AGE at FIRST BREEDING and NATAL DISPERSAL in a DECLINING POPULATION of CASSIN&Apos;S AUKLET

TheAuk 118(4):996-1007, 2001

AGE AT FIRST BREEDING AND NATAL DISPERSAL IN A DECLINING POPULATION OF CASSIN'S AUKLET

PETER PYLE 1 PointReyes Bird Observatory,4990 ShorelineHighway, Stinson Beach, California 94970, USA

ABSTRACT.--Ageof first breeding (AFB) and natal dispersaldistance (NDD) were inves- tigated relativeto proximateand ultimate factorsin 276 known-ageCassin's Auklets (Pty- chorarnphusaleuticus) breeding during 1981-1999 on SoutheastFarallon Island, California. Breedingdensity in 446study boxes declined significantly during the period 1987-1999, con- firming otherinformation indicating a long-termdecline (since at leastthe 1970s)in thepop- ulation. Mean AFB was 3.34 years when sexeswere combined,ranging from 2-10 (mean 3.36) yearsin femalesand 2-9 (mean3.36) yearsin males.AFB showeda significantlinear increasewith decreasingcolony density, suggesting that increasedstress on the entire population, suchas that causedby decreasingfood availability,may have preventedyounger birds from attainingbreeding condition at an early age.Median NDD was 8.84m, ranging from 0-448.7 m; two individuals of each sex recruited into their natal boxes. NDD showed a significantlinear increasewith decreasingbreeding density but no significantlinear or curvilinear correlationswith mate fidelity, annual reproductivesuccess, or lifetime reproductiveoutput. A significantproportion (66.9%) of movementsafter recruitment(breeding dispersal)resulted in closerproximity to the natal site.Thus, although natal philopatrywas stronglydeveloped in the Cassin'sAuklet, it did not seemto be adaptive.Those paradoxical resultssuggest that selectiveequilibrium may be actingon natal philopatryin responseto the population decline:a low-philopatry,low-cost (later breeding and increasedsurvival) strategyhas become increasingly adaptive in Cassin'sAuklets, explaining increases in AFB and NDD with the decliningbreeding densities. The lack of sex-specificeffects on AFB and NDD suggeststhat an evensex-ratio and roughly equal rolesin reproductionamong Cassin's Auklets existson SoutheastFarallon Island, and that inbreedingavoidance may be counter- balancedby relativelylow mate fidelity in this species.Received 29 June2000, accepted 11 June 2001.

DEFERREDBREEDING, DOCUMENTED in many sources(Oring and Lank 1984, Dobson and seabirds(Furness and Monaghan1987, Newton Jones1985, Johnsonand Gaines 1990, Part 1991, 1989),could be adaptivefor long-livedorgan- Perrin and Mazalov 2000). ismsbecause potential benefits of breedingear- In moststudies of long-livedorganisms, var- ly in life are outweighedby coststo early re- iation in age of first breedingor natal dispersal production (Lack 1968, Nur 1988, Partridge distancehas been documentedin populations and Harvey 1988, Reznick 1992, Pyle et al. that are stable or in which trends in size or 1997, Caswell 2000). Natal dispersal (move- breeding density are unknown. Theory pre- ment between natal site and recruitment site) dictsthat distributionsin thosetraits may vary is relativelyundocumented in seabirds(Green- in species undergoing population change wood 1980, Greenwoodand Harvey 1982, but (Mertz 1971, Caswell 2000, Ricklefs 2000). For see Chabrzyk and Coulson1976; Ainley et al. example,age of first breeding (AFB) could in- 1983, 1990; Rabouam et al. 1998; Spear et al. crease with population decline if increased 1998).Depending on populationdemography, stress on the entire population, such as that natal philopatryin birds couldbe adaptivedue causedby decreasingfood availability,prohib- to local experiencewith breedingparameters its youngerbirds from attaining breedingcon- and proximity of successfuland related indi- dition at an early age. Alternatively, if a popu- viduals, or it could be maladaptive due to a lation decline is caused by increasedadult higher risk of inbreeding and increasedcom- mortality (e.g.due to interactionswith fisheries petition for mates, nesting sites, or food re- or increasedpredation pressure), AFB could decreaseas a populationdecreases due to less competition among potential recruits for E-mail: [email protected] breeding sitesand experiencedmates. For sim-

996 October2001] Ageand Recruitment in Cassin's Auklets 997 ilar reasons, natal dispersal distance (NDD) lantic Puffin (Fraterculaarctica; Petersen 1976, could also decreasewith population decline, Kress and Nettleship 1988), whereas age of assuming that natal philopatry is advanta- youngestbreeding is known for a few other geous.Evidence based on long-livedbirds in- species(Gaston and Jones1998). Along with dicates that AFB can either increase (Wyllie Ancient Murrelet (Synthliboramphusantiquus; and Newton 1991) or decrease (Bendell et al. Gaston 1990), Cassin'sAuklet (Ptychoramphus 1972; Coulson et al. 1982; Weimerskirch and aleuticus)is the only alcid known to breed as Jouventin 1987; Kress and Nettleship 1988; young as two years (Speich and Manuwal Croxall et al. 1990, 1998) with reduced breed- 1974), but variation in that trait has not been ing densities,and that NDD tends to decline measured. with declining densities (Greenwood 1980, Cassin'sAuklet has been declining on South- Greenwood and Harvey 1982, Negro et al. east Farallon Island, off San Francisco, Califor- 1997), although sex-specificdifferences have nia, from an estimated 105,000 individuals in been noted (Greenwood et al. 1979). the breeding population in 1972 (Manual Bothage of firstbreeding and nataldispersal 1974a) to 46,000 in 1989 (Carter et al. 1992), distance can show sex-specificvariation. De- with the declinecontinuing through the 1990s, on the basis of decreases in burrow densities pendingon the population,AFB in seabirdscan be significantlyyounger in either males (e.g. within studyplots and decreasedoccupancy of Wooller and Coulson 1977, Mills 1989, Spear et nest boxes(Ainley et al. 1994, Sydemanet al. al. 1995)or females(e.g. Ainley et al. 1983,Gas- 1996,this study). Declinesmay be the result of ton et al. 1994), or it can show no sex-specific predationat the breedingcolony by an increas- bias(e.g. Chabrzyk and Coulson1976, Wooller ing (during the 1970s)population of Western Gulls (Larus occidentalis)or shifts in prey re- et al. 1989).Sex-specific variation in AFB could sourcescaused by oceanwarming (Ainley et al. relate to skewed sex-ratiosin recruiting popu- 1996,$ydeman et al. 1996).Generally, breeding lations,resulting from differential survival in densitiesand annual reproductive successby prebreeding birds (Spear et al. 1987, 1995; Cassin'sAuklets are inversely correlated with Newton 1989). In birds, females usually show sea surface temperature (Ainley and Boekel- greaternatal dispersaland breedingdispersal heide 1990, Ainley et al. 1995), an effect most (movement between successivebreeding at- pronounced during warm-water, E1-Nifio- tempts) distancesthan males whereasthe re- Southern-Oscillation (ENSO) events (see Ain- verse tends to be true in mammals (Greenwood ley et al. 1995). Cassin'sAuklets on Southeast 1980, Johnsonand Gaines 1990, Perrin and Ma- FarallonIsland exhibit moderatelylow mate fi- zalov 2000).Sex-specific differences in dispers- delity; that is, birds often divorce(and at times al havebeen explainedas a meansto avoidin- remate after divorce) during consecutive breeding (Pusey 1987), and the difference breedingattempts (Emslie et al. 1992,Sydeman between birds and mammals correlates with et al. 1996). whether or not mate selection is based on a re- In this paper,I investigateage of first breed- source-defensebreeding system(most birds) ing and natal dispersalin 276 known-ageCas- or a mate-defensesystem (most mammals). sin'sAuklets breeding on SoutheastFarallon Is- The breeding system(nomadic, polyginous, or land between 1981 and 1999. I examine those monogamous)as well asskewed sex-ratios may traits relative to eachother, sex, natal fledging alsoaffect sex-specific natal dispersaldistances date, interannual variation in colony density, (Greenwood1980, Liberg and von Shantz1985, and ultimate factors such as mate and site fi- Johnsonand Gaines 1990,Spear et al. 1998,Per- delity, annual reproductivesuccess, and life- rin and Mazalov 2000). time reproductiveoutput. I alsoassess changes Due to their habit of nesting in densecolo- in ageof first breedingand natal dispersaldis- nies and sensitivity to human disturbance,lit- tanceduring the 17 year study period as relat- tle is known about age of first breedingor in- ed to the decliningpopulation trend. tracolonynatal dispersalin alcids(Gaston and Jones1998). Frequency distributions in AFB METHODS havebeen examined only in the Razorbill(Alca The breeding biology of the Cassin'sAuklet on torda;Lloyd and Perrins 1977), Thick-billed SoutheastFarallon Island has been describedby Ma- Murre (Uria lomvia;Gaston et al. 1994), and At- nuwal (1974a,b) and Ainley and Boekelheide(1990). 998 P•T•R P¾• [Auk, Vol. 118

was monitored on a five day scheduleto determine hatching and fiedging success(see Ainley and Boe- kelheide 1990 for details). Sex of known-agebirds was determined by bill depth (Nelson 1981). Bill depths of <9.8 mm indicated females and those >10.2 mm indicated males; only a small proportion of known-age individuals could not be sexed after pairs were measured during one or more years of data collection. Natal dispersaldistance (NDD) and breeding dispersal distance(BDD) were measureddirectly as the distance (nearest 0.01 m) between entrances of natal and recruitment boxes and of nest boxes used by known-ageindividuals between years. Breeding colony density was defined as proportion of the 446 study boxes that were occupied by an incubating bird eachyear. Estimated natal fiedgingdate was cal- FIG. 1. Locationsof 10 known-ageCassin's Auklet culated to the nearest15 days for most known-age study plots on SoutheastFarallon Island including birds on the basis of information on egg-laying, number of boxesper plot (1987-1999). chick-hatching,and chick-fiedgingdates during the known-agebird's natal year.Mate fidelitywas scored (for birds with at least three yearsof breeding)as 1 Between 1978 and 1983, -700 nest boxes were in- - (total number of mates/total number yearsbreed- stalled for oil-related studies(e.g. Ainley et al. 1981) ing); thus, birds with a differentmate eachyear have and subsequentlyused by Cassin'sAuklets. Boxes mate fidelity of 0, and those with the same mate were unevenlydistributed among 10 plots through- throughouttheir lives have mate fidelity approach- out accessibleportions of the island (Fig. 1). In 1987, ing 1. Site fidelity was assessed(for birds with at boxesthat had not been occupiedby aukletssince in- least two yearsof breeding) as an individual'smean stallation were removed, resulting in a total of 446 breeding dispersaldistance during the study. boxes that were subsequentlymaintained through- Mean annual reproductive successwas deter- out the study period (Fig. 1). This study is basedon mined (in birds that had bred at least two years) as recruitmentand breeding attemptswithin those446 total number of chicksfledged divided by years of boxes,as monitored by Point ReyesBird Observatory breeding (Cassin'sAuklets can fledge up to two (PRBO)biologists. Although a few unmonitoredbur- chicks per year; Ainley ahd Boekelheide 1990). rows were presentin the vicinity of boxesduring the Fledgingstatus for attemptswith missingdata (4.8% early yearsof the study,by 1987 there were virtually of all attempts)were calculatedwith a model includ- no burrows within the maximum breeding dispersal ing year and age and experienceterms of both the distance (14.45 m; see below) of boxes;thus, it is as- known-agebird and its mate (seePyle et al. in press sumed that biases in the data due to movement be- for details). Lifetime reproductiveoutput of known- tween burrows and boxeswere negligeable. age birds was calculated, for birds that initiated Throughoutthe study period (beginningin 1978), breeding in 1993 or earlier, as mean reproductive band numbersof breeding aukletsin all boxeswere successx number of years breeding. A small pro- recorded each year. Between 1978 and 1999, all un- portion of thosebirds (7.2%) were still breeding at banded breeding adults were banded, and between the end of the study period (1998 or 1999),but there 1987 and 1999 unbanded mates of known-age birds is no reasonto assumethat the potentiallyunderes- were banded. All fiedging chickswere banded each timated lifetime reproductive output of thosebirds year. The bands were stainless steel, and a double- would be biasedwith respectto AFB or NDD. bandedbird experimentconducted by PRBO(P. Pyle Using STATA(Stata Corporation 1997), analysis of unpubl. data) from 1978 to 1988 indicated that band variance (ANOVA) and simple and multiple regres- losswas absentor virtually so (<0.1% per year). Be- sion were used to examineeffects and model depen- tween 1981 (when the first known-ageadult was ob- dent variables.To normalize natal and breeding dis- served) and 1986, annual reproductive successwas persal distance,these terms were log-transformed determined for only a few breeding attempts by (log[n + 0.5 m], becauseNDD and BDD both includ- known-age auklets.Between 1987 and 1999, breed- ed values of 0). Residuals of all variables met as- ing successwas determinedfor most or all attempts sumptionsfor normality (skewness/kurtosistests, P per year.Beginning 15 March eachyear (1987-1999), > 0.05; Stata Corporation 1997). Nonlinearity (cur- eachbox was checkedonce every 15 days for breed- vilinearity) was testedby examiningsignificance of ing auklets.When a known-agebird was found, the quadratic terms. Sex-specificeffects were tested us- band status of its mate was determined and the site ing analysesof covariance(ANCOVA) which exam- October2001] Ageand Recruitment in Cassin's Auklets 999

450 Known-agebirds.--Between 1981 and 1999, 400 276 known-ageCassin's Auklets were recorded •5o in the breeding plots (144 males, 123 females, •oo and 9 of undetermined sex). A total of 794 250 breedingattempts was recorded,by birds 2-14 yrs of age and 0-11 yearsof previousbreeding 200 experience.Annual reproductive successwas 150 determinedfor 756 of thoseattempts including •00 688 attempts(91.0%) between 1987 and 1999. 5o Among known-age birds with at least three years breeding (n = 116), mean mate fidelity was 0.484 (range 0-0.857). Mean breeding dis- FIG. 2. Number of breeding Cassin'sAuklets re- persal distance(n = 511 between-yearattempts corded per year in the 446 study boxes. Breeding by 161 individuals) was 1.16 + 2.21 (SD) m density,defined as proportion of occupiedboxes, de- with a range of 0-14.45 m; breeding dispersal clined significantlyduring this period, indicating a occurred during 34.2% of between-year at- popuationdecline of -2.8% per year. tempts.Among known-agebirds with at least two yearsof breedingdata (n = 160),mean annual reproductive success was 0.63 (+0.34 ines interaction terms between dependentvariables [SD]; range 0-1.5) chicksfledged. Lifetime re- and sex;additionally, analyses were performedsep- productive output for aukletsfirst breeding in arately on eachsex to look for significanteffects of a term on one sex but not the other. 1993 or earlier (n = 166) was 2.74 (+2.66; range 0-9.34). Age of first breeding.--MeanAFB was 3.34 RESULTS years (+1.32 SD; n =276) when sexes were Colonydensity.•umber of occupiedboxes combined.AFB ranged from 2-10 years (mean within the 10 plots (Fig. 1) varied from 115 3.357, median 3 years;n = 123) in femalesand (25.8%;1998) to 412 (92.4%;1987) during 1987- 2-9 years (mean 3.36, median 3 years;n = 144) 1999 (Fig. 2), resulting in a mean breedingden- in males (Fig. 3). The sex-specificdifference sity of 66.9%. Breeding density was signifi- was not significant(ANOVA, F = 0.60, df = 1 cantly lower during the two years affectedby and 266, P = 0.44), althougha slightly higher warmer sea s-arface temperatures associated proportion of females (29.3%) than males with ENSO events, 1992 (40.6%) and 1998 (24.1%)recruited at the youngestage, 2 years (25.8%), than in the other 11 years (mean (Fig. 3). A majority of birds (95.5%,sexes com- 73.1%; ANOVA, F -- 29.02, df = 1 and 11, P < bined) initiatedbreeding between 2 and 5 years 0.001).Breeding density declined significantly (Fig. 3); thus, in the following analyses,data during the 13 year period,both with (linearre- from birds with AFBs of 6-10 yearshave been gression,t = -2.71, P = 0.020) and without (t pooled. = -4.71, P = 0.001) inclusion of data from the Correlationsof AFB with both proximateand two ENSO years. Skipped years, defined as ultimate factorsare shown in Table 1. No sig- yearsof absencebetween years of breeding,oc- nificant linear or curvilinear correlations or curred 74 times among47 birds (23 males and sex-specificeffects were found between AFB 24 females),and included 64 skipped episodes and NDD or natal fledging date. Mate fidelity of 1-4 consecutiveyears. Thirty of the 74 showedsignificant positive linear correlations skipped years (40.5%) occurred during the with AFB, and mean BDD (log transformed) ENSO events of 1992 and 1998. Skipping did showedsignificant linear decreaseswith AFB, not increaseduring the studyperiod (linearre- overall and in both sexes. Those correlations, gression,t = 0.45, P = 0.67); thus, decreasing confoundedby affects of age and previous breedingdensity was not causedby an increas- breeding experienceon mate and site fidelity, ing rate of skipping. Without including data were discussedin more detail in a separatepa- from 1992 and 1998 (clearly years of reduced per (Pyle et al. in press). attendancerather than populationfluctuation), When sexes were combined, AFB of recruit- averagedecline was 2.8% per year. ing individuals showeda significantlinear in- 1000 PETERPYLE [Auk, Vol. 118

Females Males 60 60

50 50

,40 40

I c 30

o c 20 2O

lO 10

o ,o2 3 4 5 6 7 8 g 2 3 4 5 6 7 8 9 10 0 Age of First Breeding Age of First Breeding

FIG. 3. Age of first breeding in female and male Cassin'sAuklets. crease with year (1987-1999 only, to remove ed confoundingbetween linear effectson AFB bias associatedwith sample of potential re- of year (t = 1.24, P = 0.22) and density (t = cruits) and a significantlinear decreasewith -1.20, P = 0.23); that is, it could not be deter- colony density; in each case,quadratic terms mined which of the two variables (or both) re- were significantand negative indicating that sulted in significant correlationswith AFB. year trend was deceleratingand density trend Similar results were obtained with each sex was accelerating(Fig. 4). AFB showeda signif- separately.Examination of AFB during ENSO icant linear increase but no curvilinear rela- events (1992 and 1998; n = 17, mean AFB 3.59), tionship with year in females, whereas in when colonydensity was reduced(Fig. 2), ver- males,no linear trend but a significantnegative susthe other 11 years (n = 208, mean AFB 3.45) curvilinear trend was found; however, ANCO- indicated no significant difference (ANOVA, F VA revealed no significant sex-specificdiffer- = 0.07, df = 1 and 222, P = 0.79). ences in those relationships (Table 1). Relative Natal dispersaldistance.--Median NDD in to colonydensity, AFB showeda significantlin- known-ageCassin's Auklets was 8.84 m (n = ear and curvilinear decrease in females, and no 273; mean -- 17.6 + 36.2 [SD] m; range 0-448.7 linear trend but a significantnegative curvilin- m), 9.39 m (n = 121; mean -- 19.1 + 28.6 m) in ear trend in males;again, however,sex-specific females and 8.11 m (n = 144; mean = 16.8 + effectswere not significant(Table 1). Whensex- 44.6 m) in males. Two individuals of each sex es were combined,multiple regressionindicat- recruited into their natal boxes (NDD = 0).

TAB!,E1. Relationshipsbetween age of first breeding and natal fiedging date, year, breedingdensity, natal dispersaldistance (NDD), mate fidelity, and breeding dispersaldistance (BDD), in Cassin'sAuklets on SoutheastFarallon Island. Seetext for definitionsof variablesand certainrestrictions; AFB's of 6-10 years were pooled. For overall (both sexes),females, and males, t-valuesbased on linear regressionor multiple regressionincluding quadratic terms (e.g. X2 ) are presented.Results of analysesincluding quadratic terms are presentedonly when significantcurvilinear effectswere found. Linear effectswere calculatedwithout including quadratic terms. For sex-specificeffects, F-statistics are presentedon the basisof analysisof covariance.Significant effects are indicatedby asterisks:* P < 0.05, ** P < 0.01, *** P < 0.001.

Overall Females Males Sex-specific Variable n r t t t effect F Fledging date 270 0.07 0.73 1.47 - 1.03 3.70 Year x year 224 0.17 3.04** 2.80** 1.35 1.22 0.31 -2.02' -0.85 -2.56' 0.81 Density x density 224 0.19 -3.02** -2.83** -1.36 1.72 0.34 - 3.42'** - 2.03* - 2.58* 0.59 NDD 273 0.00 0.25 -0.13 0.78 0.45 Mate fidelity 116 0.22 3.62*** 2.59* 2.63* 0.01 Mean BDD 161 0.23 -3.17'* - 1.98' -2.23* 0.06 October2001] Ageand Recruitment in Cassin's Auklets 1001

5 o c. 4.5 4.5 o o 4 2 g•

.,._• u_ 71.5 oo• 3.5 • 3 3

o g z5 2.5

Year B•eedtng Density

FIG. 4. Linear and curvilinear correlationsbetween mean age of first breeding and year and density in Cassin'sAuklets. SeeTable 1 for statisticalanalyses.

NDD was >75 m in only seven individuals < 10 m (X2 = 0.40, P = 0.53), or NDD < 15 m (three males and four females that recruited (X2 = 0.20, P = 0.89). into nonnatalplots; see Fig. 1); excludingthose, Correlationsof NDD (log transformed)with mean NDD was 13.5 + 13.0 m in females and both proximateand ultimate factorsare shown 12.8 + 13.8 m in males. Those values were like- in Table 2. No significantlinear or curvilinear ly biased downward, comparedwith overall correlations,or sex-specificeffects, were found dispersaldistances, by the nonrandom distri- between NDD and natal fiedging date, year, bution of plots and boxeswithin plots (Fig. 1) mate fidelity, annual reproductivesuccess, or and the factthat dispersalaway from Southeast lifetime reproductive output. When sexeswere Farallon Island was unknown; however, com- combined,NDD showeda significantlinear de- parisonsof NDD betweensexes and with other creasewith breeding density(Fig. 5). Multiple parametersshould not be subjectto that bias. regressionshowed that effect remained signif- The sex-specificdifference in NDD (log trans- icant (t; -2.12, P = 0.035) with inclusion of formed) was not significant either with (F = the year term, which was not significant (t = 0.31, df = I and 263, P = 0.58) or without (F = -0.58, P = 0.56). The decrease in NDD with 0.42, df = I and 256, P = 0.52) the inclusion of density was significantin femalesand not sig- birds with NDD > 75 m. There also was no sex- nificant in males,but sex-specificdifference in specificdifference in natal philopatry when de- that correlationwas not significant(Table 2). fined as NDD < 5 m (X2 = 1.28, P = 0.72), NDD NDD also showed a significantincrease with

TABLE2. Relationshipsbetween natal dispersaldistance at recruitmentand natal fledgingdate, year, breeding density,mate fidelity, breedingdispersal distance (BDD), mean reproductivesuccess (RS), and estimated lifetime reproductive success(LRS) in Cassin'sAuklets on SoutheastFarallon Island. See text for definitionsof variablesand certainrestrictions; natal dispersaldistance (meters) represented as log (NDD + 0.5). For overall (both sexes),females, and males,t-values based on linear regressionare presented.Qua- dratic analyseswere performedon eachvariable and no significantcurvilinear effects were found. Forsex- specificeffects, F-statistics are presentedon the basisof analysisof covariance.Significant effects are indicated by asterisks:* P < 0.05, ** P < 0.01, *** P < 0.001. Overall Sex-specific Variable n r t Females t Males t effect F Fledging date 269 0.06 1.32 1.71 -0.37 1.06 Year 273 0.08 0.54 0.32 0.50 1.97 Density 223 0.16 - 2.34' - 2.00' - 1.41 0.76 Mate fidelity 114 0.00 1.07 1.23 0.41 0.43 Mean BDD 161 0.16 1.25 2.84** -0.67 5.37* Mean RS 160 0.07 -0.59 -1.15 0.06 0.75 Estimated LRS 164 0.09 0.56 -0.57 1.39 2.04 1002 PETERPYLE [Auk, Vol. 118

60 cy was strongerin males (65 closer,27 farther) than females (45 closer,28 farther), although the sex-specificdifference was not significant • •o (X2 = 1.48, P = 0.22). Mean changein distance from the natal box was -0.52 + 3.20 (SD) m •2 30 •99 89 with a range of -11.73 to 9.20 m. • 2o DISCUSSION o% •o 9694 '•7 Decliningpopulation and age of first breeding.- o The significant decreasein breeding density Breedzng Denszty between1987 and 1999,coupled with lackof an increasein skipped breeding, confirmsother FIG. 5. Mean natal dispersal distance as corn- information (Carter et al. 1992; Ainley et al. pared with annualbreeding density in Cassin'sAuk- lets on Southeast Farallon Island. See Table 2 for sta- 1994, 1996)that the Cassin'sAuklet population is declining on SoutheastFarallon Island. Our tistical analyses. calculatedannual percentage change of -2.8% is similar to the -2.6% preliminarilyreported mean breeding dispersal distance(log trans- by Nur et al. (1998)based on mark-recapture formed)in femalesbut not in males(Fig. 6) and analysis.Similar declineshave been noted for that sex-specificdifference was significant(Ta- other planktivorousspecies of alcids in the ble 2). However, the female with the largest North Pacific (Kitaysky and Golubova 2000), NDD (181.4 m) alsohad the largestmean BDD and for Cassin's Auklets in British Columbia (13.78 m; Fig. 6); removing that individual (Bertram et al. 2000). from the sampleresulted in a marginally non- The significantly lower breeding densities significantcorrelation (t; 2.01, P; 0.051)be- observed during the two years affected by tween NDD and BDD in females and no sex- ENSO support findingsby Ainley and Boekel- specificeffect (ANCOVA, F = 1.19, df = 3 and heide (1990)and Ainley et al. (1995)that breed- 155, P = 0.28). ing-populationsizes are inverselycorrelated Change in distancefrom the natal site was with sea-surfacetemperature (SST). The high analyzedafter breedingdispersal events. Dur- rates of skipping during ENSO years suggest ing 111 of 166 events (66.9%), known-age Cas- that warmer SST associated with ENSO events sin's Auklets moved closer to their natal box, result in decreasedcolony attendancerather whereas during 55 events they moved farther than reducedpopulation per se.However, cor- from their natal box; that proportionwas sig- relations between warmer SST and reduced nificantly different from 0.50 (X2 = 9.72, P = zooplanktonabundance off California(Roem- 0.002).When sexeswere separatedthis tenden- mich and McGowan 1995), coupled with the

o o 5 o .,-, o 4 8 o o o

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F•G. 6. Natal dispersaldistance (log transformed)compared with mean breeding dispersaldistance in female and male Cassin'sAuklets. SeeTable 2 for statisticalanalyses. October2001] Ageand Recruitment in Cassin's Auklets 1003 fact that SST in the eastern North Pacific has in- NatalPhilopatry.--The median natal dispersal creased during this study (McGowan et al. distanceof 8.84 m is likely biaseddownward by 1998) suggeststhat the long-termpopulation the nonrandom distribution of study boxesand decline is correlated with ocean-temperature- lack of information on intercolony dispersal. related shiftsin prey resources(primarily krill However,several lines of evidencesuggest that [Euphausia])as suggestedby Ainley et al. (1995, natal philoparry is strongly developedin Cas- 1996; see also Bertram et al. 2000), rather than sin's Auklets on Southeast Farallon Island (cf. (or in addition to) predation by WesternGulls Baker 1978, Greenwood and Harvey 1982). (cf. Nelson1989, Eroslieand Messenger1991). What little is known about natal dispersalin al- Nur et al. (1988)also report that adult survival cids suggeststhat most breedersrecruit into was lower during ENSO events. their natal colony,at least in tourres(Birt-Frie- Results of this study support the premise sen et al. 1992, Gaston and Jones 1998). The that shiftsin prey resourcesare contributingto nearestsignificant colony of Cassin'sAuklets to the populationdecline. Mean AFB in Cassin's Southeast Farallon Island is that of Castle Rock, Auklets increasedsignificantly with declining almost 400 km to the north (Carter et al. 1992), breeding density during 1987-1999.Although suggestingthat the SoutheastFarallon Island confoundedwith year, we believe that density colonylikely experienceslittle immigrationor effects on life-history traits in the population emmigration. That four birds recruited into are genuine,as shown for NDD in this study their natal sitesand that a significantproportion of auklets moved closer to their natal site (see also Pyle et al. in press more details). Thesefindings are oppositeto resultsof other after breeding dispersal events further sup- studies indicating decreasedAFB with lower ports the conclusionthat natal philopatry is ad- breeding densitiesin BlueGrouse (Dendrgapus vantageousin the population. Paradoxically, obscurus;Bendell et al. 1972, Zwickel 1972), however,neither AFB nor any of the proximate or ultimate estimatorsof fitness (mate fidelity, Herring Gulls (Larusargentatus; Coulson et al. annual reproductive success,lifetime repro- 1982), Atlantic Puffins (Kress and Nettleship ductive output) were correlatedwith NDD, ex- 1988), and Wandering Albatross(Diomedea except for a marginally significantrelationship ulans;Croxall et al. 1990), but similar to results betweenbreeding dispersaldistance and NDD suggestingincreased AFB in a decliningpop- in females. Furthermore, NDD increased with ulation of European Sparrowhawks(Accipiter decreasing breeding density, a result that nisus;Wyllie and Newton 1991). Explanations might be expectedif NDD was constrainedby for a positive relationship between AFB and mate availabilitybut is oppositeto that expect- breeding density have included reduced com- ed in most birds (Greenwood1980), assuming petition for nest sitesand food resourcesand a auklets are selectedfor natal philopatry and greater availability of experiencedmates, situ- NDD is constrainedby competition. ations that arise due to decreased survival or A study of NDD in WesternGulls on South- increased (often human-induced) mortality of eastFarallon Island from 1979to 1994 (Spearet breeding adults, as was the case in the study al. 1998) indicated similar results: natal philo- populationsof grouse,gulls, puffins, and al- patty was well developedbut appeared to be batross mentioned above. A similar relation- nonadaptive,if not realadaptive,due to lower ship in Cassin'sAuklet might be expectedif lifetime productivityin more philopatricgulls. predationby WesternGulls on breeding adults Spear et al. explained those results in terms of was the primary factor causing decline. In- relatively poor food availability in the Gulf of stead, increasein AFB with decreaseddensity the Farallonesduring 1989-1994. Natal philo- in Cassin'sAuklets suggeststhat increased patty may have been selectedfor, especiallyin stress on the entire population, such as that male WesternGulls, as part of a high-costre- caused by decreasing food availability, may productivestrategy (earlier breeding and poor- have preventedyounger birds from attaining er long-term survival) developedduring a pe- breeding conditionat an early age. A similar riod of good food availabilityin the 1970sand situation may have occurredin the declining early 1980s.Once food availability decreased, populationof sparrowhawks(Wyllie and New- however,that strategywas no longer adaptive. ton 1991). A similar situation may be occurring in Cas- 1004 PETERPYLE [Auk, Vol. 118 sin's Auklets, given decreasingbreeding den- wood 1980, Spear et al. 1998). Sex-specificdif- sitiesthat are likely related to decreasesin food ferencesin natal dispersalhave beenexplained availability (seeabove). Thus, as in the Western as a meansto avoidinbreeding, and female-bi- Gull, a low-philopatry,low-cost strategy (later aseddispersal in mostbirds correlateswith the breeding and increasedsurvival) may now be fact that mate selectionis basedprimarily on adaptive in Cassin'sAuklets, explaining both resource-defensebreeding systems (Green- acceleratingincreases in AFB and linear in- wood 1980, Pusey 1987). During territory es- creasesin NDD with declining breeding den- tablishment and defense, males may benefit sities, and lending support to the theory that from natal philopatry due to local experience selectiveequilibrium may be acting on such with breeding parameters and proximity of traits as natal philopatry, depending on peri- successful and related individuals, whereas fe- odicitiesin breeding densitiesand food avail- males disperseto avoid inbreedingwith phil- ability (Stearns1992, Spear et al. 1998). opatricmale siblings. Sex-specificeffects.--A sex-specificeffect in Cassin'sAuklets show little tendencytoward the positive correlation between NDD and sex-specificroles in reproduction.Unlike West- meanbreeding dispersal distance became non- ern Gulls,where quality of territory (amountof significantwith the removal of the individual cover for chicks) correlates with successfulre- femalewith the greatestNDD. Otherwise,there production,nest boxes(and perhapsbreeding were no sex-specificeffects on AFB, NDD, or burrows) are all relatively similar in quality. their correlationswith proximateand ultimate Lack of sex-specificeffects in both AFB and factors in known-age Cassin's Auklets on NDD in Cassin'sAuklets may support a theory Southeast Farallon Island. proposed by Johnson(1986) correlatingsex- It is again instructive to contrast those re- specificeffects in philopatry with thoseof de- sults with the many sex-specificresults found layed breeding in mammals (see also Johnson in Western Gulls breeding on SoutheastFaral- and Gaines 1990). In addition, the relatively lon Island during roughly the sameperiod of low matefidelity in Cassin'sAuklets may lessen time. Age of first breeding was significantly importance of inbreeding avoidance in that lowerin male than femaleWestern Gulls (Spear species(cf. Liberg and von Schantz1985, but et al. 1995,Pyle et al. 1997).That differencewas see also Marks and Redmond 1987). Should in- correlated with a skewed sex ratio in the re- breeding occur at a low level in Cassin'sAuk- cruiting population, the result of differential lets, as has been documentedin Cory's Shear- survivalamong prebreeding birds (Spearet al. waters (Calonectrisdiomedea; Rabouam et al. 1987), and with differencesin roles of repro- 1998), frequent mate switching ensures in- duction(Pierotti 1981,Pyle et al. 1991;see also creased genetic heterogeneity at the level of Chabrzyk and Coulson 1976, Wooller and both the individual and the population.Rela- Coulson1977, Ainley et al. 1983, Mills 1989). tionshipsbetween mate fidelity and age,breed- The lack of sex-specificeffects on AFB in Cas- ing experience,site fidelity, and ultimate mea- sures of fitness in Cassin's Auklets on Southeast sin'sAuklets may indicatean evensex-ratio and roughly equal roles in reproductionon South- Farallon Island, which shed further light on east Farallon Island. That correspondswith those issues,have been treated in a separate what we know of the breeding biology of that analysis(Pyle et al. in press). species(and of alcids in general; Gaston and ACKNOWLEDGMENTS Jones1998): incubation and chick-provisioning are shared(Manual 1974a,b) and the relatively I thank the U.S. Fish and Wildlife Service/San low (comparedwith other long-lived seabirds) FranciscoBay National Wildlife Refuge,managers of mate and site fidelity (Emslie et al. 1992, Sy- the FarallonIsland NationalWildlife Refuge,for fa- demanet al. 1996,Pyle et al. in press)suggests cilitating and supporting PRBO researchon the is- that territoriality and sex-specificroles in re- land. I am grateful to the numerousinterns that col- lectedthe known-ageCassin's Auklet data,under the productiveeffort are not well developed. supervisionof Farallonbiologists David Ainley,Bob Except in nomadic species and waterfowl Boekelheide,Teya and Jay penniman, Harry Carter, (Anatidae), most birds show female-biased na- William J.Sydeman, Steven Emslie, Joan Walsh, Eliz- tal dispersal;that is, femalesdisperse in greater abethMcLaren, Michelle Hester,Kelly Hastings,and proportions and farther than males (Green- Kyra Mills. I especiallythank David Ainley for initial October2001] Ageand Recruitment in Cassin'sAuklets 1005 encouragementof this project,and Larry Spear,Na- JORY,AND D. L. WHITWORTH.1992. Breeding day Nur, and William J. Sydeman for much discus- populations of seabirds on the Northern and sion on life-history traits in Farallonseabird s. Ainley, Central California coastsin 1989-1991.Report Spear, Sydeman,Scott Hatch, David Nettleship, and no. 14-12-001-30456,U.S. Department of Interi- an anonymousreviewer provided commentsthat im- or, Mineral Management Services,Los Angeles, proved this paper. This is contribution#927 of the California. Point ReyesBird Observatory. CASWELL,H. 2000.Matrix PopulationModels. Sinaur Associates, Sunderland, Massachusettes. LITERATURE CITED CHABRZYK, G., AND J. C. COULSON. 1976. Survival and recruitmentin the Herring Gull Larusargen- AINLEY, D. G., AND R. J. BOEKELHEIDE.1990. Seabirds tatus.Journal of Animal Ecology45:187-203. of the Farallon Islands. Ecology,Dynamics, and COULSON, J. C., N. DUNCAN, AND C. THOMAS. 1982. Structureof an Upwelling-systemCommunity. Changesin the breedingbiology of the Herring StanfordUniversity Press, Stanford, California. Gull Larusargentatus induced by reductionin the AINLEY, D. G., C. R. GRAU, T. E. ROUDYBUSH,S. H. size and densityof the colony.Journal of Animal MORRELL,AND J. M. UTTS. 1981. Petroleum in- Ecology51:739-756. gestion reduces reproduction in Cassin'sAuk- CROXALL,J.P., P. ROTHERY,S. P. C. PICKERING,AND lets. Marine Pollution Bulletin 12:314-317. P. A. PRINCE.1990. Reproductiveperformance, AINLEY, D. G., R. E. LERESCHE,AND W. J. SLADEN. recruitment and survival of Wandering Alba- 1983. Breeding Biology of the Adelie Penguin. tross Diomedea exulans at Bird Island, South University of California Press, Berkeley, Georgia.Journal of Animal Ecology59:775-796. California. CROXALL,J.P., P. A. PRINCE, P. ROTHERY,AND A. G. AINLEY, D. G., C. A. RIBIC, AND R. C. WOOD. 1990. A WOOD.1998. Populationchanges in albatrosses demographicstudy of the SouthPolar Skua Ca- at South Georgia. Pages68-83 in Albatross Bi- thatactamaccormicki at Cape Crozier. Journalof ology and Conservation(G. Robertsonand A. Animal Ecology59:1-20. Gales, Eds.). Surrey Beatty and Sons,Chipping AINLEY, D. G., L. B. SPEAR,AND S. G. ALLEN. 1996. Norton, Australia. Variation in the diet of Cassin's Auklet reveals DOBSON,E S., ANDW. T. JONES.1985. Multiple causes spatial, seasonal,and decadal occurrencepat- of dispersal.American Naturalist 126:859-866. ternsof euphausiidsoff California, USA. Marine EMSLIE, S. D., AND S. MESSENGER. 1991. Pellet and Ecology Progress Series 137:1-10. bone accumulationat a colonyof WesternGulls AINLEY, D. G., W. J. SYDEMAN, S. A. HATCH, AND U. (Larus occidentalis).Journal of Vertebrate Pale- L. WILSON. 1994. Seabird population trends ontology 11:133-136. along the west coastof North America: Causes EMSLIE,S. D., W. J. SYDEMAN,AND P. PYLE. 1992. The and the extent of regional concordance.Studies in Avian Biology15:119-133. importanceof materetention and experienceon AINLEY, D. G., W. J. SYDEMAN,AND J. NORTON. 1995. breeding successin Cassin'sAuklet (Ptychoram- phusaleuticus). Behavioral Ecology 3:189-195. Upper trophic level predatorsindicate interan- FURNESS,R. W., AND P. MONAGHAN. 1987. Seabird nual negativeand positiveanomalies in the Cal- ifornia Current food web. Marine Ecology Pro- Ecology.Blackie, Glasgow, Scotland. gressSeries 118:69-79. GASTON,A. J.1990. Population parameters of theAn- cient Murrelet. Condor 92:998-1101. BAKER,R. R. 1978.The EvolutionaryEcology of An- imal Migrations. Hodder and Stoughton, GASTON,A. J., L. N. DEFOREST,G. DONALDSON,AND London. D. G. NOBLE.1994. Populationparameters of BENDELL,J. E, D. G. KING, AND D. H. MossoP. 1972. Thick-billed Murres at Coats Island, Northwest Removaland repopulationof Blue Grousein a Territories, Canada. Condor 96:935-948. declining population. Journal of Wildlife Man- GASTON,A. J., AND I. L. JONES.1998. The Auks: Ali- agement 36:1153-1165. cidae.Oxford University Press,Oxford. BERTRAM, D. E, I. L. JONES, E. C. COOCH, H. A. GREENWOOD,P. J. 1980. Mating systems,philopatry KNECHTEL,AND E COOKE. 2000. Survival rates of and dispersal in birds and mammals. Animal Cassin'sand Rhinocerosauklets at Triangle Is- Behaviour 28:1140-1162. land, British Columbia. Condor 102:155-162. GREENWOOD,P. J., AND P. H. HARVEY. 1982. The natal BIRT-FRIESEN,V. L., W. A. MOTEVECCHI,A. J. GASTON, and breeding dispersalof birds. Annual Review AND W. S. DAVIDSON. 1992. Genetic structure of of Ecologyand Systematics13:1-21. Thick-billedMurre (Uria Iomvia)populations ex- GREENWOOD,P. J., P. H. HARVEY, AND C. M. PERRINS. amined using direct-sequenceanalysis of am- 1979.The role of dispersalin the Great Tit (Parus plified DNA. Evolution46:267-272. major):The causes,consequences and heritabil- CARTER,H. R., G. J. MCCHESNEY, D. L. JAQUES,C. S. ity of natal dispersal.Journal of Animal Ecology STRONG,M. W. PARKER,J. E. TAKEKAWA, D. L. 48:123-142. 1006 PETERPYLE [Auk, Vol. 118

JOHNSON,C. N. 1986.Sex-biased philopatry and dis- NUR, N., W. J. SYDEMAN, M. HESTER,AND P. PYLE. persal in mammals.Oecologia 69:626-627. 1998. Survival in Cassin's Auklets on Southeast JOHNSON,M. L., AND M. S. GAINES. 1990. Evolution Farallon Island: Temporalpatterns, population of dispersal:Theoretical models and empirical viability, and the costof double-brooding.Pacif- testsusing birds and mammals.Annual Review ic Seabirds 25:38-39. of Ecologyand Systematics21:449-480. ORING,L. W., AND D. B. LANK. 1984. Breedingarea K1TAYSKY,A. S., AND E.G. GOLUBOVA.2000. Climate fidelity,natal philopatry,and the socialsystems changecauses contrasting trends in reproduc- of sandpipers.Pages 125-147 in Behaviorof Ma- tive performanceof planktivorousand piscivo- rine Animals, vol. 5 (J. Burger and B. L. Olla, rous alcids.Journal of Animal Ecology69:248- Eds.). Plenum Press, New York. 262. PART,T. 1991. Philopatry pays: A comparisonbe- KRESS,S. W., AND D. N. NETTLESHIP. 1988. Re-estab- tween Collared Flycatcher sisters. American lishment of Atlantic Puffins (Fraterculaarctica) at Naturalist 138:790-796. a former breeding site in the Gulf of Maine. Jour- PARTRIDGE,L., ANDP. HARVEY.1988. The ecological nal of Field Ornithology59:161-170. context of life history evolution. Science241: LACK,D. 1968.Ecological Adaptations for Breeding 1449-1455. in Birds. Methuen, London. PERRIN,N., AND V. MAZALOV.2000. Local competi- LIBERG, O., AND T. VON SHANTZ. 1985. Sex-biased tion, inbreeding,and the evolutionof sex-biased philopatry and dispersalin birds and mammals: dispersal.American Naturalist 155:116-127. The Oedipus hypothesis.American Naturalist PETERSEN,A. 1976.Size variables in puffinsFratercula 126:129-135. arctica from Iceland and bill features as criteria LLOYD, C. S., AND C. M. PERRINS.1977. Survival and of age. Ornis Scandinavica7:185-192. age of first breeding in the Razorbill (Alcatorda). PIEROTTI,R. 1981.Male and femaleparental roles in Bird-Banding48:239-252. the Western Gull under different environmental MANUWAL, D. A. 1974a. Effects of territoriality on conditions. Auk 98:532-549. breeding in a population of Cassin'sAuklet. PUSEY,A. E. 1987.Sex-biased dispersal and inbreed- Ecology55:1399-1406. ing avoidancein birds and mammals.Trends in MANUWAL,D. A. 1974b.The natural history of Cas- Ecologyand Evolution2:295-299. sin's Auklet (Ptychoramphusaleuticus). Condor PYLE, P., N. NUR, W. J. SYDEMAN,AND S. D. EMSLIE. 76:421-431. 1997.Costs of reproductionand the evolutionof McGOwAN, J. A., D. R. CAYAN, AND L. M. DORMAN. deferred breeding in the WesternGull. Behav- 1998. Climate-oceanvariability and ecosystem ioral Ecology8:140-147. responsein the Northeast Pacific.Science 281: PYLE, P., L. B. SPEAR,W. J. SYDEMAN,AND D. G. AIN- 210-217. LEY.1991. The effectsof experienceand age on MARKS, J. S., AND R. L. REDMOND. 1987. Parent-off- the breedingperformance of WesternGulls. Auk spring conflictand natal dispersalin birds and 108:25-33. mammals:Comments on the Oedipushypothe- PYLE, P., W. J. SYDEMAN, AND M. HESTER.2001. Ef- sis. American Naturalist 129:158-164. fectsof age,breeding experience, mate experi- MERTZ,D. B. 1971. Life history phenomenain in- ence,and site fidelity on breedingperformance creasingand decreasingpopulations. Pages 361- in a declining population of Cassin'sAuklet. 369 in StatisticalEcology (G. P.Patil, E. C. Pielou, Journalof Animal Ecology70:in press. and W. E. Waters,Eds.). PennsylvaniaState Uni- RABOUAM,C., J.-C. THIBAULT, AND V. BRETAGNOLLE. versity Press,University Park. 1998.Natal Philopatryand closeinbreeding in MILLS,J. A. 1989.Red-billed Gull. Pages387-404 in the Cory's Shearwater (Calonectrisdiomedea). LifetimeReproduction in Birds(I. Newton,Ed.). Auk 115:483-486. Academic Press, New York. REZNICK,D. 1992.Measuring the costsof reproduc- NEGRO,J. J., F. HIRALDO, AND J. A. DONAZAR. 1997. tion. Trends in Ecologyand Evolution 7:42-45. Causesof natal dispersalin the LesserKestrel: RICKLEFS,R. E. 2000.Density dependence, evolution- Inbreeding avoidanceor resourcecompetition? ary optimization,and the diversificationof avi- Journalof Animal Ecology66:640-648. an life histories. Condor 102:9-22. NELSON, D. A. 1981. Sexual differences in measure- ROEMMICH, D., AND J. McGOWAN. 1995. Climate ments of Cassin's Auklet. Journal of Field Orni- warming and the declineof zooplanktonin the thology 52:233-234. California Current. Science 267:1324-1326. NEllSON,D. A. 1989.Gull predationon Cassin'sAuk- SPEAR,L. B., H. R. CARTER,T. M. PENNIMAN, J.P. PEN- let varieswith lunar cycle.Auk 106:495-497. NIMAN, AND D. G. AINLEY.1987. Survivorship NEWTON,1., Ed. 1989. Lifetime Reproduction in and mortality factorsin a populationof Western Birds. Academic Press, New York. Gulls. Studiesin Avian Biology 10:44-56. NUR, N. 1988.The costof reproductionin birds: Ex- SPEAR,L. B., P. PYLE, AND N. NUR. 1998. Natal dis- amining the evidence.Ardea 76:155-168. persalin the WesternGull: Proximalfactors and October2001] Ageand Recruitment in Cassin's Auklets 1007

fitnessconsequences. Journal of Animal Ecology WEIMERSKIRCH,H., AND P. JOUVENTIN.1987. Popu- 67:165-179. lation dynamicsof the WanderingAlbatross, Di- SPEAR,L. B., W. J. SYDEMAN,AND P. PYLE. 1995. Fac- omedeaexulans, of the Crozet Islands: Causes and tors affecting recruitment age and recruitment consequencesof the populationdecline. Oikos probabilityin theWestern Gull Larusoccidentalis. 49:315-322. Ibis 137:352-359. WOOLLER,R. D., J. S. BRADLEY,I. J. SKIRA, AND D. L. SPEICH,S., AND D. A. MANUWAL.1974. Gular pouch SERVENTY.1989. Short-tailed Shearwater.Pages developmentand populationstructure in Cas- 405-417 in Lifetime Reproduction in Birds (I. sin's Auklet. Auk 91:291-306. Newton, Ed.). Academic Press, New York. STATA CORPORATION 1997. STATA statistical soft- WOOLLER,R. D., AND J. C. COULSON.1977. Factors af- wear: Release 5.0. Stata Corporation, College fectingthe age of first breedingof the Kittiwake Station, Texas. Rissatridactyla. Ibis 119:339-349. STEARNS,S.C. 1992. The Evolution of Life Histories. WYLLIE,I., AND I. NEWTON.1991. Demographyof an Oxford UniversityPress, Oxford. increasingpopulation of Sparrowhawks.Journal SYDEMAN,W. J., p. PYLE, S. D. EMSLIE,AND E. B. of Animal Ecology 60:749-766. MCLAREN.1996. Causes and consequencesof ZWICKEL,E C. 1972. Removal and repopulationof long-termpartnerships in Cassin'sAuklets. Pag- Blue Grousein an increasingpopulation. Journal es 211-222 in Partnershipsin Birds (J. M. Black, of Wildlife Management 36:1141-1152. Ed.). Oxford University Press,Oxford. AssociateEditor: D. Nettleship