedin both of the other studies. In this study, weused the same oiling index as Day et al., but thencategorized each bay as oiled or unoiled (Fig. 2)
to accommodate the BACI-like analytical design. Klosiewski and Laing (1994) also categorized areas as oiled and unoiled for use in a
BACI-like analysis, but they considered any

shoreline that was in the general path of the spill
to be oiled, whether or not field surveys showedthat it actually was oiled. Although the analytical procedures used in this study and by Klosiewski and Laing were similar, differences in
impact classifications for individual taxa probably were related to differences between the studies in the classification of shoreline oiling. The
distribution of oil on shorelines in the generalpath of the spill was highly variable (to the extent that some entire bays were unoiled), and aquantitatively based measure such as the oilingindex we used would seem to provide a moreaccurate assay of oiling conditions. Indeed, Baker et al. (1990) considered this level of resolution to be critical, stating that “considerationof the overall, long-term impact of a particularEXXONVALDEZOIL SPILL AND BIRDS

311

noted that most communities of organisms intemperate waters will recover from massive oilspills within 2 years, with some effects detectable up to 10 years after the spill.
CONCLUSIONS
Our researchindicates that, despite substantial
Our evaluation of pre- and post-spill abundances mortality in the aftermath of the spill and sigof birds in 10 bays in Prince William Sound in- nificant impacts to the distribution of up to 40%
dicated that 3 of 11 taxa evaluated had declined of the taxa in PWS during the first summer folsignificantly from numbers recorded 4-5 years lowing the spill, the abundance of birds during
before the spill. Although this analysis could not mid-summer generally was not significantly affactor out natural sourcesof variation, there was fected and redistribution of birds back intono evidence of large-scale, post-spill changesin heavily oiled bays was occurring for nearly all
overall abundance of most taxa. Our inability to taxa by the second summer after the spill. The
reject the null hypothesisof no large-scale, post- redistribution of birds into the heavily oiled bays
spill changesin overall abundance of most taxa indicates that these habitats are at least in thein our 10 study bays (even using a broad statis- process of recovering and that the birds themtical definition of impact) suggeststhat most bird selves find the habitat suitable for occupancy.
populations in PWS during mid-summer were Becausehabitat recovery is a prerequisite for renot impacted catastrophically. The Pigeon Guilcovery of population or reproductive measureslemot was the one species that was both com- (Morrison 1986, Wiens 1995), the prognosis formon during summer in PWS and showed per- recovery in these other dimensions appears tosistent declines in overall abundance relative to be good.
the pre-spill baseline.
Our analysis of oiling impacts indicated that ACKNOWLEDGMENTS
the effects of the spill on bird abundancein oiled We thank the U.S. Fish and Wildlife Service, and esbays during mid-summer were most pronounced pecially D. B. Irons, for granting permission to use
in 1989 and that, by 1991, most marine bird spe- data collected from PWS prior to the spill. We also
cies in PWS were using oiled bays at levels that thank D. C. Zwiefelhofer, U.S. Fish and Wildlife Service, for allowing us to use data from Kodiak Island.
were indistinguishable from levels of use in un- We gratefully acknowledge the personnel of ABR,
oiled bays. Once again, Pigeon Guillemots
Inc.,who worked on this project:L. N. Smith, D. A.
showed the greatest negative impacts and the Flint, L. C. Bvme. S. G. Sueckman. J. R. Rose. B. K.
Lance, C. B. johnson, A. i. Zusi-Cobb, B. A. Cooper,
fewest signs of recovery.
R. M. Burgess, R. J. Ritchie, A. A. Stickney, T. D.
The conclusions from both analyses are Davis, E L. Craighead, T. J. Mabee, M. K. MacDonald,strengthenedby the fact that we sampled many D. C. Anthon, and I. E. Kellogg. This manuscript wasof the most heavily oiled bays in the entire spill critically reviewed by B. E. Lawhead, R. J. Ritchie andarea, thereby increasing our chancesof detecting B. A. Anderson (ABR, Inc.) and D. B. Irons, S. I?
impacts if they were present. These conclusions Klosiewski, and D. C. Zwiefelhofer (U.S. Fish andWildlife Service). Exxon Company, USA funded thisalso are supportedby Day et al. (1995, in press), research. The views expressed here are our own andwho evaluated effects of the spill on habitat use, do not necessarily reflect those of the reviewers or ofand by Wiens et al. (1996), who evaluated ef- Exxon.
fects on bird communities. The analyses supporting those papers were based upon the same LITERATURE CITED
spill must take into account the relative proportions of oiled and unoiled habitats in the area ofinterest.”

post-spill data used in this paper, although theiranalyses did not incorporate comparisons withpre-spill data. Regardless, both Day et al. andWiens et al. found that impacts were substantial
and most apparent in 1989, but that recovery
was well underway by 1991. Our conclusions
also are in agreement with assessmentsof impacts and recovery by researchers who havestudied other large marine oil spills (Clark 1982,
Baker et al. 1990, Mielke 1990, Ritchie andO’Sullivan 1994). For example, Clark (1982)

BAKER,J. M., R. B. CLARK,P. E KINGSTON,
AND R. H.
JENKINS.1990. Natural recovery of cold water
marine environmentsafter an oil spill, p. 173-177.
In Proceedings of the Thirteenth

Annual Arctic
and Marine Oilspill Program Technical Seminar,
6-8 June 1990, Edmonton, Alberta, Canada. Environment Canada, Ottawa, Ontario, Canada.

CLARK,R. B. 1982. The long-term effect of oil pollution on marine populations, communities, andecosystems: some questions. Phil. Trans. Roy.
Sot. 297B:185-192.
DAY, R. H., S. M. MURPHY,J. A. WIENS,G. C. HAYWARD,E. J. HARNER,AND L. N. SMITH. 1995. Use

312

STEPHEN

M. MURPHYET AL.

of oil-affected habitats by birds after the Exxon
Vuldez oil spill, p. 726-761. In F?G. Wells, J. N.
Butler, and J. S. Hughes [eds.], Exxon Vuldez oil
spill: fate and effects in Alaskan waters. ASTM
Spec. Tech. Publ. 1219, Am. Sot. Testingand Materials, Philadelphia, PA.
DAY, R. H., S. M. MURPHY,J. A. WIENS,G. C. HAYWARD, E. J. HARNER,AND L. N. SMITH.In press.
Effects of the Exxon Valdez oil spill on habitat
use by birds in Prince William Sound, Alaska.
Ecol. Appl.
DWYER,T J., l? ISLEIB,D. A. DAVENPORT,AND J. L.
HADDOCK. 1976. Marine bird populations in
Prince William Sound,Alaska. Unpubl. Rep., U.S.
Fish Wildl. Serv., Anchorage, AK.
EBERHARDT,
L. L., AND J. M. THOMAS.1991. Designing environmentalfield studies.Ecol. Monogr. 61:
53-73.
ECOLOGICALCONSULTING,
INC. 1991. Assessmentof
direct seabird mortality in Prince William Soundand the western Gulf of Alaska resultingfrom theExxon Valdez oil spill. Unpubl. report preparedfor
the Exxon Valdez Oil Spill Trustees,Anchorage,
Alaska, by Ecological Consulting, Inc., Portland,
OR. (Available from the Oil Spill Public Information Office, 645 G St., Anchorage,AK 995013451)
ERIKSON,D. E. 1995. Surveys of murre colony attendance in the northern Gulf of Alaska following
the Exxon Valdez oil spill, p. 780-819. In P (?
Wells, J. N. Butler, and J. S. Hughes [eds.], Exxon
Valdez oil spill: fate and effects in Alaskan waters.
ASTM Spec. Tech. Publ. 1219, Am. Sot. Testingand Materials, Philadelphia, PA.
FRY. D. M. 1993. How do YOUfix the loss of half a‘million birds?, p. 30%i3. In Exxon Valdez Oil
Spill Symposium, Abstract Book. Exxon Valdez
Oil Spill TrusteesCouncil, Anchorage, AK.
GALT, J. A., W. J. LEHR, AND D. L. PAYTON. 1991.
Fate and transportof the Exxon Valdez oil spill.
Environ. Sci. Technol. 25:202-209.
IRONS,D. B., D. R. NYSEWANDER,
AND J. L. TRAPP.
1988. Prince William Sound waterbird distribution in relation to habitat type. Unpubl. rep., U.S.
Fish Wildl. Serv., Anchorage, AK:_ISLEIB.M. E.. AND B. KESSEL. 1973. Birds of the North
Gulf Co&t-Prince William Sound region, Alaska.
Biol. Pap. Univ. Alaska 14:1-149.
KLOSIEWSKI,
S. P, AND K. K. LAING. 1994. Marine
bird populationsof Prince William Sound,Alaska,
before and after the Enwon Valdez oil spill. Unpubl. report. Exxon Valdez Oil Spill State andFederal Natural Resources Damage Assessment
Final Report, preparedfor Exxon Valdez Oil Spill
Trustees Council, Anchorage, AK. (Available
from the Oil Spill Public Information Office, 645
G St., Anchorage, AK 99501-3451)
MAPSTONE,B. D. 1995. Scaleable decision rules forenvironmentalimpact studies:effect size, Type I,
and Type II errors. Ecol. Appl. 5: 401-410.
MIELKE,J: 2. 1990. Oil in the-ocean: the short- andlong-term impacts of a spill. CongressionalRes.
Serv. Rep., Library of Congress,Washington,DC.
MORRISON,
M. L. 1986. Bird populationsas indicatorsof environmental change. Current Omithol. 3:
429-45 1.
MOSTELLER,
E, AND J. W. TUKEY. 1977. Data analysis
and regression:a second course in statistics.Addison-Wesley,Reading, MA.
NEFF,J. M., E. H. OWENS,S. W. STOKER, AND D. M.
MCCORMICK.1995. Conditions of shorelines in
Prince William Soundfollowing the Exxon Valdez
oil spill: Part l-shoreline oiling, p. 312-346. Inl? G. Wells, J. N. Butler, and J. S. Hughes [eds.],
Exxon Valdez oil spill: fate and effects in Alaskan
waters. ASTM Spec. Tech. Publ. 1219, Am. Sot.
Testing and Materials, Philadelphia, PA.
OAKLEY, K. L., AND K. J. KULETZ. 1993. Effects ofthe Exxon Valdez oil spill on Pigeon Guillemots(Cepphus columba), p. 144-146. In Exxon Valdez
Oil Spill Symposium,Abstract Book. Exxon Valdez Oil Spill TrusteesCouncil, Anchorage,AK.
OSENBERG,
C. W., R. J. SCHMITT,S. A. HOLBROOK,K.
E. ABU-SABA,AND R. FLEGAL.1994. Detection ofenvironmental impacts: natural variability, effectsize, and power analysis.Ecol. Appl. 4: lk30.
PARRISH,J. K., AND I? D. BOERSMA.1995. Muddy waters. Am. Sci. 83:112-115.
PIAT~,J. E, C. J. LENSINK,W. BUTLER,M. KENDZIOREK,AND D. R. NYSEWANDER.
1990. Immediate impactof the ‘Exxon Valdez’ oil spill on marine birds.
Auk 107~387-397.
RITCHIE,W., AND M. O’SULLIVAN. 1994. The environmental impacts of the wreck of the Braer. The
ScottishOffice. Edinburgh, Scotland.
ROYER,T. C., J. A. VERMERSCH,
T. J. WEINGARTNER,
H.
J. NIEBAUER,AND R. D. MUENCH. 1990. Ocean
circulation influencing the Exxon Valdez oil spill.
Oceanography3:3-10.
SCHROETER,
S. C., J. D. DIXON, J. KASTENDIEK, AND R.
0. SMITH. 1993. Detecting the ecological effects
of environmental impacts: a case study of kelp
forest invertebrates.Ecol. Appl. 3:331-350.
SHRADER-FRECHETTE,
K. S., AND E. D. MCCOY. 1993.
Methods in ecology: strategiesfor conservation.
Cambridge Univ. Press,Cambridge.
SKALSKI,J. R., AND D. H. MCKENZIE. 1982. A design
for aquaticmonitoringprograms.J. Environ. Manage. i4:237-251.
- _ STEWART-OATEN.
A.. J. R. BENCE.AND C. W. OSENBERG. 199i.
Assessing effects of unreplicatedperturbations:no simple solutions. Ecology 73:
1396-1404.
AND K. R. PARSTEWART-OATEN,
A., W. H. MURDOCH,
KER. 1986. Environmental impact assessment:“pseudoreplication” in time? Ecology 67:929940.

TASKER,M. L., P H. JONES,
T. DIXON,AND B. E BLAKE.
1981. Counting seabirdsat sea from ships: a review of methods employed and a suggestionfora standardizedapproach.Auk 101:567-577.
WIENS, J. A. 1995. Recovery of seabirdsfollowing
the Exxon Valdez oil spill: an overview, p. 854893. In P G. Wells, J. N. Butler, and J. S. Hughes[eds.], Exxon Valdez oil spill: fate and effects in
Alaskan waters. ASTM Spec. Tech. Publ. 1219,
Am. Sot. Testingand Materials, Philadelphia,PA.
WIENS,J. A. In press. Coping with variability in en-

EXXONvironmental impact assessment,p. 55-70. In D.
Baird, L. Maltby, I? W. Greig-Smith, and I? E. T.
Douben [eds.], Ecotoxicology: ecological dimensions. Chapman and Hall, London.
WIENS,J. A., T 0. GRIST,R. H. DAY, S. M. MURPHY,
AND G. D. HAYWARD.1996. Effects of the Exxon
Valdez oil spill on marine bird communities in
Prince William Sound,Alaska. Ecol. Appl. 6:828841.
WIENS,J. A., AND K. R. PARKER. 1995. Analyzing theeffects of accidental environmental impacts: approaches and assumptions.Ecol. Appl. 5:10691083.
WINER,B. J., D. R. BROWN,AND K. M. MICHELS.1991.
Statistical principles in experimental design. McGraw-Hill, New York.
ZAR, J. H. 1984. Biostatisticalanalysis.2nd ed. Prentice-Hall, Englewood Cliffs, NJ.
1989. Marine
ZWIEFELHOFER,
D. C., AND D. J. FORSELL.
birds and mammals wintering in selectedbays of
Kodiak Island, Alaska: a five-year study.Unpubl.
Rep., U.S. Fish Wildl. Serv., Kodiak, AK.

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APPENDIX 1. Scientific names of bird speciesmentioned in text.
Commonname

Scientificname

Red-necked Grebe
Podiceps grisegena
Pelagic CormorantPhalacrocorax pelagicus
Harlequin Duck
Histrionicus histrionicus
Common Merganser
Mergus merganser
Bald Eagle
Haliaeetus leucocephalus
Black Oystercatcher Haematopus bachmani
Wandering Tattler
Heteroscelus incanus
Spotted Sandpiper
Actitis ma&aria
Red-necked Phalarope Phalaropus lobatus
Mew Gull
Lams canus
Glaucous-wingedGull Lams glaucescens
Black-legged Kittiwake Rissa triductyla
Arctic Tern
Sterna paradisaea
Pigeon Guillemot
Cepphus columba
Marbled Murrelet
Brachyramphus marmoratus
Kittlitz’s Murrelet
Brachyramphus brevirostris
Homed Puffin
Fratercula comiculata
Belted Kingfisher
Ceryle alcyon
Steller’s Jay
Cyanocitta stelleri
Black-billed Magpie
Pica pica
NorthwesternCrow
Corvus caurinus
Common Raven
Corvus cot-ax