SIGNIFICANCE OF THE PATTERN OF NEST DISTRIBUTION IN THE PIGEON ( COL UMBA )

S. K. EMMS • AND N. A.M. VERBEEK Departmentof BiologicalSciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6,Canada

ABSTRACT.--Westudied nest distribution in the (Cepphuscolumba), and three possibleselective factors that influence it: nest predation, the use of nest groupsas information centers for food finding, and spatial variation in nest-hole quality. Quadrat surveysin 2 yr suggestedthat nestswere dispersedrandomly with respectto the distribution of suitableholes, but on a larger spatialscale, holes on the south shorewere preferred to those on the north. Predation intensity was largely independent of nest density. This was true whether egg predationalone, chickpredation alone, or total predationwas considered. Nestingclose to other , however,had a slight advantageon a very local scale.Reduced predationon south-shorenests seemed to be due to their beingbetter protected from predators ratherthan to specificadvantages to aggregation.Neither chickfeeding rates nor chickgrowth ratesincreased with nestgroup size, indicatingthat nestgroups did not serveas information centersfor food finding. We concludethat the observedclumping of nestswas due to a clumpeddistribution of suitablesites and spatialvariation in nest-holequality. Received18 February1988, accepted3 October1988.

COLONI^Lnesting is a highly conspicuous knowledgeableroost mates, and recentlyBrown pattern of breedingin birds, and many studies (1986), Greene (1987), and Waltz (1987) provid- havebeen devotedto understandingits adap- ed strong evidence that Cliff Swallow (Hirundo tive significance(see Wittenberger and Hunt pyrrhonota), Osprey (Pandion haliaetus), and [1985]for review). Lack(1968), in a majorcom- Common Tern (Sternahirundo) colonies serve as parative study, found that coloniality was dis- information centers. proportionately common in speciesthat eat A secondmajor explanation for the evolution aquaticprey, seeds,or fruit, and that it is often of coloniality is that predation pressure de- associatedwith flockfeeding. He suggestedthat creasesas nest density increases(Patterson 1965, the main advantageof coloniality, at least for Robertson 1973, Parsons 1976, Hoogland and species that feed in flocks, was an increased Sherman 1976, Birkhead 1977, G0tmark and An- efficiencyof food exploitation. Ward and Za- dersson 1984, Robinson 1985). Several mecha- havi (1973) argued explicitly that many nisms have been invoked to explain this fact, speciesused breeding colonies as "information although alternativesare often difficult to sep- centres"for food finding. Severalstudies have arate in the field. First, predator mobbing often supportedWard and Zahavi'shypothesis (Horn increaseswith colony size or nesting density 1968, Siegfried 1971, Krebs 1974, Emlen and (Patterson 1965, Tenaza 1971, Balda and Bate- Demong 1975, Erwin 1978, Loman and Tamm man 1972, Hoogland and Sherman 1976, GOt- 1980, Ydenberg et al. 1983), but Bayer (1982) mark and Andersson 1984, Robinson 1985). In- arguedthat other hypotheses explained equally creasedmobbing may drive off predators or well much of the data, and Andersson et al. lower their hunting success.Second, the per in- (1981) found that Black-headedGull (Larusri- dividualprobability of being caught decreases dibundus)colonies did not appear to serve as ascolony size increasesthrough a prey dilution information centers.However, De Groot (1980) effect (Bertram 1978). Third, a "selfish herd" showedunequivocally that captiveQuelea que- effect may operate (Hamilton 1971). Fourth, lea learned the location of food or water from predator satiation may occur if prey are suffi- ciently abundant. Support for this last mecha- nism comes from studies that show that the • Present address:Department of Biology, Prince- proportion of individuals killed decreasesdur- ton University, Princeton, New Jersey 08544-1003 ing the peak of the breeding season(Patterson USA. 1965, Nisbet 1975, Daan and Tinbergen 1979,

193 The 106:193-202. April 1989 194 EMMS^•qD VERSEEK [Auk,Vol. 106

Findlay and Cooke 1982, Nisbet and Welton The north and east shores are much lower and more 1984). gently sloping. Exfoliation of the rocks is most ex- A third possibleinfluence on the pattern of tensive on the east and south shores, where there are a number of boulder tumbles below the cliffs, and nestdistribution is spatialvariation in siteavail- here the majorityof the guillemotsbreed. Details about ability. Birds may nest in clumpsbecause suit- the islandare given in Butler (1974, 1980)and Brooke able sitesare limited and patchily distributed et al. (1983). Thomson (1981) provides detailed in- (Lack 1968, Snapp 1976), or becausehigh-qual- formation about the oceanographyof the Strait of ity sitesare clumpedeven though suitablesites Georgia. are not limited. Approximately200 pairsof PigeonGuillemots nest Most studiesof colonialityhave concentrated on Mitlenatch. We determined their breeding distri- on highly colonialspecies (e.g. Patterson1965, bution by systematicallysearching the island shore Tenaza 1971, Parsons1976, Hoogland and Sher- for nestsand mapping their locationsonto a 50 m2 man 1976, Birkhead 1977). Loosely colonial quadrat system.(We did not searchthe cliffs on the species(those that nest in smaller, less dense south shore to avoid disturbing breeding cormo- rants.)We found nestsduring the egg-layingperiod, aggregations)have received lessattention, but or occasionallylater in the season,by observingfood the factors that influence their nest-distribution deliveries to chicks.We searchedall quadratsfor un- patternsmay be very different.Studies of loose- occupiedbut apparentlysuitable nest holes. We mea- ly colonial speciesmay also help to illuminate suredthe height and width of the entrancehole, the the problem of the evolutionof coloniality,be- distancefrom the hole entranceto the nest,the height causetheir nest-distribution patterns are prob- and width of the nest chamber, and the distance from ably similar to those on which selectiononce the hole entrance to the rear of the chamber. We acted in speciesthat are now highly colonial. scored concealment of the hole entrance on a scale Thusone canpartially avoid the argumentthat of one to five, and complexity of the nest chamber on a scale of one to six. Holes were considered suitable by studying highly colonial speciesone can if the measurementsfell within the range of the same demonstrateonly the selectivefactors that main- measurementsof occupiedholes. Birds might have tain coloniality. We studied the Pigeon Guille- used other criteria to decide whether a hole was suit- mot (Cepphuscolumba) because it nestsat a wide able or not. Although our criteria might have led to range of densities, from solitary individuals to an under- or overestimation of the total number of loose aggregationsand even dense colonies in suitableholes, it is unlikely that we createda system- somehigh arcticregions (Nettleship and Evans atic bias in the data. 1985).Our purposewas to determinethe factors To look for nonrandompatterns of nest distribu- responsiblefor the nest-distributionpattern of tion, we plotted nestdensity per quadratagainst suit- this species. able-holedensity and comparedthe fitted slopeto the slopeexpected if nestswere distributedrandomly. A random distribution of nestsimplies that the same STUDY AREA AND METHODS proportion of suitableholes is occupiedat all hole densities,so the expectedslope has a value equal to The Pigeon Guillemot is a medium-sized(400 g) the overallproportion of occupiedholes. If birdschose alcid that breeds on the coasts of the northern Pacific. to nest colonially, proportionatelymore holes than It nests in holes under boulders and rock tumbles on expectedwould be occupiedat high hole densities the lower shore, in crevices in rock faces, in driftwood and proportionatelyfewer holesthan expectedwould piles, and in a variety of similar locations(Drent et be occupiedat low hole densities.The fitted slope al. 1964),including man-madesites (Campbell 1977). would then be steeperthan expected.Conversely, if It is unusualamong the alcidsbecause it nestslargely birdswere territorial, the fitted slopewould be shal- in small,loose aggregations, feeds mainly on inshore, lower than expected.The data were transformedbe- benthic organisms,and lays normally two eggsin- fore analysisbecause nest density per quadratis bi- stead of one (Storer 1952, Thoreson and Booth 1958, nomially distributed (a hole is either occupiedor Drent 1965, Koelink 1972, Kuletz 1983). unoccupied).Its variance therefore increasesas suit- The study was done on Mitlenatch Island, British able hole density increases.Homogeneity of vari- Columbia(49ø57'N, 125ø00'W), a small(35.5 ha) rocky ances was obtained by dividing both sides of the islandat the northernend of the Strait of Georgia,in regressionequation by the squareroot of the inde- 1984(between 20 April and 24 August) and 1985(be- pendentvariable: the regression was transformed from tween 20 April and 18 August). The island is com- N = bSto N/V'• = bV'•, whereN = nestsper quadrat posedlargely of basalticrock that formstwo low hills and S = suitable holes per quadrat. Becauseb is the (<60 m) and a seriesof rocky bluffs. The south shore samein both cases,the slopefitted to the transformed consistsof low, vertical cliffs up to 20 m in height. data can still be compareddirectly to the expected April1989] PigeonGuillemot Nesting Dispersion 195 slope.One potential problem with this analysisis the N appropriatechoice of quadratsize. We arbitrarilychose 50 m2, and although a random distribution cannotbe made to appearclumped by adjustingquadrat size, a clumped distribution could appear random if the quadrat size chosen were too small. We therefore pooleddata from adjacentquadrats and repeatedthe analysison the new quadrat size of 50 x 100 m. We used the same method to look for variation in pre- dation intensity with nest density. We visited all occupied nests every 1-5 days throughoutthe breeding seasonand recordedtheir fates.The majorcauses of nestfailure were predation, 0 200 400 desertion, and failure of the eggs to hatch. A small number of chicksdied in the nest each year. m We useddiscriminant function analysis t_o separate nestsinto depredatedand nondepredatedgroups on the basis of the hole measurementslisted above plus three extravariables: size of the entrancehole (height x width), the ratio of entrance width to nest depth B (i.e. depth to the nest),and the ratio of entrance-hole size to nest depth. We used the last two variables to allow for the conflicting effectsof nest depth and entrance-holesize on predation risk: i.e. a nest with a large entrancehole might be well-protectedif the nest were deep inside, while a shallow nest might be protected if it had a narrow entrance hole. All vari- ableswere logetransformed before analysis;but even after transformation, nest-chamberheight showed a strongpositive skew and wastherefore dropped from the analysis. We weighed chickswith 0-100 g and 0-500 g Pesola scalesuntil they fledged,died, or were eaten. Weigh- ings were done in the afternoon, every 3-5 days in 1984 and every day in 1985. We observedfeeding ratesto nestsfrom a blind, or from locations distant or concealed from the nest group. Observationperiods lasted 2 h. We recorded the number of pairsin the nest group that fed chicks, Fig. 1. Mitlenatch Island, showing the distribu- the number of food deliveries to each nest, the time tion of (A) guillemotnests in 1984;(B) guillemot nests at the start of each observationperiod, and the state in 1985; (C) total number of suitable holes. Quadrat of the during the period. We chosestarting times size is 50 m2. The size of the dot in each quadrat so that the tidal height halfway through the period indicates the number of nests or suitable holes. In (A), was either 0.75 m, 2.30 m, or 3.85 m. These were the dotted line shows north-south division of the classifiedas low-, mid-, and high-tide periods, re- island, 1 = Camp Bay;2 = F-Island; 3 = West Hill. spectively. We also kept records of the number of chicksin each nest in the nest group. We used the BMDP statisticalsoftware programs fitting linear regressionsto the linear phaseof growth, (Dixon 1985) for all analyses.We calculatedfledging when chicks were 5-20 days old. This is preferable massesof chicksby fitting logisticequations to the to using the growth curve constantK of the logistic growth curves of individual chicks and using the equation,as recommended by Ricklefs(1967), because asymptoticmass parameter A asa measureof fledging K is a measureof the speedwith which the asymptote mass(Ricklefs 1967). Because nestling reach is reachedand can therefore give misleading results their maximum massat fledging, A is a reasonable if comparisonsare madebetween chicksreaching dif- measureof fledging mass.The masson the last day ferent asymptotes(Hussel 1972, Gaston1985). in the nest was not a good measurebecause masses We mostly usedparametric statistics, transforming varied considerably(up to 30-40 g) from day to day the datato obtainnormal distributionswhere appro- during the final part of the fledging period. We cal- priate.We usednonparametric statistics if normal dis- culated growth rates as massincreases in g/day by tributions could not be obtained by transformation. 196 EMMSAND VERBEEK [Auk,Vol. 106

TABLE1. Fitted and expectedslopes for regressions TABLE 3. Predation on individual nests in 1984 and of nests per quadrat vs. total number of suitable 1985.Only nestswhose complete fates were known holes per quadrat. Analyses were done on trans- in both years were used. Nests that were depre- formed data; t-tests are two-tailed. dated in 1984were likely to be depredatedin 1985. x 2 = 9.12, P < 0.01. Slope Not Ex- Depredated depredated Year Fitted _+SE pected t n P (1985) (1985) 50 m 2 Quadrat Depredated(1984) 18 2 1984 0.547 _+ 0.113 0.464 0.735 37 >0.40 Not depredated (1984) 8 10 1985 0.700 _+ 0.110 0.495 1.860 37 >0.05 50 x 100 m Quadrat 1984 0.638 _+ 0.112 0.468 1.520 22 >0.10 1985 0.750 + 0.091 0.503 2.710 22 <0.05 deep and complex nest chambers were some- what better protected from predators (Table 2), but for none of the measuresof predation did RESULTS the same characteristicsenter the analysis in both years. In addition, the percentageof cor- Thedistribution pattern of nests.--Inboth 1984 rect classificationsof nestsinto depredatedand and 1985, nestswere clumped into three main nondepredated groups was never very high areas:Camp Bay, F-Island, and West Hill (Fig. (64.9-73.2%).Thus, the probability of a nest 1A, B), but suitable holes were also clumped being depredated cannot be predicted with (Fig. 1C), and in neither year was the distri- much confidence from measurements of the nest bution of nests within 50 m 2 quadrats signifi- hole. However, neststhat were depredated in cantlydifferent from that expectedif birdschose 1984 were also highly likely to be depredated nest holesrandomly (seeStudy Area and Meth- in 1985 (Table 3). ods).The sameresult held for 50 x 100 m quad- The differences in nest-hole vulnerability rats in 1984, but in 1985 the fitted slope was shown by the discriminant function analyses significantly steeperthan expected,indicating were partially reflected in the choice of nest that occupiedholes were more clumped than holes by breeding pairs. Over the island as a suitableholes (Table 1). On a larger scale,birds whole, used holes were more complex (•: 3.09 preferred holes on the southside of the island vs. 2.45; n = 218, t = 3.55, P = 0.0005) and had in both years (1984: 74/136 vs. 16/58; x2 = 11.75, smaller ratios of entrance size to depth to nest P < 0.001; 1985:81/138 vs. 15/58; x 2 = 17.60, P cup (11.55 vs. 13.98; n = 216, t = -2.35, P = < 0.001). 0.02) than unused holes. Suitable holes on the Theeffect of spatialvariation in nest-holequali- south shore were better concealed (2.25 vs. 1.75; ty.--Several nest-holecharacteristics affected the n = 223, t = 3.79, P = 0.0002) and had more risk of predation.Deeper nests,nests with small complex chambers (3.05 vs. 2.28; n = 217, t = and concealed nest entrances, and those with 4.18, P < 0.0001) than those on the north shore.

TABLE2. Discriminantfunction analyses separating nests into depredatedand nondepredatedgroups on the basisof nest-holecharacteristics. Means and standarderrors are for raw data,but data were 1ogetransformed beforeanalysis. See text for the completeset of predictorvariables.

Cor- Significant rect predictor Depredated Nondepredated cases Groupingvariable variables nests(œ + SE) nests(œ + SE) n F-value df (%) Total predation1984 ------71 ------Totalpredation 1985 Depthto nest 43.34+ 24.83 48.44_+ 20.46 77 5.86 1, 75 64.9 Eggpredation 1984 Concealment 1.67_+ 1.05 2.26 + 0.69 70 4.95 1, 68 68.6 Eggpredation 1985 Complexity 2.06 + 1.08 2.92 + 1.00 82 10.30 2, 79 73.2 Depth to nest 34.00 + 14.53 44.91 + 18.90 Chickpredation 1984 Entrancesize/ 9.71 + 4.56 14.96+ 8.44 48 7.53 1, 46 62.5 depth to nest Chick predation1985 ------33 ------April 1989] PigeonGuillemot Nesting Dispersion 197

T^BLE4. Fitted and expectedslopes for regressionsof depredatednests per quadratvs. numberof nestsper quadrat.Analyses were done on transformeddata; t-testsare two-tailed.

Predation Slope intensity estimate Fitted _+SE Expected t n P 1984 Upper 0.686 + 0.123 0.614 0.585 27 >0.50 Lower 0.584 + 0.121 0.545 0.322 27 >0.70

1985 Upper 0.424 + 0.164 0.479 -0.335 25 >0.70 Lower 0.275 + 0.142 0.396 -0.852 25 >0.40

Occupied holes were also better concealed on unableto pair quadratsand repeatthe analyses the south shore (Mann-Whitney U-test, P = on the large quadrat size of 50 x 100 m. 0.014),but there were no significantdifferences In neither 1984 nor 1985 were the fitted and between unoccupiedholes in the south and oc- expectedslopes significantly different (Table 4), cupied holes in the north. None of the other suggestingthat predation pressurewas inde- nest-hole characteristics differed between pendent of nest density.We alsoanalyzed egg groups in any of the analyses. predation alone and chick predation alone, be- Theeffect of nestdensity on predation intensity.- causeeggs and chickswere taken largely by The two main predatorsof guillemoteggs and different predators,but found no consistentef- chicks on Mitlenatch were Northwestern Crows fectsof density (Tables5, 6). In only one of the (Corvuscaurinus) and gartersnakes (Thamnophis eight analyses--egg predation in 1985 using elegans).Crows were probably responsiblefor lower estimatesof predation--was there a sig- all egg predation,because it is unlikely that nificant effect. The slope was shallower than even the largest could swallow expected,indicating that clumped nests suf- guillemoteggs (Campbell 1969, P. W. Gregory fered less predation. pers. comm.). Conversely,chick predation was We found no consistent differences in the risk probably almost exclusively by snakes, al- of predation to nests on the north and south thoughcrows sometimes took young chicks from shores(Table 7). In 1984 the only significant very exposednests. effect was a higher egg predation on the north We plotted the number of nests depredated shorewhen lower estimatesof predationwere per quadrat against the total number of nests used.In 1985both egg predationand total pre- per quadrat and comparedthe fitted slope to dation were higher on the north shore.No anal- that expectedif predation were occurringran- ysesof chick predation were done in 1985 be- domly. For some nests we were uncertain cause no birds on the northern shore hatched whetherpredation had occurred, so we did sep- their eggs. arateanalyses for upper and lower estimatesof One problem with the above analyseswas predationintensity. The numberof quadratsfor that the number of nestsper quadratreveals which we had useful data was small, so we were little aboutthe proximityof nestswithin quad-

TABLE5. Fittedand expected slopes for regressionsof depredatednests per quadratvs. number of nestsper quadratfor egg predationonly. Analyseswere done on transformeddata; t-tests are two-tailed.

Predation Slope intensity estimate Fitted _+SE Expected t n P 1984 Upper 0.248 + 0.121 0.264 -0.132 27 >0.80 Lower 0.153 + 0.122 0.230 -0.631 27 >0.50

1985 Upper 0.051 + 0.130 0.266 -1.654 24 >0.10 Lower -0.066 + 0.1280 0.213 -2.180 24 <0.05 198 EmmsAND VERBEEIC [Auk,Vol. 106

TABLE6. Fitted and expectedslopes for regressionsof depredatednests per quadratvs. number of nestsper quadratfor chick predationonly. Analyseswere done on transformeddata; t-testsare two-tailed. Predation Slope intensity estimate Fitted + SE Expected t n P 1984 Upper 0.852 + 0.186 0.600 1.355 19 >0.10 Lower 0.690 + 0.191 0.533 0.822 19 >0.40

1985 Upper 1.083 + 0.208 0.636 2.149 12 >0.05 Lower 0.753 + 0.205 0.455 1.454 12 >0.10

rats. Spacing nestson a very local scale is ad- Nestgroups as informationcenters for foodfind- vantageousif predators show area-restricted ing.--We testedthe informationcenter hypoth- searching (Tinbergen et al. 1967; Croze 1970; esisindirectly through feeding ratesand chick Smith 1974a, b; Zach and Falls 1977). Area-re- growth rates.If nest groupsserve as informa- strictedsearching requires that predatorstake tion centers, the amount of information avail- more than one prey item at a time. Although able to unsuccessfulforagers should increase snakesprobably do not do this, crowscertainly with nest-groupsize. Consequently, both feed- do (Verbeek pets. obs.). Spacing nests is also ing ratesto nestsand chickgrowth ratesshould advantageousif predatorsreturn to areaswhere increasewith nest-group size. When we con- they were successfulin the past.We strongly trolled for date, time of day, tidal state, chick suspectedthat both crowsand snakesdid this. age,and numberof chicksper nestin a multiple We analyzed the frequencyof predation on regressionanalysis, nest-group size did not sig- nearestneighbors of depredatednests for two nificantlyaffect feeding rates to nests(b = -0.008 arbitrarydistance categories, 0-10 m and 10-20 + 0.005, t = -1.69, P = 0.091, n = 524). Chick m. If predatorsshowed area-restricted search- growth parametersdid not differ betweenyears, ing, or returnedto areaswhere they had been so data from 1984 and 1985 were pooled. Nest- successful,nests close to depredatedneighbors group size had no effecton either growth rates shouldhave sufferedmore predationthan those or fledging mass(Fig. 2). farther away.In 1984there was no significant effect(9 / 16 vs. 5 / 7; Fisher'sExact Test, P = 0.657), DISCUSSION but in 1985 the observed effect was oppositeto that predicted.Nearest neighbors 10-20 m away Thepattern of nestdistribution.--Cepphus guil- from a depredatednest were more likely to be lemots nest in small colonies or loose aggre- depredatedthan those0-10 m away (10/13 vs. gations(Belopol'skii 1957; Preston 1968; Cairns 2/13; Fisher's Exact Test, P = 0.005). 1980, 1981). This pattern was found on Mitle-

TABLE 7. Predation intensities on north and south shore nests. P-values are for Fisher's Exact Test. No tests were done for chick predationin 1985 becauseno birds on the north shorehatched eggs.

Proportion of depredated nests Lower estimate Upper estimate Predation measure North South P North South P

1984 Total 10 / 16 38 / 72 0.584 I0 / 16 44 / 72 1.000 Egg 7 / 16 13/71 0.046 7/16 16/71 0.116 Chick 4 / 8 28 / 52 1.000 4 / 8 32 / 52 0.702 1985 Total 10/15 28/81 0.024 11 / 15 34/81 0.046 Egg 9 / 14 11/ 80 0.0002 9 / 14 16/ 80 0.0015 Chick ...... April1989] PigeonGuillemot Nesting Dispersion 199 hatch, presumablydue to a clumped distribu- 3O A tion of suitable holeswhich birds are occupying at random. However, the evidence was some- what equivocal.In one of the four surveys(50 x 100 m quadratsin 1985),nests were signifi- cantly more clumpedthan expected.In addi- 20 tion, holes on the south shore, where total hole density was much higher, were preferred to ..,-lO those on the north shore in both years. There aretwo possibleexplanations for thenorth-south effect. First, birds benefit from nesting coloni- ally, and they do this only on the southshore, o 5 10 15 where holesare sufficientlydense. Second, south shore holes are better than north shore holes 6.4- for reasonsunassociated with aggregation. B Theinformation center hypothesis.--Guillemots on Mitlenatch did not appear to use their nest- -•6.2- ing groupsas information centers.An infor- mation center is only of value if food is dis- =6.0 tributedunpredictably in spaceand time (Ward _z and Zahavi 1973). Although the distribution pattern of the inshore,benthic on which •5.8 Cepphusguillemot chicks feed is not well-known, it is probably relatively uniform and predict- z able (Cairns 1984). Kuletz (1983) provided evi- 5.4 dence that food is predictablydistributed: in- 0 5 10 15 dividual parent guillemots had preferred NEST GROUP SIZE foragingareas to which they returnedregularly within a seasonand faithfully from year to year, Fig. 2. The effectof nest-groupsize on (A) chick and from which they may have excludedother growth rate:y = 0.122x- 13.37,r 2= 0.055,P = 0.094; birds.On Mitlenatch,foraging birds always flew (B) fledging weight (logetransformed): y = -0.002x + 6.05, r 2 = 0.007, P = 0.563. east from the island. This suggestedthat they knew where to find food. Food could be dis- tributedunpredictably on a very localscale, but then birds could find it by locatingflocks of intensity was generally independent of nest foraging individuals on the feeding grounds density, and only in 1985 did nests close to rather than by getting informationat the colony depredatedneighbors suffer less predation than (Bayer1982). Theoretically the benefitsof using thosefarther away. The latter effectmay be due colonies as information centers decline as for- to satiationof snakepredators (Emms and Mor- agingrange decreases and inshorefeeders such gan in press).However, there was significantly asPigeon Guillemots should have little usefor morepredation on the north shoreof the island information centers(Waltz 1982). In a compar- in 1985.This could imply an advantageto nest- ative study of six tern species,Erwin (1978) ing colonially becausethe density of nestsin found that colonysize was positively correlated the north was much less than in the south. with both foraging range and gregariousness However, we believe that the effect was due to when feeding.It seems,therefore, that Pigeon variation in nest-hole quality. Guillemots have no need to obtain information Variationin nest-hole quality. mNest holes could about the locationof good feeding sitesfrom differ in quality in two ways. First, the micro- conspecificsat the breedingsite. Cairns(1984) climate could be different on the two sides of arguessimilarly. the island. The south shore is sunnier, and holes Predationpressure.--Predation only weakly there may be slightly warmer. The south shore influenced the distribution of nests through is lesssheltered, especially early in the breeding variation in predationintensity with nest den- seasonwhen the prevailingwinds (which often sity.The quadratsurveys showed that predation bring strongrain) arefrom the southeast.Storey 200 EMMSAND VERBEEOC [Auk, Vol. 106

and Lein (1985) found that the temperature in- are already nesting, becauseif there are some side an artificialManx Shearwater(Puffinus puf- suitable sites in the area, there are probably finus)burrow on the leeward side of Middle others. Lawn Island, Newfoundland, was slightly Lack (1968) arguedthat many island-nesting higher than inside a similar burrow on the seabirdsnest in groups primarily becauseof a windward side. Most occupied burrows were shortageof suitablenesting habitat. We believe on the leeward side of the island. Holes could that, even when suitable sites are not limiting, alsovary in the degreeof protectionfrom pred~ nestscan be clumped becauseof a clumped dis- ators.South shore nestswere significantly bet- tribution of sites and spatial variation in site ter concealedand had more complexchambers quality. Thus, to show true coloniality(i.e. that than north shore nests. Both these factors had birds choose to nest close to other birds), one some effect in reducing predation risk. must show that nestsare clumped significantly South shore nestsare both more clumped and more than suitablesites and that the clumping of better quality than north shore nests,and of nestsis not due to a clumpingof high-quality either clumping or quality could be responsible sites.Wittenberger and Hunt (1985) argue sim- for the reducedpredation. We believe that the ilarly. It may be easierto show true coloniality associationbetween predation on individual for species(like guillemots)that occupydiscrete nestsin 1984 and 1985 showsnest-hole quality natural cavitiesthan for open-nestingspecies, to be the important factor. If clumping were or for hole nestersthat dig their own burrows. responsible for the benefit, those nests Even so, showing that birds nest in tighter depredated within clumps should be random. clumps than would be predicted from the spa- Individual nestswithin clumps should not be tial distribution of sitesof varying quality poses especiallyvulnerable, unless there was a center- substantialdifficulties. The null hypothesisfor edge,or someother sort of position,effect. The sucha testis that during colonyformation birds degreeof clumping of nestson Mitlenatch was occupy sites in order of decreasing intrinsic not sufficientlygreat to allow us to assignnests quality, irrespectiveof local site density. Com- to "center"and "edge" categories,but neststhat paring data with this null hypothesisis com- were depredated in both years were not sig- plicatedby such factorsas nest-sitefidelity and nificantly farther from their nearestneighbor the effect of age and experience on breeding (•: 22.22 m vs. 23.17 m; n = 15, 10; P = 1.000, date. Mann-Whitney U-test), nor did they have sig- Of course, the fact that a species does not nificantly fewer nestsin their quadrat(4.60 vs. choose to nest in "true colonies" does not mean 6.20;n = 15, 10;P = 0.262,Mann-Whitney U-test), that there are no benefitsto group nesting.Dur- than those that were not depredated in both ing the initial evolutionof colonialityin species years.However, sample sizes were small. that are now highly colonial,selection presum- Cairns (1980) also addressedthe pattern of ably actedon groupsbrought togetherfor rea- nest distribution in guillemots. Black Guille- sonsunrelated to the advantagesof group liv- mots (Cepphusgrylle) on Brandypot Island, St. ing. For example,there must have been some Lawrence River estuary,nested colonially, but initial variation in nesting density on which the birds did not use their colonies as infor- selectioncould work. The distribution of guil- mation centers,nor was predation important on lemot nests on Mitlenatch is largely random Brandypot(Cairns pers.comm.). with respectto the distribution of suitable nest One explanation for Cairns's observations holes,but variationsin the local densityof nests (and for our datathat birds preferredsouth shore are produced becauseof variations in the dis- nests) is that first-time breeders searched for tribution of suitable holes and spatial variation nest holes near to established birds because this in nest-hole quality. It was possiblethat birds wasthe mostefficient way of finding a suitable exploited thesevariations by using nest groups site. Guillemots are social birds early in the as information centers or benefited from re- breeding season,display in groups,and aggre- duced predation at certain densities. Selective gate on the seaand the lower shore.First-time pressurescould have acted in opposite direc- breedersare probablyattracted to these aggre- tions. One possibility is increasedefficiency of gations.After pairing, the quickestway to find food exploitation by using colonies as infor- a nesthole may be to searchwhere other birds mation centers,while higher predationon dense April1989] PigeonGuillemot Nesting Dispersion 201 aggregationswas another reasonfor territorial on Mitlenatch Island, British behavior. In fact, none of these alternatives was Columbia. Can. Field-Nat. 88: 313-316. true, but there was no a priori reason to prefer 1980. The breeding ecologyand social or- one of them over the otherssimply on the basis ganizationof the NorthwesternCrow (Corvus caurinus)on Mitlenatch Island, British Columbia. of the distribution pattern of nests.All potential M.S. thesis, Burnaby,British Columbia, Simon selectivepressures should be consideredto un- Fraser Univ. derstand patterns of breeding dispersion. CAIRNS,D. 1980. Nesting density, habitat structure and human disturbance as factors in Black Guil- ACKNOWLEDGMENTS lemot reproduction.Wilson Bull. 92: 351-362. ß 1981. Breeding,feeding, and chick growth This work was funded by NSERCgrant No. A0239 of the BlackGuillemot (Cepphusgrylle) in south- to Verbeek, and by the following scholarshipsto ern Quebec. Can. Field-Nat. 95: 312-318. Emms: The William and Ada Isabelle Steele Memorial 1984. The foraging ecology of the Black GraduateScholarship (1984-1985), the Anne Vallee Guillemot (Cepphusgrylle). Ph.D. dissertation.Ot- EcologicalFund Scholarship(1986), and the S.F.U. tawa, Carleton Univ. President'sResearch Stipend (1986). Emms's research CAMPBELL,g. W. 1969. Notes on some foods of the supervisorycommittee (Larry Dill, JaimieSmith, Kees wandering garter snake on Mitlenatch Island, Vermeer, and Ron Ydenberg) gave much invaluable British Columbia. Syesis2: 183-187. advice during the development of this work. Stim- 1977. Use of man-made structures as nest ulation was alsoprovided by the membersof the As- sitesby PigeonGuillemots. Can. Field-Nat. 91: sociation of Serious Scientists for Holistic and Or- 193-194. ganismalLandmark EcologicalStudies at Simon Fraser CROZE,H. 1970. Searchingimage in Carrion Crows: University. The British Columbia ParksBranch kind- hunting strategyin a predator and some anti- ly allowed us to work on Mitlenatch. Anne and Art predator devicesin camouflagedprey. Z. Tier- Lechasseurat Miracle BeachResort gave us much lo- psychol.Beih. 5. gistical support,and John Wieczorek, Rod Malcolm, DAAN,S., & J. TINBERGEN.1979. Young guillemots and Joel Hagan helped with someof the fieldwork. ( lomvia)leaving their arcticbreeding cliffs: Peter Grant, Tony Ives, BruceLyon, and two review- a daily rhythm in numbers and risk. Ardea 67: ers(Douglas Siegel-Causey and Anne Storey)read the 96-100. manuscriptand made numerous suggestionsthat DEGROOT, P. 1980. Informationtransfer in a socially greatly improvedits quality. roosting Weaverbird (Queleaquelea: Ploceinae): an experimentalstudy. Anim. Behav.28: 1249- LITERATURE CITED 1254. DIXON,W. J. (CHIEFED.). 1985. BMDP statisticalsoft- ANDERSSON,M., F. GOTMARK,& C. G. WIKLUND. 1981. waremanual: 1985 printing. Berkeley, Univ. Cal- Food information in the Black-headed , Larus ifornia Press. ridibundus. Behav. Ecol. Sociobiol. 9: 199-202. DRENT,R.H. 1965. Breedingbiology of the Pigeon BALDA,R. P., & G. C. BATEMAN.1972. 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