POPULATION STUDY HEBREEDING RANGE OF LEACH'S Storm-Petrel( Oceanodroma leu- The world'slargest known corhoa) in the Northwest Atlan- tic is centered in eastern Newfound- land. Owing to the species'nocturnal nestingcolony of and burrow-nestinghabits, the breed- ing populations and distributions of storm-petrels, first documented in Leach's Storm-Petrels Newfoundlandby Petersand Burleigh (1951), have historicallybeen grossly underestimatedand are not presently on BaccalieuIsland,, well known. Storm-petrel coloniesin Newfoundland are vastly larger than all others in the Northwest Atlantic Newfoundland (Huntington 1963; Montevecchi, in prep.), but few quantitative data on colony populationshave been gener- ated. The colony of Leach's Storm-Pe- trels on Baccalieu Island, Newfound- B. O. Sklepkovych and IV. A. Montevecchi land, hasgone largely unrecognized as an important nestingsite for the spe- des (Brown et al. 1975; Nettleship 1980). In the present paper, we esti- mate the breeding population of storm-petrelsbased on measurements of burrow densitiesin all nestinghab- itats and on quantitative estimatesof the island'ssurface areas. Our popu- lation figure, which is compared with thoseof other coloniesthroughout the world, yields new perspectivesof the energy consumption of these birds in the marine ecosystemof the North- west Atlantic. Study Area BaccalieuIsland (-6.3 x I kilome- ter, 48'07'N, 54ø12'W) is situatedoff the northeastern tip of Newfound- land's Avalon Peninsula (Figure 1). Leach'sStorm-Petrels nest in a variety of habitats over the entire island, but principallyoccupy grassy-hummock- ed slopes,boreal forest and Empetrum heaths. Smaller, less defined habitats suchas herb (grassyareas with more than 60 percent fern or tall herb cover), grassy meadow, and heath Leach• Storm-Petrel (Oceanodromaleucorhda). Photograph/GeoffLeBaron/VIREO mixed with herb or grassesare also 105/7/009. utilized by nesting storm-petrels (Sklepkovych1986). The densestcon- centrations of storm-petrelsoccur in the grassy-hummockedareas along 38 AmericanBirds, Spring 1989 TABLE 1. Percentage(number) of burrowoccupancy in grassy-hummocked,forest, and heath habitat plots. Habitat Contour % Occupied% BurrowsWith Burrows Eggs/Chick Grass Level 74.6 (47/63) 69.8 (44/63) Slope 67.5 (56/83) 60.2 (50/83) Total 70.5 (103/146) 64.4 (94/146) Forest Level 71.4 (10/14) 50.0 (7/14) Slope 72.4 (21/29) 72.4 (21/29) Total 72.1 (31/43) 65.1 (28/43) Heath Plots 55.6 (10/18) 50.0 (9/18) Transects 52.4 (11/21) 52.4 (11/21) Total 53.8 (21/39) 51.3 (20/39) Grand Mean (Sum) 68.0 (155/228) 62.3 (142/228) the eastern side of the island (Figure longesttransverse axis of the islandat 2), and medium and low densitycon- approximatelyequal intervals.Cross- centrations are found in forest and sectionsof the island were plotted and heath regions,respectively. Northern scaledrelative to the planar distance Fulmars (Fulrnarusglacialis), North- (X), and then measuredusing the Nu- ern Gannets (Sula bassanus),Black- monics digitizer (Y). The ratio of Figure 1. Aerial photographof Baccalieu leggedKitfiwakes (Rissa tridactyla), thesevalues (Y/X) providedan esti- Island, located off the northeasterntip of NewfoundlandgAvalon Peninsula between Common and Thick-billed tourres mate of the proportional increasein Conceptionand Trinity bays.Photograph/ ( Uria aalgeand U. lornvia),Razorbills surfacearea of each segmentresulting Canadian Dept. of Mines, Energy, Re- (Alca torda),Atlantic Puffins(Frater- from the island's contour, and the sources. cula arcfica),Black Guillemots( Cep- mean of these ratios was used as an phusgrylle), Herring Gulls (Larus ar- estimate of overall increase in surface tained from data collected in associa- gentatus), and Great Black-backed area due to slope.This procedureas- tion with studiesof red fox hunting Gulls (L. marinus) also nest on Bac- sumed that habitats are equally dis- strategy(Sklepkovych 1986). Both the calieu Island, making up the greatest tributed over different contours and number of burrowsat fox diggingsites diversityof breedingseabirds at any as a consequencetends to underesti- and at random adjacent control sites site in Newfoundland and Labrador mate grassy-hummockedand forest were included in the estimates of hole (Montevecchiand Tuck 1987). Red habitats, because much of the rela- density, which were similar to those foxes(Vulpes vulpes)are the island's tivelylevel terrain of the islandis com- at randomly selected sites (Sklepko- only resident mammals (Maccarone posedof heath. Owing to differences vych 1986). and Montevecchi 1981; Sklepkovych in the nestingdensities associated with During May and June 1984, five- 1986). these habitats (see Results), the pro- by-ten meter occupancy plots were cedure will also tend to underestimate established in typical grassy-hum- the breedingpopulation. Two-dimen- mocked (n = 4) and forest (n = 4) Methods sional considerations of habitat areas habitats. Each plot was subdivided would generateerrors that greatly ex- into four equal quadrats(12.5m 2) of A Numonics digitizer(Model 1210) ceed those produced by our contour which one was chosen at random. was used to estimate the two-dimen- estimation procedure. Two plots in each habitat were in sionalsurface area and major habitats A census of holes was made in each areas with _>30* slope and two were of Baccalieu Island from a 1:8650 habitat to estimatethe densityof po- on areaswith little or no slope(0-5ø). scaledmap. For comparisonand ad- tential storm-petrel burrows. To cover In heath habitat, the density of holes ditional information, three 1:25,000 most areas of the island, 57 relatively (potentialburrows) was very low and aerial photographs(Canadian Dept. equidistantsites were preselectedon a variable, and therefore, two 100-me- Energy, Mines, Resources)compris- map. The samplingscheme was strat- ter-squareplots were set up in heath ing the southern,central, and north- ified according to slope and geo- areaswith high burrow densities(i.e. ern portions of the island were also graphiclocation. Randomness of the on the island'speriphery), and two 4 used.A grid of 150-1 cm2 quadrats exact location of studyplots and tran- x 100-meter transects were set out was laid over the aerial photographs, sects at these sites was attained by randomly in typical heath habitat. and each quadrat was examined with travelling a random compassdirec- Holes were examined betweenJuly 12 a magnifying lamp or stereoscopic tion (N, E, S, W) for a distancedeter- and 22, early in the chick period ap- glasses,and the percentageof each mined from a random numbers table proximatelyfive to sixweeks after egg- habitat type was visuallyestimated. between 100 and 500 meters in heath, laying (Monteve•chiunpubl. data; To estimate the island's surface area 50 and 100 meters in forest and 25 Morse and Buchheister 1979). Dia- contour analysiswas performedon a and 50 meters both in mixed heath gramsof plots were drawn indicating 1:50,000 map (Canadian Dept. En- and in grassymeadow habitats.Esti- hole position, distance to adjacent ergy,Mifies, Resources). Fifteen tran- matesof hole densitiesin grassy-hum- holes, connections between burrows, sectionswere drawn perpendicularto mocked and herb habitats were oh- burrow lengths,general vegetational Volume43, Number 1 39 features and the presenceof nest ma- terial, adults, chicks, and eggs.Holes <20 centimeters long were typically inactive and were not considered to be burrows. Nest chambers with mul- tiple entrances were counted once. The number of confirmed nests ex- cluded burrows that were probable nests but in which nest chambers were inaccessible. For heath, forest, and grasshabitats exact occupancyvalues were used in calculating density; for all other sub-habitat estimates, the .$ overall proportion of nestingburrows and grand mean of the occupancy rates of the three sampled habitats were used. Results PhysicalHabitat Characteristics-- The total two-dimensional surface area of Baccalieu Island was calcu- lated to be 523 hectares, approxi- mately 60 percent of which is _•75 meters above sea level. Analysis of Figure 2. The grassy-hummockedslopes on the easternside of BaccalieuIsland, where island contour revealed 31-100 per- Leach• Storm-Petrelsnest in abundance.PhotographlB. O. Sklepkovych. centincreases in area(2 __SD = 59 _+19%) due to topography,suggesting that the surface area of Baccalieu Is- and occupancy by known nestersat occupancyx % breeding)of all acces- land is actually 832 hectares,with 688 plots and transects in grassy-hum- sibleburrows (N = 228) wasestimated hectares providing potential nesting mocked, forest, and heath habitats. to be occupied by breeding pairs. habitat for Leach's Storm-Petrels. Sixty-eight percent (155/228) of all Nearly identicaloccupancy rates have Analysis of aerial photographsand accessible nests were occupied by been found on Great Island, Witless maps yielded estimates of 315 hec- either an adult, chick and/or egg.Per- Bay, Newfoundland and on Little tares (38%) of heath, 286 hectares centageof occupancywas similar be- Duck Island, Maine. (R. Butler [De- (34%) of forest,and 46 hectares(6%) tween grassy-hummocked(71%) and partment of Science & Math, Univ. of grassy-hummocked habitats. forest (72%) habitats and lower, Maine, Farmington]pers. comm.). Smaller, lessdefined habitatsmade up though not significantly,within the Study plots varied in size and num- 41 hectares(5%), and non-nestingre- heath habitat (54%). The presenceof ber within and between habitats and gions(including exclusive puffin nest- an