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Estimates of Changes in Seabird Numbers in the Grand Manan Archipelago, New Brunswick, Canada

R

OBERT

A. R

ONCONI

1

AND

S

ARAH

N. P. W

ONG

1,2

1

Grand Manan Whale and Seabird Research Station, 24 Rte. 776, Grand Manan, NB, E5G 1A1, Canada

2

University of Calgary, Department of Anthropology, 2500 University Dr. NW, Calgary AB, T2N 1N4, CanadaInternet: snpwong@ucalgary.ca

Abstract.—

A census of the Grand Manan Archipelago, New Brunswick, Canada, found nine seabird species nest-ing at 22 sites. Herring Gull (

Larus argentatus

) and Common Eider (

Somateria mollissima

) were the most widely dis-tributed species, with a combined 89% of all breeding seabirds in the region. Although fewer, the Great Black-backed Gull (

Larus marinus

) and Black Guillemot (

Cepphus grylle

) were widely distributed. Other seabirds had morelimited distributions; one Common Tern (

Sterna hirundo

) colony, three Double-crested Cormorant (

Phalacrocorax au-ritus

) colonies and one mixed colony of Razorbill (

Alca torda

) and Common Murre (

Uria aalge

). Leach’s Storm-Petrel(

Oceanodroma leucorhoa

) numbers were not determined, but were found nesting on three islands and were suspectedto be nesting on two others. Comparisons with historical data showed increases in gulls, Razorbill and CommonMurre, whereas eiders had remained relatively stable over the past two decades, but had decreased in recent years.Insufficient data are available to assess trends of other seabird species nesting around Grand Manan. Fluctuationsin cormorant numbers may be a result of moving breeding sites. The recent decrease in eider numbers may be aconsequence of low survival rates of young and increasing numbers of predatory gulls. Considerable discrepancieswere observed between recent and historical gull numbers which may have resulted from 1) differences in surveymethodology, and 2) survey timing. This study provides estimates of breeding seabirds, which can be used as baselinedata for future monitoring in the Grand Manan Archipelago.

Received 30 December 2002, accepted 7 June 2003

.

Key words.

—Seabirds, census, Grand Manan, numbers, trends, gulls, alcids, cormorants, eiders, storm-petrels.

Waterbirds 26(4): 462-472, 2003

Grand Manan Island and its surroundingislands provide important habitat for breed-ing seabirds in the Bay of Fundy region ofCanada. Over the past two decades, GrandManan has experienced steady increases inhuman activities such as aquaculture, rock-weed harvesting, and eco-tourism, yet little isknown about the potential effects of these ac-tivities on colonial nesting seabirds. Contin-ued monitoring of breeding seabird numbersis a useful tool for identifying risks to seabirdscaused by coastal development. Furthermore,seabirds are recognized as monitors of marineecosystems (Kushlan 1993; Diamond 2000);acting as indicators of marine environmentalchange (e.g., changes in response to El Niñoevents; Schreiber and Schreiber 1989), ma-rine pollution (Furness and Camphuysen1997), and changes in fish stocks (Montevec-chi 1993). However, little concurrent data forthe Grand Manan region is available to assessmarine ecosystem changes or long-term af-fects of coastal development. Past seabird sur-veys in the Grand Manan Archipelago (Lock1982; Cannell and Maddox 1983; Lock 1986;Hébert 1989; Lock

et al.

1994; Mawhinney

et al.

1999; Chapdelaine

et al.

2001) includedHerring Gull (

Larus argentatus

), Great Black-backed Gull (

Larus marinus

), Common Eider(

Somateria mollissima

), Double-crested Cormo-rant (

Phalacrocorax auritus

), Razorbill (

Alca tor-da

), Common Murre (

Uria aalge

) and Leach’sStorm-Petrel (

Oceanodroma leucorhoa

). Someof this information, however, is now out ofdate, and incomplete for some species. Fur-thermore, estimates for some colonies andspecies remain undocumented and/or un-published, e.g., Black Guillemot (

Cepphusgrylle

) and Common Tern (

Sterna hirundo

).The objective of this study was to obtain

current and complete census data of all sea-bird colonies in the Grand Manan Archipel-ago during the 2001 breeding season. Thesedata are compared to previous surveys to ex-amine regional trends in seabird numbers.Results of this study could be used as base-line data for measuring future changes inBay of Fundy seabirds.

M

ETHODS

The Grand Manan Archipelago (Fig. 1) is located inthe Bay of Fundy, New Brunswick, Canada (44°42’N,

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66°48’W). All islands in the archipelago were surveyedexcept Cheney Is. and Machias Seal Island. Cheney Is.has few historical records of nesting seabirds (two gullpairs in 1998; Mawhinney

et al.

1999). Machias is a wild-life sanctuary maintained and studied by the CanadianWildlife Service (CWS) and a census of Machias was be-yond the scope of this project.

This study used standardized census techniques(Nettleship 1976; Bibby

et al.

1992; Walsh

et al.

1995;

Diamond 2000) and adhered to the American Ornithol-ogists’ Union (1988) guidelines to minimize distur-bance. Frequency and length of visits were minimizedand no colony was entered in poor weather conditions.Trends in seabird numbers were assessed by compari-sons with available historical records.

Black Guillemot

—Guillemots were surveyed from23 April to 11 May 2001. Counts of individuals andcourtship pairs within 300m of the shore were conduct-

Figure 1. Locations of seabird colonies in the Grand Manan Archipelago, NB. See Table 1 for key to location num-bers. Yellow Ledge (25) is 13 km south of Western Green Is. (18). Machias Seal Is. is approx. 19 km southwest ofSouthwest Head (24).

464 W

ATERBIRDS

ed in calm seas (< Beaufort 2) from sunrise to 10.00 h.Shore-based counts at two colonies (Bishop and South-west Head) were made from cliff-top vantage points,while all other colonies were surveyed from a boat trav-eling at low speed, ~400 m from shore. A colony was de-fined when at least one courtship pair was present, andtotal colony size is reported as the sum of adult-plum-aged individuals, both paired and unpaired (Bibby

et al.

1992; Walsh

et al.

1995). Surveys were repeated for eacharea, and final estimates used highest counts from con-tinuous stretches of shoreline or groups of adjacent is-lets (Walsh

et al.

1995).

Herring Gull and Great Black-backed Gull

—Surveyscounted active nests (containing eggs, chicks or evi-dence of hatched eggs) and well-constructed nestsjudged capable of holding a clutch (Walsh

et al.

1995)from 9 May to 26 June 2001. Direct nest counts wereconducted on small islands, small colonies or shorelineareas of larger islands. For large colonies, 10 m widetransects were randomly placed at right angles to thelength of the colony. Nests were marked non-invasivelywith paint to avoid double counting and to obtainmarked vs. unmarked ratios. These ratios were appliedto initial counts to give corrected estimates of colonysize (Walsh

et al.

1995). Species composition was deter-mined by binocular counts from vantage points ofadults on regularly spaced territories within the breed-ing area; periphery birds were omitted. This ratio wasapplied to final nest estimates to give species estimatesfor each site. In some cases, where better informationwas available, species composition was determined fromdata collected during nest counts.

Collection of gull eggs occurs regularly in the GrandManan Archipelago, but egg re-laying behavior in re-sponse to this is unclear. Paynter (1947) found 43% ofnests empty 15 days after egg removal, while Spear(1942) found that gulls relayed soon after. Egging activ-ity was recorded and results were adjusted as follows. In2001, egging began 6 June, and empty nests were in-cluded in estimates for islands surveyed prior to thisdate. Estimates for islands surveyed after June 6 exclud-ed empty nests to reduce the probability of includingnests, which may or may not become active again.

Common Eider

—Eiders were surveyed during gullsurveys using the same methodology. Both inactive andactive nests were counted, however, only active nestswere included in final estimates since Lock (1986)found a close correlation between active nest countsand aerial surveys of adult male eiders.

Double-crested Cormorant

—In late June, aerialphotographs were taken of the largest cormorant colo-ny on North Green Is. to count active nests. For all othercolonies, direct counts of nests occupied by an adult,and other well-built nests capable of holding a clutchwere conducted from vantage points at sea. To confirmthese numbers, direct nest counts of used nests weremade in late August, post fledging.

Common Tern

—Apparently incubating adults werecounted (Walsh

et al.

1995) in June with a spotting scopeat weekly intervals. A single flush count (Walsh

et al.

1995) was conducted to confirm scope counts. Colonyvisits were limited to a maximum of 20 minutes.

Razorbill and Common Murre

—Counts of individu-al adults were made from aerial photographs of thebreeding site on Yellow Ledge (44°31’N, 66°51’W). On29 June, photographs were taken from a Cessna 172(elv. ~150 m) with a 35 mm SLR camera and 80-300 mmzoom-lens. The species ratio was determined from pho-

tos of the colony taken by boat on 19 June and appliedto the total aerial count for species estimates, providingan estimate of individuals but not breeding pairs.

Leach’s Storm-Petrel

—Surveys aimed to confirmthe presence or absence of breeding petrels, but esti-mates of numbers were not possible due to time limita-tions. In July, observers searched for petrel burrows aminimum of 1.5 h on each island considered to be suit-able petrel habitat. Potential burrows were searched forfresh nesting material, eggs, incubating adults, orchicks. The large size of Wood Is. made a total burrowsearch impractical, and observers spent a night listeningfor calls of petrels returning to burrows.

R

ESULTS

Current Species Composition and Distribution

In 2001, nine seabird species were foundnesting at 22 sites in the Grand Manan Archi-pelago (Table 1). Grand Manan breedingseabirds totaled 33,915 individuals, whichconsisted predominately of Herring Gull(70%) and Common Eider (20%). HerringGulls and eiders were the most widespread,nesting at 18 sites. Although comprisingsmall proportions of the total breeding num-bers, Great Black-backed Gulls (3.6%) andBlack Guillemots (3.9%) were widely distrib-uted, nesting at 17 and 13 sites respectively.Four other species comprised the remainingnumbers: Common Tern (<0.05%, 1 site),Double-crested Cormorant (0.9%, 3 sites),Razorbill (0.8%, 1 site), and CommonMurre (1.3%, 1 site). Leach’s Storm-Petrelswere found nesting at three sites and pres-ence (calling petrels or predated petrel partsin gull nests) was recorded at two other sites.

Throughout the archipelago, certainsites are identifiable as important seabirdhabitat (i.e., having high proportions of thetotal breeding seabirds and/or high abun-dance for particular species). Three islandsprovided habitat for 72% of all nesting sea-birds: Kent Is. (39%), Great Duck Is. (17%),and Outer Wood Is. (16%). These three is-lands were used by nearly 80% of all HerringGulls and over 70% of all eiders. The GreatBlack-backed Gull nested predominantly atthree sites: Great Duck Is. (47%), NorthGreen Is. (14%) and Sheep Is. (9%). Thetwo largest guillemot colonies were found onKent Is. and Outer Wood Is. with other largecolonies at the Bishop and Southwest Head.Most cormorants (77%) nested at North

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Table 1. Estimates of seabirds nesting in the Grand Manan Archipelago, New Brunswick, 2001. (Numbers are given in pairs except for * given as individuals. Parentheses indicatethe no. of courtship pairs observed at each guillemot colony. Location no. correspond with Figure 1.)

Location** CoordinatesDouble-crested

CormorantLeach’s

Storm-PetrelCommon

EiderHerring

GullGreat Black-backed Gull

Common Tern Razorbill*

Common Murre*

Black Guillemot*

The Bishop (1) 44°47'N, 66°46'W 149 (40)Long Is. (2) 44°43'45N, 66°43'20W 21 118 13High Duck Is. (3) 44°42'50N, 66°43'40W 10 16 56 1Low Duck Is. (4) 44°42'30N, 66°42'40W 55 48 4Nantucket Is. (5) 44°41'50N, 66°43'20W 1 28 6Gull Islet (6) 44°41'15N, 66°43'25W 0 38 9Great Duck Is. (7) 44°41'15N, 66°41'40W 750 1,910 285 5 (1)Gull Rocks (11) 44°37'51N, 66°40'58W 15 3 38 16 (2)SW Gull Rock (12) 44°37'39N, 66°41'12W 1 6 0Pumpkin Islet (13) 44°37'09N, 66°43'46W 30 0 9North Green Is. (14) 44°37'10N, 66°45'40W 113 36 259 86 122 (24)South Green Is. (15) 44°36'40N, 66°45'10W nesting 137 144 34 60 (14)Wood Is. (16) 44°37'00N, 66°49'30W present 221 587 0 52 (13)Outer Wood Is. (17) 44°36'30N, 66°49'00W present 1,147 1,404 14 248 (57)Western Green Is. (18) 44°36'15N, 66°50'30W 125 192 4 101 (22)West Pumpkin Is. (19) 44°36'38N, 66°50'30W 14 16 0 20 (7)White Horse Islet (20) 44°36'10N, 66°48'25W 24 1 59 3 43 (8)Hay Is. (21) 44°35'40N, 66°45'50W nesting 230 573 18Sheep Is. (22) 44°35'00N, 66°46'10W 90 442 55 8 114 (30)Kent Is. (23) 44°35'00N, 66°45'20W nesting 480 5,926 23 256 (61)Southwest Head (24) 44°36'N, 66°54'W 152 (45)Yellow Ledge (25) 44°31'N, 66°51'W 281 424

Total 147 3,370 11,809 602 8 281 424 1,338

**Cheney Is. (9) 44°39'N, 66°43'W was not surveyed. No colonies were found on Ross Is. (8) 44°40'N, 66°44'W or White Head Is. (10) 44°38'N, 66°42'W.

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Green Is. and terns nested only on Sheep Is-land. Yellow Ledge is currently the only reg-ular breeding colony of Common Murres inthe Bay of Fundy or Gulf of Maine (J. Drury,pers. comm.), although a few breeding pairswere found on Machias Seal Is. in 2002 (A.Diamond, pers. comm.).

Seabird Trends

Of the nine species surveyed, increasesare apparent for four: Herring Gull, GreatBlack-backed Gull, Razorbill and CommonMurre. Common Eiders appear to be rela-tively stable, although they have decreased inrecent years. Insufficient data are availableto assess trends for other species. Details oftrends for each species are presented below.

Black Guillemot

—Little or no historicaldata exist to assess trends for many sitesaround Grand Manan. The earliest guillem-ot counts of the major colonies in 1936 (Pet-tingill 1939, in Drury 1973-74) estimatedGrand Manan numbers as near 700 pairs;similar to 2001 totals of 1,338 individuals. Atthe Bishop, the highest count was 171 indi-viduals in 1999 (Ronconi and St. Clair 2002),and 118 individuals in 2000 (R. A. Ronconi,unpubl.); consistent with 2001 counts (149individuals). Estimates from Kent Is. havefluctuated between 45 and 100 pairs from1936 to 1956, 30 pairs in the vicinity of KentIs., Sheep Is. and Hay Is. in 1973 (Winn 1950;Drury 1973-74), to the present count of 61courtship pairs in 2001. In 1973, Drury(1973-74) reported 15 pairs on Outer WoodIs., 100 pairs on North and South Green Is.,and 15 pairs on White Horse Islet. At South-west Head, counts increased slightly from129 in July 2000 (R. A. Ronconi, unpubl.) to152 at present. There seems to be little, ifany, indication of a trend in numbers.

Herring Gull and Great Black-backedGull

—Despite an overall increase of HerringGulls in New England from 1900 to 1972(Drury 1973-74), Cannell and Maddox (1983)and Hébert (1989) demonstrated a steady de-cline in Kent Is. gull numbers over the mid20th century. However, 2001 results show thehighest numbers of gulls breeding on Kent Is.since 1972 (Table 2). Moreover, total Herring

Gull numbers for the archipelago have nearlyreturned to 1962, 1965, and 1972 numbers of12,300, 11,900, and 14,100 pairs respectively(Drury 1973-74). A comparison with the mostrecent estimates in 1998 (Mawhinney

et al.

1999) illustrates a six-fold increase on Kent Is.as well as dramatic increases throughout thearchipelago (Table 3). By contrast, compari-sons with 2001 and 1979 surveys (Lock 1982)show similar Herring Gull numbers on severalislands, although Great Black-backed Gullnumbers have increased ten-fold (Table 3).Historically, Great Black-backed Gulls havenested in small numbers in the Grand MananArchipelago with four, 100, and 60 pairs in1930, 1965 and 1972, respectively (Drury1973-74). Since 1979, Great Black-backedGulls have expanded from five colonies to tenin 1998 and to 16 sites in 2001. However, thedramatic increase of both gull species since1998 seems an improbable change in justthree years. Possible reasons for these discrep-ancies are discussed later.

Common Eider

—Few records are avail-able for assessing eider trends around GrandManan. Estimates on Kent Is. from 1969 to2001 show a decreasing trend, however thisdifference could partially reflect the differ-ent survey techniques used in each study (Ta-ble 2). There was also a decrease in GrandManan eider numbers. Nest estimates andaerial surveys in 2001 showed a 16% and 13%decrease respectively since aerial estimates in1998, and a subsequent drop in 2002 (Table4). Mawhinney

et al.

(1999) suggested thatGrand Manan eider numbers remained rela-tively stable from 1987 to 1998, yet decreasesin 2001 and 2002 are suggestive of a decline.

Double-crested Cormorant

—Records ofcormorant numbers are available for thethree nesting sites in the archipelago (Table5), yet trends are uncertain. Cormorantnumbers on White Horse Islet appeared rel-atively stable from 1975 to 1992 with a meanof 61 pairs, until the numbers dropped by59% of this mean in 1995 and remainedclose to this level in 2001. The North GreenIs. colony has steadily increased since 1989and presently has the largest numbers of cor-morants. The High Duck Is. colony has fluc-tuated greatly between 1987 and 2001.

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Common Tern

—Although terns havenested on Sheep Is. irregularly over the pasttwo decades, estimates are only available forrecent years, and show a possible decline innumbers. In 1999 and 2000, 25 and 15 pairsrespectively were nesting at the colony (B.Dalzell, pers. comm.) while eight pairs werefound in 2001. Expansion of the colony inthe future may be limited by the distributionof nesting gulls on the island.

Razorbill and Common Murre

—Scatteredrecords of Razorbill and Common Murrenumbers illustrate an increase for both spe-

cies in recent years. Previous Razorbill sur-veys on Yellow Ledge estimated 25 pairs in1987 (CWS 2001), 50-60 pairs in 1998(J. Drury, pers. comm.) and 150 individualsin 1999 (Chapdelaine

et al.

2001). Therewere 50 pairs of Common Murre on YellowLedge in 1981 (Nettleship and Evans 1985),120 pairs in 1993 (Lock

et al.

1994) and 50-60pairs in 1998 (J. Drury, pers. comm.). Colonyestimates for Razorbills and CommonMurres in 2001 are the highest ever record-ed (281 and 424 individuals) at YellowLedge.

Table 2. Breeding pair estimates of the Herring Gull and Common Eider on Kent Island: past and present. Updatedfrom Cannell and Maddox (1983).

Date Location No. pairs Method Reference

Common Eider1873 Kent Is. in abundance not stated Herrick 1873*1923 Kent Is. nearly gone not stated Townsend 1923*1930 Kent/Hay/Sheep 30 not stated Huntington 1956*1932 Kent Is. 178 nest count Gross 1938*1937 Kent Is. >300 nest count Gross 1938*1944 Kent/Hay/Sheep 1000 direct count CWS 20011947 Kent Is. 1000 not stated Gross 1947*1948 Kent Is. 750 not stated Paynter 1951*1955 Kent Is. 1000-3000 not stated Huntington 1956*1960 Kent Is. **1226 not stated Lock 19821961 Kent Is. **1450 not stated Lock 19821969 Kent Is. 2000 direct count CWS 20011973 Kent Is. **1228 not stated Lock 19821979 Kent Is. 1200 direct count CWS 20011980 Kent Is. 893 quadrat Cannell and Maddox 19831987 Kent Is. 900 not stated BSS 19872001 Kent Is. 480 nest count/transects present study

Herring Gull1935-1937 Kent/Hay/Sheep 20,000 not stated Pettingill 1939*1940 Kent Is. 11,672 nest count Crystal 1941*1940 Kent Is. 16,000 nest count + estimates Crystal 1941*1945 Kent Is. 25,000 not stated Cameron 1945*1947 Kent Is. 30,000 not stated Gross 1947*1948 Kent Is. 20,500 count? Boyer 1948*1949 Kent Is. 12,000-15,000 not stated Paynter 1949*1949 Kent Is. 17,000 sample plots Pimlott 1952*1963 Kent Is. 10,000 air estimate Drury pers. comm.*1965 Kent Is. 7,500 air estimate Drury pers. comm.*1967 Kent Is. 8,347 nest count + estimates Ainley 1967*1972 Kent Is. 8,000 air estimate Drury pers. comm.*1973 Kent Is. 4,000 air estimate Drury pers. comm.*1979 Kent Is. 4,560 quadrat + estimates Lock 19821980 Kent Is. 4,979 quadrat Cannell and Maddox 19831984 Kent Is. 1,441 nest count Hébert 19891998 Kent Is. 940 aerial Mawhinney

et al.

19992001 Kent Is. 5,926 nest count/transects present study

*Taken from Cannell and Maddox (1983). **No. of females counted.

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Leach’s Storm-Petrel

—Though no dataare available to assess trends in petrel num-bers, some distributional changes are appar-ent. Between 1932 and 1946 petrels werenesting on Kent Is., Outer Wood Is., SouthGreen Is. (Gross 1947) and North Green Is.(CWS 2001). Previously, petrels nested onWhite Horse Islet (Townsend 1923 in Drury1973-74). In 1959, nests were found on manyof the southern islands and some of the north-ern islands (C. Huntington, pers. comm.). In2001, no nests were found on any of the

northern islands and nests were confirmed ononly three of the southern islands (Table 1).Petrel presence was documented on OuterWood Is. and Wood Is., but no nests werefound. These limited results suggest a possibledecrease in petrel distribution since 1959.

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ISCUSSION

Species composition and distribution

—Numbers presented above (Table 1) illus-trate Herring Gull and Common Eider as the

Table 3. Changes in gull numbers in the Grand Manan Archipelago from 1979, 1998 and 2001. Numbers in boldindicate estimates from ground counts.

Location

Herring Gull pairs Great Black-backed Gull pairs

1979* 1998† 2001‡ 1979* 1998† 2001‡

Long Is. 70 22

118

0 8

13

High Duck Is. 70 90

56

0 9

1

Low Duck Is. 85 44

48

0 0

4

Nantucket Is.

20

2

28 0

0

6

Gull Islet — 25

38

— 0

9

Great Duck Is. 120 525

1,910

0 25

285

Cheney Is. — 2 — — 0 —Gull Rocks

27

0

3 27

94

38

SW Gull Rock — 33

6

— 1

0

Pumpkin Islet

7

0

0 0

19

9

North Green Is. 93 101 259 9 23 86South Green Is. 128 55 144 13 12 34Wood Is. — 232 587 — 0 0Outer Wood Is. 1,600 448 1,404 2 0 14Western Green Is. 100 57 192 0 0 4West Pumpkin Is. 56 11 16 0 0 0White Horse Islet 31 39 59 0 0 3Sheep Is. 400 202 442 0 1 55Hay Is. 350 147 573 0 0 18Kent Is. 4,560 940 5,926 10 10 23

Total 7,717 2,975 11,809 61 202 602

*Combination of aerial visual estimates, ground counts, estimates from adult to nest ratios (Lock 1982), †aerialvisual estimates (Mawhinney et al. 1999), and ‡ground counts (present study).

Table 4. Common Eider estimates in the Grand Manan Archipelago, 1982-2002.

Date Breeding pairs Method Reference

1982 2,763 aerial, adult males Lock, 198612 May 1987 5,237 aerial, adult males Mawhinney et al., 199915 May 1990 3,060 aerial, adult males Mawhinney et al., 19994 May 1995 3,092 aerial, adult males Mawhinney et al., 199914 May 1996 4,069 aerial, adult males Mawhinney et al., 199919 May 1998 4,030 aerial, adult males Mawhinney et al., 19998 May 2001 3,516 aerial, adult males K. Connors, pers. comm.May/June 2001 3,370 nest count/transects present study12 May 2002 3,180 aerial, adult males K. Connors, pers. comm.

GRAND MANAN SEABIRD TRENDS 469

most abundant breeders in the archipelago,yet these results are somewhat misleading.More accurate assessment of compositionand distribution statistics would benefit frominclusion of storm-petrel data, however thesedata are only available for one island. An es-timated 25,400 pairs of petrels nested onKent Is. in 2000/01 (L. Minich, pers. comm.),suggesting that petrels are the most abun-dant breeding seabird in the archipelago.Furthermore, species composition would al-so change with the inclusion of Machias SealIs. by increasing Common Tern and Razor-bill numbers and including two additionalspecies (Arctic Tern [Sterna paradisaea] andAtlantic Puffin [Fratercula arctica]).

Fluctuations in Grand Manan numbers—Although trends could not be assessed forguillemots, terns, or storm-petrels, trendswere apparent for gulls, murres and razor-bills. Trends are less clear for eiders and cor-morants.

The relatively stable and recent decreas-ing trend for eiders is illustrated by compari-sons between aerial and ground surveys.Although aerial surveys have traditionallybeen used to estimate Grand Manan eidernumbers (Mawhinney et al. 1999), ground es-timates in 2001 showed a close correlationwith aerial estimates (Table 4). The ratio of

adult males to nests was 1.04:1, which was sim-ilar to ratios of 0.98:1 and 1.21:1 in Labrador(Lock 1986). Ground estimates from 2001should be comparable with historical aerialdata, suggesting that a genuine decline maybe occurring. In 1998, Mawhinney et al.(1999) reported low eider duckling survivalrates for Grand Manan, and warned that per-sistently low rates could eventually lead to de-clines. These low survival rates in combinationwith increasing gull numbers (known preda-tors of eider eggs and young) may explain ei-der decreases in 2001 and 2002. Yetelsewhere, long-term studies of eiders haveshown frequent years of poor breeding andconsiderable year-to-year variation in nesting(Coulson 1984), thus some natural variationin numbers should be expected.

Table 5 illustrates varying trends of thethree cormorant colonies showing increases(North Green Is.), decreases (White Horse Is-let) and fluctuations (High Duck Is.). Theseobservations may be due to adult breedersmoving among these colonies, rather thanchanges in overall Grand Manan numbers.Over time, cormorants may have emigratedfrom White Horse Islet and High Duck Is. toNorth Green Is., however lack of concurrentcolony records on all three islands make thisdifficult to confirm. Cormorants are sensitive

Table 5. Double-crested Cormorant numbers at three colonies in the Grand Manan Archipelago.

Date No. pairs Method Reference

White Horse Islet1975 66 direct count CWS 20011987 69 not stated Lock et al. 19941991 46 direct count CWS 20011992 62 direct count CWS 20011995 25 direct count CWS 20012001 24 direct count present study

North Green Is.1989 18 direct count N. Stobie, field notes1995 25 direct count CWS 2001ca. 1999 50-60 not stated B. Dalzell, pers. comm.2001 113 aerial photo present study

High Duck Is.1987 38 not stated Lock et al. 19941989 16 direct count B. Cronk & D. Green, field notes2000 40 direct count B. Dalzell, pers. comm.2001 10 direct count present study2002 0 personal observations

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to human disturbance, which can lead to per-manent displacement of colonies (Cairns etal. 1998). High Duck Is. receives a high de-gree of human disturbance, which may ex-plain the recent decline in nesting activity,and site abandonment in 2002.

Discrepancies in gull numbers—Consid-erable changes in gull numbers have beenobserved in the Grand Manan region overthe past two decades (Tables 2 and 3) and2001 estimates show improbable changes onsome islands (e.g., Great Duck) in just threeyears. While some moving among islands mayaccount for increases and decreases on spe-cific islands, this cannot account for the over-all increase in total numbers. Inaccuratecalculations of Great Black-backed Gull num-bers from species ratios may partially explaindiscrepancies for this species, but this errorwas reduced by omitting “loafing” or “periph-ery” birds from species ratio counts and by us-ing ground count data of confirmed GreatBlack-backed Gull nests. Furthermore, theproportion of Great Black-backed Gull toHerring Gull remained similar between 1998(0.068:1) and 2001 (0.051:1), suggesting thatboth species have increased at comparablerates in recent years. For both gull species,possible explanations for discrepancies be-tween 1998 and 2001 numbers are differenc-es in survey methodology, and the effects ofsurvey timing.

Ideally, assessment of trends should bebased on surveys by identical methodology;however, such data are not available forGrand Manan. Ground techniques were em-ployed in 2001, whereas the 1998 surveysused aerial estimates (Mawhinney et al.1999) and a mix of aerial estimates, directcounts, quadrat sampling, and flush countswere used in 1979 (Lock 1982). Direct nestcounts generally yield most accurate results;Hutchinson (1979a, b) found lower errorrates with direct nest counts (±5%) than withaerial photos (±33%) or aerial visual esti-mates (±140%). Aerial surveys are less accu-rate because nests can be difficult todistinguish from natural features or may beconcealed by vegetation, and nests unattend-ed by adults are often omitted (Hutchinson1979b; Nisbet 2001). Nisbet (2001) cautions

the use of aerial surveys for monitoringtrends, and stresses the importance of stan-dardized protocols and calibration with si-multaneous ground counts. Moreover, Lock(1982) suggests that aerial estimates calibrat-ed by ground counts may adequately esti-mate regional numbers, but are not reliablefor single islands. Although Mawhinney et al.(1999) corrected their aerial estimates withground count data, their ground counts in-cluded only two colonies (Table 3) in GrandManan, since much of their ground calibra-tion data came from elsewhere in New Brun-swick. Mawhinney et al. (1999) surveys wouldhave benefited from further calibration ofground counts from within the archipelago.

Survey timing is an important factor inestimating and monitoring gull numbers.Johnston and Krohn (2001) propose a mon-itoring scheme for gulls in the Gulf of Mainewith annual aerial surveys between 23 Mayand 23 June, or biennial surveys from 2-17June. In 1979 surveys were conducted from8-14 June, while 1998 surveys were done on25/26 May. With the exception of two GreatBlack-backed Gull colonies surveyed 9 May,surveys in 2001 extended from 24 May to 26June, with the largest colonies surveyed be-tween 7-22 June. Surveys in 1998 fall outsideof the proposed biennial monitoring time-line (Johnson and Krohn 2001), potentiallymissing peak nesting dates. For example, on26/27 May 2001, 82% and 79% of the gullnests on High Duck Is. and Low Duck Is. re-spectively were empty. Regular annual or bi-ennial surveys under the proposed timelineby Johnson and Krohn (2001) could elimi-nate timing issues in future monitoring. Fortrend assessment, 1998 results should be in-terpreted with caution, since surveys wereconducted relatively early and few groundcounts were used to calibrate for the GrandManan region.

Some plausible biological factors may alsoexplain some of the observed differences ingull numbers. Immigration may have oc-curred from other regions, however this can-not be assessed from the present study.Intermittent breeding is known to occur inHerring Gulls (Drost et al. 1961; Pons andMigot 1995) with 15-30% non-breeding adults

GRAND MANAN SEABIRD TRENDS 471

at colonies in New England (Kadlec andDrury 1968). With Herring and Lesser Black-backed Gulls (Larus fuscus), Calladine andHarris (1997) report that 33-40% of previous-ly breeding adults failed to breed in two years.For Lesser Black-backed Gulls, O’Connell et al.(1997) found multiple nest building and 27%non-breeding of gulls that established territo-ries. Coulson et al. (1982) suggests intermit-tent breeding as a likely cause for suddendeclines in Herring Gull breeding numberson the Isle of May, Scotland. Nevertheless, nodata are available to assess intermittent breed-ing in 1998 or 2001. Alternatively increases ingull numbers may reflect long-term changesin food availability from human sources. Withthe decrease in groundfish fisheries, gullshave had less offal to feed on in recent years,but increases in lobster fisheries may compen-sate for this with bait discards (L. Murison,pers. comm.). Moreover, the local salmonaquaculture industry has grown considerablyover the past decade (pers. obs.), potentiallyattracting gulls to spilled fish feed (Pillay1992; Furness 1996).

Development may also have negative ef-fects on certain seabirds. Furness (1996) sug-gests that salmon farms may alter thedistribution of natural seabird prey. Distur-bance from mechanical equipment maycause displacement of seabirds, and mortali-ty may increase from deliberate killing or en-trapment in anti-predator nets (Pillay 1996).Furthermore, eiders are sensitive to distur-bance and studies have shown increased pre-dation rates by gulls on eider ducklings dueto boat disturbance (Ahlund and Gotmark1989; Keller 1991). The effects of aquacul-ture and other development on the seabirdsof Grand Manan are currently unknown.Monitoring of seabird numbers using datafrom this study and previous surveys, willhelp assess changes in the face of develop-ment and help plan for future developmentthat ensures the long-term sustainability ofboth industry and seabirds in the region.

ACKNOWLEDGMENTS

Financial support was provided by New BrunswickWildlife Trust Fund, New Brunswick EnvironmentalTrust Fund, Canadian Council for Human Resources in

the Environment Industry, and TD Friends of the Envi-ronment Foundation. Logistical and technical supportwas provided by the Grand Manan Whale and SeabirdResearch Station. A special thanks to our research assis-tant, L. Thorne. A. W. Diamond provided invaluableguidance and advice throughout the project and manu-script reviews. Equipment donations were provided byIrving Oil’s Atlantic Emergency Response Team, Inter-national Marine Mammal Association, C. St. Clair (Uni-versity of Alberta), Haltech Environmental andMountain Equipment Co-op. Eastern Charlotte Water-ways provided support for GIS work and base maps. Wethank the following people for substantial support inthe field and/or reviewing proposals and drafts of relat-ed documents: T. Cox, K. Ingersoll, D. Johnston, H.Koopman, L. Murison, and A. Westgate. The followingpeople provided much appreciated guidance, support,and/or help in the field: A. Boyne, J. Chardine, K. Con-nor, B. Dalzell, J. Drury, S. Farquharson, B. Guptil, C.Huntington, M. Janowicz, S. Jarvis, B. Mauck, S. Mc-Donald, L. Minich, K. O’Reilly, N. Wheelwright, and stu-dents from the Bowdoin Scientific Station. Thank youto the residents and landowners of Grand Manan.Thanks to a referee and J. Coulson for comments thatimproved this paper.

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