Gene Flow Between Breeding Populations of Lesser Snow Geese

GENE FLOW BETWEEN BREEDING POPULATIONS OF LESSER SNOW GEESE

F. COOKE,C. D. MACINNES,AND J.P. PREVETT

THE measurementof geneflow betweennatural populationsof animals in the field has rarely been achievedin populationbiology. The nesting coloniesof Lesser Snow Geese (Chen c. caerulescens)in the Hudson Bay area providea favorablesituation for such measurements--theyare relatively discrete,often separatedby hundredsof miles (Fig. 1), and, at manyof the colonies,large numbers of geesehave beenmarked with leg bands,facilitating the detectionof movementbetween colonies. The Lesser Snow Gooseis dichromatic,comprising blue and white (snow) phases; this dichromatismis controlledby a single gene or tightly linked groupof genes(Cooke and Mirsky 1972). In this paper the names Lesser Snow Goose or Snow Goose will be used to denote the species,and blue and white to denote the color phases. The ratios of blue to white phaseindividuals differ significantlyfrom colonyto colony and have changedwithin coloniesin the recent past (Cooch 1963). A westwardspread of the blue phaseled Coachto postulatesome exchange between colonies. The lack of morphologicaldifferentiation between LesserSnow Geese from different colonies(in contrastto that of Canada Geese(Branta canadensis)nesting in the sameregion that showsignificant regionalmorphological variation (Macinnes 1966)) is consistentwith the considerablegene exchangebetween different Snow Goosecolonies. The two colorphases mate assortatively(Coach and Beardmore1959). Cookeand Cooch(1968), on the basisof a geneticanalysis of the Boas River colony,postulated that individualsselect mates accordingto the color phase of their parents. Mate selectionmight be modified by the relative availability of each color phase at the time of mate selection. Lemieux and Heyland (1967) and Cooch (1961) showedthat white phase birds from the Koukdjuak and Boas River colonies,respectively, tended to have a more westerly fall migration route and wintering range than blue phase birds from the same colonies,although separationwas far from absolute.This phenomenonis hereafterreferred to as differential phasemigration. The migration and winteringdistributions of the two coloniesshared large areasof overlap,indicating that birds from at least two coloniesmixed at this time. If pair formation occurred during this period, interchange between colonies would result unless the geese possesseda specialmeans of recognizingbirds from their own colony or unlessbirds from a singlecolony maintained exclusive flocks. The purposeof this paper is to determine the amount of gene flow

493 The Auk 92: 493-510. July 1975 494 Coo•:E, M^clNN•S, ^NI) PREWTT [Auk, Vol. 92

BREEDING COLONIES OF LESSER SNOW GEESE

BowmonBoy 9 McConnell River Cope Dominion I0 Tho-Anne River KoukdjuakRiver II Lo PdrouseBay Easf Boy 12 Cope Henriella Moria Bear Cove 13 Perry River Boos River 14 Anderson River MaguseRiver 15 Kendoll Islond Wolf Creek 16 Egg River 800 BLUE SNOW miles km

Fig. 1. Showing the distribution, approximate size and phase ratio of all the major (> 1000 birds) Lesser Snow Goose breeding coloniesin North America. The area of the circles indicates relative size of the colonies,with the smallest circles representingcolonies with 5000 breeding birds. The numbers of birds in the larger colonies are extremely difficult to estimate (modified from Dzubin et al. 1974). July 1975] Lesser Snow Geese 495 between the different breeding coloniesand the extent to which Lesser Snow Goose breeding colonies can be consideredas discrete breeding units. To accomplishthis we shall examine: (1) the phase ratios of Hudson Bay Lesser Snow Goosepopulations in breeding and wintering grounds and the relationshipbetween the two, (2) the effects of differential phase migration and the resulting intermingling of geesefrom different colonieson mate selectionand assortativemating, and (3) the extent to which geesereturn to their natal colony or emigrate to another.

BREEDING COLONY DISTRIBUTION An excellent summary of the proportionsof blue and white phase Lesser Snow Geese at the various breeding coloniesup to 1963 is given by Cooch (1963). Dzubin et al. (1975) brought this informationup to date and tried to assessthe size of the various breeding colonies. Fig. 1 summarizes the more recent information and shows the distribution of all Lesser Snow Goose colonies in North America. Occasional nestings and coloniescontaining fewer than 1000 birds have been excluded. From the map we see that the westerncolonies are essentiallymonochromatic and the coloniessurrounding the Hudson Bay drainage are dichromatic. With a few exceptionsblue phase frequencyis higher in the east. Cooch (1963) showed that blue phase geesewere increasingin all colonieswhere estimatesof phaseratio were made for more than a single season,and on the basis of these estimateshe predicted that if present conditionsprevailed, most Hudson Bay colonieswould exceed75% blue phase by 1980. The predicted rate of increasehas not occurred,and present ratios at McConnell River are similar to those describedprior to 1963. Phase ratio counts from the La P•rouse Bay, Manitoba colony from 1968 through 1972 have revealed no changesin the color composi- tion (Table 1). Phase ratios from McConnell River, collectedby several techniques,showed significant differences between years (Table 1), yet no orderly pattern of change,as Cooch (1963) suggested,was evident. Thesedifferences could reflect the methodsof collectingthe data or may indicatereal fluctuationsin phaseratio. To summarize,at the presenttime, white phasebirds predominatein the westerlyparts of the breedingrange, and blue birds in the east. Almostall the birds breedingin the HudsonBay drainagemigrate down the Mississippiand Central Flyways and winter along the Gulf Coast in Texas and Louisiana.

WINTERING GROUND DISTRIBUTION Cooch (1961) indicated that the discrete breeding colonies had relatively distinct migrationroutes and winteringgrounds. When Cooch 496 CooKe, MACI•ES, A•I) PREVETX [Auk, Vol. 92

TABLE 1 PItASERATIOS, PAIR BONDS•AND POPULATION SIZES OE TWO I-IUDSONBAY LESSER SNOW GOOSE COLONIESx

Location White Mixed Blue % Blue Total breeding and year pairs pairs pairs phase pairs

La PdrouseBay 1968 242 73 70 28 1200 ñ 200 1969 439 91 98 23 1960 ñ 100 1970 619 147 162 25 2500 ñ 300 1971 411 91 102 24 1500 ñ 500 1972 850 205 231 26 2500 ñ 500

McConnell River 1964 126 25 41 28 -- 1965 354 59 102 26 -- 1966 270 49 96 29 43,0002 1967 384 91 86 23 -- 1968 5269 a -- 20503 28 -- 1969 1007 165 273 25 75,0002 1970 231 45 44 21 -- 1971 145 32 38 25 92,0002 Blue:whiteratio La P•rouseBay gs•= 7.33 0.I % p % 0.5; McConnellRiver g2, z 34.7 p • 0.005. From Macinnes and Kerbes (MS). Counts of arriving migrants. No pair counts recorded. carried out his studies,Snow Geesedid not regularly stop over in the Dakotas and Missouri. They do so annually now, and this may result in a greater mixing of coloniesthan occurredpreviously. In recent years bandingof LesserSnow Geese has intensified,and samplesof birds have been bandedfrom most of the coloniesin the Hudson Bay drainage. We have analyzedsome of theseband recoveriesin terms of the numbersof each phase that have been recoveredalong the traditional Gulf coast winteringgrounds, defined as 25ø 00' through 30ø 59' N and 85 through 98ø 59' W. All direct recoveries(recovered between the time of bandingand the end of the followinghunting season)were combinedfor this analysis. Table 2 shows the number of geese from each breeding colony reportedrecovered and their phaseratio for each degreeblock of the Gulf coast wintering range. For severalreasons this table does not necessarilyindicate the actual phase ratio or the relative numbersof Snow Geeseusing these areas. Two features stand out. First the winter range of each breeding colony extendsover a considerableexpanse of the Gulf coast, and the range of each breedingcolony overlaps with that of every other colony. Secondlyas a result of differentialphase migration from each breeding colony,the white phasebirds have a more westerlywintering distribution than do the blue phasebirds. Thus althoughclines in the phaseratios July 1975] LesserSnow Geese 497

o • • ',q od •,• ',•' ooo (-.•.u•

11 498 Coox•, MACI•t•ES, At•V P•wtt [Auk, Vol. 92 exist in both breedinggrounds and wintering grounds,geese cannot be identified as to colony or origin by the phase ratios in winter flocks. Two explanationsof differential phase migration have been proposed. Lemieux and Heyland (1967) suggestedthat the two color phases are distinct speciesand thus one might expect their migration routes to be different. Cooch (1958), on the other hand, felt that "like attracts like" in that geeseof each color phase would tend to join flocks comprising birds predominantlyof their own color. The developmentof concepts concerningthe role of early learning (Cooke and Cooch 1968) would modify this theory to suggestthat individuals associatedwith geeseof a color similar to their parents and/or siblings. Becausefew biologists today would agreethat the two color phasesare distinct species,we prefer Cooch'stheory, althoughit too may not be the total explanation. Cooke et al. (1972) showedthat young birds in a captive situation will run to an unknown bird of a color phase similar to that of their parent. This could provide a mechanismwhereby blue phase birds from western Hudson Bay (predominantly white phase), if they had blue phase parents, could become associatedduring migration and on the winteringgrounds with birds from the predominantlyblue phasedeasterly colonies. Reciprocally,it could explain the associationof white phase birds from easterly colonies with birds from the predominantly white phase western colonies.

PAIR FORMATION

Becausethe winter ranges of the various Hudson Bay Lesser Snow Goose colonies overlap, possibly more interchange exists between the breedingcolonies than was hitherto suspected.If pair formation always or usually occurred between birds from the same breeding colony, then the geneticintegrity of the breedingcolonies would be maintainedeven though birds mixed with birds from different colonieson the wintering grounds. This could be achieved,for instance,if pairing occurredin the summer months. Cooch (1958) theorized that first pairing occurred while the 2-year-old subadultscongregated around the periphery of the breedingcolony in associationsof nonbreeders. Observationsof yearling Snow Geesemarked with individually coded neck collars at the McConnell River (Macinnes et al. 1969) revealed no evidenceof pairing at this time. During late winter and while on spring migration, someof these samebirds (now 17-18 monthsof age) were seen engaged in pairing activity (Prevett 1972). Geese from McConnell River were also mixed during spring migration with Snow Geese from other colonies. Hence pairing seems to occur before the nesting seasonat a time when geesefrom several coloniesare freely July 1975] Lesser Snow Geese 499 intermingled. Still the possibilitycannot be excludedthat these pairs representresumptions of associationsoriginally formed in the preceding summer,though this is not supportedby observationsof marked geese. Also, in a captive flock of Snow Geeseat the Niska Waterfowl Research Centre, Guelph, Ontario, the pairing of 2-year-old birds occurred in early March and the pairing did not reflect earlier latent pairings, at least insofar as one could detect by a nearestneighbor analysis (Cooke and McNally 1975) that measuredthe frequencywith which particular birds associatedin the summerbefore visible pair formation occurred. If pair formation doesindeed occur when birds from different breeding coloniesare associating,it does not necessarilyfollow that geese from different coloniespair with one another. Geese from a single colony might associatein exclusiveflocks that would not be detectableat the level of the 1ø longitude block. Sightingsof neck-collaredbirds from the McConnell River during migration and on the wintering ground showedno evidenceof segregation;marked geesewere presentin nearly all large flocks encounteredbetween 92ø 30• and 97ø 30• W along the Gulf coast. Even if there were true mixing of different colonieswhen pair formation occurs, mechanismsmight exist permitting birds to recognizeand pair only with birds from their own breedingcolony. They might perhapsrespond to auditory or visual cuesundetectable by man. Pair bonds in Lesser Snow Geesepersist over several years. Prevett (1972) found no case of a neck-bandedadult paired with a new bird while its mate was known to be still alive. He also showed that if one memberof a pair died, the survivorre-paired.

COLONY INTEGRITY

Although banding has been carried out in most Snow Goosebreeding coloniesaround Hudson Bay, direct quantitative information on the amount of genetic exchangebetween colonies,measured by recapture of banded geeseat different coloniesis very difficult to obtain. Most bandingsat different colonieshave not been coincident,so detectionof two-wayinterchange was not alwayspossible. Nonetheless Table 3 shows interchangeoccurred in mostcases where it couldhave beendetected. We have color marked and banded Lesser Snow Geese with standard aluminumleg bandsat McConnellRiver since1966 and at La P•rouse Bay since1969. The returnof bandedbirds to the coloniesin subsequent years has been assayedin two ways, first by analyzing the previously banded birds captured in mass banding drives of molting geese,and secondlyby countingthe color-bandedgeese among nesting birds. The methodscomplement each other and checkagainst hidden biases. 5OO COOKE,MACINNES, ANn PREVETT [Auk, Vol. 92

TABLE 3 INTERCi:IANGE O1•'SNOW GEESE BETWEEN BREEDING. COLONIES IN Ti:IE HunSON B^¾ ALOE^•

Recaptured at Colony Banded at No. 1 2 3 4 5 6 7

Koukjauk River and 1 -- + + + + q- + Cape Dominion Bowman Bay 2 East Bay 3 q- O -- q- Boas River 4 q- q- q- -- q- q- C) McConnell River 5 q- q- q- q- -- q- q- La P•rouse Bay 6 X X X X q- -- q- Cape Henrietta Maria 7 X X X X q- q- -- •Total geese banded in one colony and recaptured in another: 94 adults and 43 banded as goslings. + = movement is known to have occurred through recapture of geesebanded at a different colony; O = no movement was detected; X = movement could not have been detected.

In terms of colonyintegrity, figureson the rate of return of goslings to the breedingcolony are of particular interest. Table 4 showsthe number of birds of each sex that were banded as goslingsand seen in subsequentyears at the La P•rouse Bay and McConnell River colonies. In contrastto the large proportionof femalegoslings returning to breed, very few males returned to their natal colony. The rate of return re- mained low for males up to 5 years of age, the oldest birds of known age and sex for which bandedsamples were available; 16 females3 years old, caught in 1972 at La P•rouse Bay, represent5.6% of the female goslingsbanded there in 1969. As approximately 25% of the adults presentin the colony in 1972 were caught in the banding drives, this suggeststhat about 22% (5.6% X 4) of the female goslingsbanded in 1969 were breeding at La P•rouse Bay in 1972. This calculation is admittedly crude, but it does indicate that a large proportion of those female goslingshatched at La P6rouseBay, and that were still alive 3 yearslater, werepresent and breedingin their natal colony. A number of explanationsfor the lower return of males are possible: (1) differential loss of bands by male and female, (2) differential mortality of male and female, (3) later nesting of males, (4) bias in sampling techniques,or (5) emigration of males from natal colony. Explanation (1) can be eliminated readily. Aluminum band loss is low at least for the first 5 years for all speciesexamined (Hickey 1952). Mortality figuresfor goslingsbased on hunter kills are given in Table 5. The frequenciesof males and femalesshot showno significantdifferences. Other forms of mortality are more difficult to assess.Most dead adult birds foundat the breedingcolony are females,but total numbersare low. Cooch (1958) believedthat malesnested for the first time when 3 years old, whereasfemales could nest at 2 years of age. Becausethe relative July 1975] Lesser Snow Geese 501

TABLE 4 SEX or BIRDS BANDED AS GOS•;INCS RET•rRNINC TO T•r• NATAl; LA P•ROtrSZ BAY AND McCoNNELL RIVER

La P•rouse Bay McConnell River How Age detected (years) Female Male Female Male

Retrapped 1 19 23 --• -- 2 51 1 10 0 3 84 1 27 3 4 17 -- 16 1 5 -- -- 10 2

Seen nesting 1 0 0 0 0 2 54 4 25 4 3 18 2 26 4 4 -- -- 12 2

Seen, 1 __• __ __a __ not nesting 2 -- -- 39 14 3 -- -- 11 5 4 -- -- 1 0

• Banding drives at McConnell River were not designed to catch nonbreeding geese that molt earlier in flocks largely segregated from breeders at this colony. Nonbreeders associated with breeders more commonly at La P•rouse Bay. 2 Nonbreeding geeseat La PdrouseBay were not studied. a Sex of yearlings observedat McConnell River was not determined. numbersof males in the 2-, 3-, 4-, and 5-year-old age classesdid not change,and as the sex ratio of breeding and nonbreeding2- and 3-year- olds was similar (0.05 < p < 0.10), the probability that males return to their natal colonybut at a more advancedage is remote. By analogy with what is known of other geese (e.g. Bauer and Glutz 1968), that most males delay nesting until they are more than 5 years old seems inconceivable.Our data show that 2-year-old males can breed. Bias in samplingis unlikely to explain the discrepancies.Because two distinct methods were used to collect the data and they agree so well, we feel that the difference in return rate between the sexes is real. The sex ratio of birds in the banding drives is approximately1:1, and we have noted no obvioussurplus of either sex on the breedingcolony. As the above hypothesesare unlikely, the most plausible explanation for the disappearanceof banded male goslingsfrom the breedingcolonies is emigration. Becausethere are equal numbersof each sex in the breeding population, one must infer that there has been compensatingim- migration of males from other colonies. Direct evidenceof immigration is difficult to obtain becausemuch of the arctic bandinghas concentrated on banding as many birds as possible,and consequentlythe sexeswere usually not determined. Four birds of known sex banded as goslings have been recaptured in different colonies2 or more years later and three of these were males. 502 COOKE, MACINNES, AND PREVETT [Auk, Vol. 92

TABLE 5 PROPORTION O•F MALE AND FEMALE SNOW GEESE BANDED AS GOSLINGS

Number Reported Colony Sex banded shot %

La P•rouse Bays Male 2103 204 9.7 p • 0.10 1969-1971 Female 2127 192 9.0

McConnell Male 1797 303 16.9 p • 0.10 River • Female 1816 301 16.6 1966-1968

Just direct recoveries. Includes indirect recoveriesup to 1971.

It seems that females hatched in a particular colony generally will return to that colony for at least the first nesting, but that the males do so only rarely. This suggestsstrongly that pair formation does not usually occur between membersof the same breedingcolony, and implies that mate selectionprobably occurs outside the breeding season,when birds from different colonies are associating. A further conclusionis that when two birds from different coloniespair, the female determines the colonyin which the pair will nest. The phenomenonof differential return to the breedingcolony is not restrictedto birds banded as goslings.Table 6 showsthe numbersand sexesof geesebanded as adults at McConnell River and at La P•rouse Bay that were subsequentlyretrapped at those colonies. The proportion of malesamong the retrappedbirds decreasesprogressively. Adult females from McConnell River were more vulnerable to hunting than males. Of 974 adult females 189 (19.4%) banded in the years 1966 through 1968 were subsequentlyreported shot as againstonly 173 of 1139 adult males (15.2%). This difference is significant at the 5% level, which makes the preponderanceof females retrapped in later years even more meaningful. At La P•rouse Bay the sexesdo not differ significantlyin numbers reported shot. More data on the samephenomenon came from visual sightingsduring the periods of incubation and hatch of mated pairs marked with colored leg bands (La P•rouse Bay) or neck collars (McConnell River) (Table 7). Banding drives (Cooch 1956) were designedto catch flocks com- posed of adult pairs and their goslings. Every adult was not always caught and somepairs had just one bandedbird. As almost equal numbers of males and females were banded at each location, these pairs should have contained equal numbers of banded males and females. Recapture in subsequentyears, however, showsa large excessof pairs where only the female was banded. The excesswas greater in the second July 1975] Lesser Snow Geese 503

TABLE 6 SEX RATIO O• ADULTSBANDED AND SUBSEQUENTLYRECAPTURED AT McCoNNELL RIVER AND LA P•ROUS• BAY

Number captured Number banded Banded % at Year Female Male % Male Female Male Male

McConnell 1966 261 280 52 46 22 32 • River 1967 417 593 59 34 28 452 1968 296 266 47 20 11 35 TOTAL 974 1139 54 100 61 38 •

La P•rouse 1969 45 28 38 17 5 23 Bay 1970 539 504 48 149 99 40• 1971 275 286 51 55 42 ' 43 TOTAL 859 818 49 221 146 41 •

•p < 0.05. 2 p < O.Ol. and subsequentyears than in the year immediatelyfollowing banding, indicatinga continuingdisappearance of markedmales. The data in Table 7 thereforestrongly suggesta loss of male birds among previouslymated pairs. As male mortality is not substantially higher than female mortality, at least as evidencedby reportedhunter kills, the greaterloss of malesamong previously mated pairs suggests that someof the malesthat did not return to the colonywhere banded are still aliveand breedingelsewhere. A workinghypothesis to explain the greater loss of adult malesis as follows:when a pair is broken(usually by deathof onemember), a new mate is acquired.A femalewill return with her new mate to the colony in whichshe nested the previousyear; a male will accompanyhis new mate to where she previouslynested. BecauseLa P•rouse Bay is a small colony and pair formation takes places in flocks of mixed origin, the probability of a La P•rouse Bay female choosinga new mate from her own colony is small. Consequentlyif a male dies, he is lost to the breeding colony but his mate is not, whereasif a female dies both birds are usuallylost to the breedingcolony. If the chanceof death of one member of a pair were independentof the death of the other (this is unlikely to be strictly true), then the rate of return of malesto the colonywould be the squareof the return of the females. This relationshipassumes that the size of the colony is small relative to the size of the total Hudson Bay population. The McConnell River colony is much larger than that at La P•rouse Bay, and hencemore widowedmales from this colony could be expected to remate with McConnell River geese. This expectationis borne out 504 COOKE,MACINNES, AND PREVETT [Auk, Vol. 92

TABLE 7 PAIRS OF SNOW GEESEBANDED AS ADULTS AND FOUND NESTING IN SUBSEQUENTYEARS AT COLONY WHERE BANDED

Year banded Sightedin subsequentyears Mc- La Connell P•rouse Both Only 9 Only • Both Only 9 Only • River Bay banded banded banded banded banded banded

1 year later 2 years later

1966 -- 11 19 18 6 14 4 1967 -- 30 $ 14 10 18 8 1968 -- 74 18 6 15 29 4 1969 -- 54 25 10 ------

TOTAL 67 48 61 16 (42%) (21%)

-- 1970 ------58 43 3 -- 1971 58 23 5 41 30 2 -- 1972 73 66 15 TOTAL 89 20 73 5 (18%) (6%)

3 years later 4 years later

1966 -- $ 10 7 2 16 2 1967 -- 1 14 7 ------TOTAL 24 14 16 2 (37%) (11%)

-- 1970 44 60 3 TOTAL 60 3 (5%) by the data in Table 7. A higher proportion of pa/rs containing just color marked males was found at McConnell River than at La P•rouse Bay 1 and 2 years after banding (p < 0.05). If one makesa few simplifyingassumptions, the relative rate of male and femalereturn to nestingcolony can be formulatedmathematically for any size of colony. Assumethat if a female survivesbetween breeding seasonsshe will return to the breedingcolony. Assumealso that if death of one memberof a pair occurs,a new pair will be formed. Let p = probabilityof male survival: probability of female survival. No = number of birds from particular breedingcolony where mate selectionis occurring. NT = total number of birds where mate selectionis occurring. Return of femalesis by our simplifyingassumption p. A male will return if: July 1975] LesserSnow Geese 505

(1) both he and his mate survive; probability = p x p = p• (2) his mate is killed (1 - p) but he survives (p) and pairs with another bird from the same breeding colony as his former mate;

probability= • (p-p2) Return of males is then: + Nc ( Ne '• 2 If the colonynumber were small relative to the sizeof N•< NT; p2+(•--•) Nc p_ [Ne•,•-• ]p'• •pS If N• were very large relative to N•; Nc • N•:

N• Nc

T•s formula can be refined as more is learned about the validity of the assumptionsmade. As an exampleof how this formula can be used, if a sample of breeding adult geesewere marked at a small breedingcolony such that N• N•, the proportionof marked geesestill nestingat the colonyin the t th year after marking would be approximatelypt for females and p2t for males, where p is the annual survival rate. It must be rememberedthat it is assumedthat all survivingfemale geesereturn to the breedingcolony and that all geesethat lose their mates pair again. As neither of these assumptionsis strictly correct,p representsa minimal estimateof survival. In Table 8 the percentageof male and femalesremaining on the breeding colony after a number of years has been calculated theoretically on the assumptionof p = 0.75 and from actual data from the La P•rouse Bay colonybased on birds bandedin 1970 and 1971 and resightednesting on the colony in 1972 and 1973. The percentageswere calculated from counts of a large proportion of the nesting birds (30-50%), assuming that they representeda random sampleof the colony, and extrapolating from the number of birds seencarrying bands of the particular year class to estimate the total number of birds nesting on the colony. Although the evidence given so far shows that exchange between colonies is much more prevalent in the males, we have some direct evidence that females also occasionallymove from one colony to another. Three breeding femalesbanded at McConnell River were found nestingat La P•rouse Bay in 1972, and another female nestedat McCon- nell River and Bowman Bay in successiveyears. Two adult females 506 COOKE,MACINNES, AND PREVETT [Auk, Vol. 92

TABLE 8 PERCENTAGE O1* MALE AND FE1YI•ALE ADULT GEESE FROM A MARKED SA3A'PLE STILL PRESENTAT A BREEDINGCOLONY IN SUBSEQIJENTYEARS

Sex 0 1 2 3 4

Theoretical example (p = 0.75): Female 100 75 56.2 42.1 31.6 Male 100 56.2 31.6 17.8 10.0

Percentages observed at La P•rouse Bay: Banded in 1970 Female 100 -- 54 -+- 3 40 -+- 2 Male 100 -- 32 -+- 2 20 -+- 2 Banded in 1971 Female 100 80 __+4 55 -+- 3 Male 100 65 __+3 37 __+2 bandedat La P•rouse Bay in 1970 were caughtin the bandingdrives at McC0nnell River in 1971. Because the La P•rouse Bay colony is on the migration route of birds from McConnell River, it is perhaps not surprisingthaf some exchangeof females occursbetween the colonies. There seemsto be a similar exchangebetween the Cape Henrietta Maria and Baffin Island colonies.

DISCUSSION This discussionis based on four major premises,the evidence for which is given in this or earlier papers. (1) Mate selectionoccurs during spring migration or on the wintering groundwhen birds from severalcolonies are mixed (this paper). (2) When birds from different coloniesform pairs, each pair return to the female'snatal colonyto breed (this paper). (3) Differential phase migration occurs such that white phase birds from each breedingcolony have a more westerlyfall and winter distribution than blue phase birds (Cooch 1963, Lemieux and Heyland 1967, this paper). (4) Mate selectionin terms of color is determinedlargely by an early learning processwhereby birds select mates of a color phase similar to that of oneof their parents(Cooke and Cooch1968). In a speciessuch as the Lesser Snow Goosewhere pair bonds usually last from year to year, the timing and nature of the first pairing is important to an understandingof the genetic integrity of the various breedingcolonies. Evidencepresented above showsthat a large proportion of the female goslingshatched at La P•rouse Bay return to breed at the natal colony. In contrast the return rate of male goslingsis extremely low at both coloniesinvestigated. If one analyzesretrap figures only, of thosebirds bandedas goslingsand recoveredat the natal colony July 1975] Lesser Snow Geese 507

2, 3, 4, or 5 years later, 8 were males and 215 were females (Table 4). This strongly suggeststhat males return to breed in their natal colony only if they happen to pair with a female from the same colony. This appearsto be a rare event, although it is more commonin the larger colonyas wouldbe expected. SnowGeese and perhapsall geeseseem to be similar to many ducksin which pair formation often takes place in winter quarters and during migration (Weller 1965), in that mates are often natives of placesremote from each other and usually the male returns with the female to her natal area. Banding returns show that females do not always return to their natal colony to breed, which is consistentwith the facts that severalnew colonieshave been establishedrecently and other sites may be used intermittently as breedinggrounds (Hanson et al. 1972). Perhapsthe most important feature of these findingsis the realization that the amountof interchangeof breedingbirds betweengeographically distinctbreeding colonies is extremelyhigh. This geneflow is largely a functionof the male pattern of dispersion,whereby most malesbreed in a different colony from the one where they were hatched, but some contributionto geneflow is providedby femalesthat nest in a nonnatal colony. At La P•rouse Bay almost all the malesand perhapssome of the femaleshatched at a different colony. This suggeststhat approximately 50% of all the birds breedingfor the first time are immigrants, hatched elsewhere.The colony at La P•rouse Bay may have a high immigration and emigrationrate in that it is relatively small and is on the migrationroute of severalmore northerly colonies. Birds banded in one colony and recoveredin another give information suggestingthat althoughexchange is more frequentbetween geographically closer colonies,exchange occurs between nearly all colonies,indicating little barrier to geneflow. Restrictionsto geneflow do occuras a result of the tendencyof femalesto return to the samecolony year after year, and from the fact that pair bondslast for more than one breedingseason. Adults also move between colonies,but we have not incorporatedthis into our calculationsof gene flow. The effects of this are at present difficult to assess,but they may be considerable. With such a large amount of gene flow between the distinct breeding colonies,it is likely that few geneticdifferences exist between colonies, apart from any genesthat may be locatedon the W chromosomeof the heterogameticfemale geese. The Hudson Bay and Foxe Basinpopulations of SnowGeese may be usefullyconsidered as a large interbreedingpopu- lation that is largely homogeneousin its geneticcomposition. The lack of morphologicaldifferences between birds from these coloniesis con- sistent with this contention. 508 COOKE,MACINNES, AND PR•V•TT [Auk, Vol. 92

The realization that the Hudson Bay populationcould be considered as one population from the genetic standpoint suggeststhat genetic differences should not be found between the various colonies. This con- clusion is in contradiction to the known facts summarized earlier in the paper regardingfrequencies of the gene controllingthe color phase. In other words, the one gene frequencywhose value is known contradicts the evidenceof extensivegene flow in that not all colonieshave a similar color phase ratio. How then, assuminga large amount of gene flow, have different coloniesbeen able to maintain their characteristic color phase ratios? Differential phase migration and assortativemating may provide the answer to this paradox. Both these phenomenacan be explainedon the basisof early learning wherebygoslings will usually associatewith and later pair with birds of a color phasesimilar to that of their parents. Mate selectionin terms of color dependson preference and prevalence. Birds pairing for the first time not only choosemates accordingto the color phase of their parents but also, becauseof differential phase migration, they are located in flocks where that phase predominates. The relative effects of preference and prevalence are at present difficult to assess. Althoughgenes are able to flow rapidly throughthe wholepopulation becauseof the considerablegene flow, apparently the movementof the allelesconcerned with color do not result in changesin phase ratio at a colonyas the genefrequency (in terms of color) of the birds enteringa colony will be determinedby the nonimmigrantbirds of that colony, which choose the color of their mates. To be more specific, in a hypothetical colony consistingonly of white birds that mixed on the winteringground with blue and white phasebirds from severaldifferent colonies,the femaleswould choose mainly white mates,and would return to their natal colonies. The immigrant males could be from any of the other colonies,so there would still be considerablegene flow between colonies,but alleles controlling the blue color would rarely enter the colony, thus maintaing the phase ratio in the face of considerablegene flow. The present distribution of the color phasesis partly determined by the historical events that led to the origin of the two phasesand partly by the traditions (of mate selectionand migration) that slow down the equilibrationof the phaseratios.

ACKNOWLEDGMENTS

These studies formed part of an investigation being carried out under contract with the Canadian Wildlife Service. Financial assistance was also provided by the National Research Council; the Manitoba Department of Mines, Resources and Environmental Management; the Arctic Institute of North America; and the Canadian National Sportsmen's Show. We also thank T. Bargiello, H. Boyd, P. July 1975] LesserSnow Geese 509

Colgan,G. Finney, R. Rockwell,J. P. Ryder• and W. N. Holsworthfor their helpful commentson an earlier draft of this paper. Considerablelogistic help was provided by the personnelof the N.R.C.'s Churchill Research Range. Special thanks are due to Father Lionel Ducharme, O.M.I., and to the personnelof the Departmentof Indian and NorthernAffairs, and the Hudson'sBay Company at Eskimo Point, who went out of their way to help. To the numerousassistants who survivedmosquitoes, polar bears,and banding driveswe are mostgrateful. We thank F. G. Cooch,T. W. Barry, H. G. Lumsden,L. Lemieux,and R. H. Kerbesfor permissionto usetheir unpublishedbanding data. The CanadianWildlife Service,through A. Dzubin, compiledFig. 1.

SUMMARY

Colorphase ratios of the HudsonBay and Foxe Basinpopulations of the LesserSnow Goose(Chen c. caerulescens)were tabulatedon the breedinggrounds and winteringareas. Differentialphase migration (i.e. the tendencyof whitephase birds to havea morewesterly migration than blue phasebirds) occurs,and birds from the different breedingcolonies mix considerablyon the winteringgrounds and in migration. Returnsof bandedgoslings show that whereasfemale geesefrequently return to the natal colony to breed, males seldomdo so. The lack of return of males is attributed to emigration. Most, if not all, pair formation occurson the winteringgrounds and during springmigration, when birds from different breedingcolonies are associating.When two birds from different coloniespair, the female usually returns with her mate to her natal colony. Geneflow per generationat the La P•rouseBay colonyis estimated to be approximately50%. At larger coloniesthe value is expectedto be somewhatlower. In view of this large õene flow, the Hudson Bay populationsof the LesserSnow Geese perhaps should be consideredfrom the geneticstandpoint as a singleinterbreeding population. In view of this, the differencesin color phaseratios at different breedingcolonies are puzzling. This paradoxcan be resolvedby assumingthat the early learningexperience that resultsin bothassortative mating and differential phasemigration restricts the rates at whichcolor phase ratios changeat the different colonies.

BA•r•R,K. M., A•D U. N. G•.•rrz vo• B•.•rTZ•r. 1968. Handbuchder Viigel Mitteleuropas.Bds. Anseriformes(1. Teil). Frankfurt am Main, Akademische Verlagsgesellschaft. Cooc•, F. G. 1956. Mass ringingof flightlessBlue and LesserSnow Geesein Canada'seastern arctic. Wildfowl Trust Ann. Rept. 8: 58-67. Cooc•, F. G. 1958. The breedingbiology and managementof the Blue Goose. UnpublishedPh.D. dissertation,Ithaca, New York, Cornell Univ. 510 COOKE,MACINNES, ANY PREVETT [Auk, Vol. 92

Cooca, F. G. 1961. Ecological aspectsof the Blue-Snow Goose complex. Auk 78: 72-89. Cooca, F. G. 1963. Recent changesin distribution of color phasesof Chen c. caerulescens.Proc. 13th Intern. Ornithol. Congr.: 1182-1194. Cooca, F. G., ^Ni• J. A. Br^RV•ORr. 1959. Assortative mating and reciprocal difference in the Blue-Snow Goose complex. Nature 183: 1833-1834. Cooxr, F., ^Nv F. G. Cooca. 1968. The genetics of polymorphism in the goose Anser caerulescens. Evolution 22: 289-300. CooTs, F., AND C. M. MCNALLY. 1975. Mate selectionand color preferencesin LesserSnow Geese.Behaviour, in press. CooTs, F., ANI• Mmsx¾, P. 1972. A genetic analysis of Lesser Snow Goose families. Auk 89: 863-871. CooxE, F, P. J. Mmsx¾, ANY M. B. S•XC•R. 1972. Color preferencesin the Lesser Snow Goose and their possiblerole in mate selection.Canadian J. Zool. 50: 529-536. DZUBXN,A., H. BoYv, AND W. J. D. ST•PaEN. 1975. Blue and Snow Goose distribution in the Mississippi and Central Flyways: a preliminary report. Progr. Notes, Canadian Wildl. Serv., in press. HANSON,H. C., H. G. LU•SV•N, J. J. LYNch, AND H. W. NORTON. 1972. Popula- tion characteristics of three mainland colonies of the Blue and Lesser Snow Geese nesting in the southern Hudson Bay region. Res. Rept. (Wildl.) 93, Ontario Min. Nat. Resources. H•cx•¾, J. J. 1952. Survival studies of banded birds. U.S. Fish Wildl. Serv., Spec.Sci. Rept., Wildl. No. 15. LEM•EtrX, L., ANI) J. HEYLAND. 1967. Fall migration of Blue Geese and Lesser Snow Geese from the Koukdjuak River, Baffin Island, Northwest Territories. Natur. Canadien 94: 677-694. MAClNN•S, C. D. 1966. Population behavior of eastern arctic Canada Geese. J. Wildl. Mgmt. 30: 536-553. MACINNES, C. D., J.P. PRIWF.TT, ANY H. A. EDN•¾. 1969. A versatile collar for individual identification of geese. J. Wildl. Mgmt. 33: 330-335. PR•w:r:r, J.P. 1972. Family behavior and age-dependent breeding biology of the Blue Goose (Anser caerulescens). Unpublished Ph.D. dissertation, Univ. Western Ontario, London. W•;•;ER, M. W. 1965. Chronology of pair formation in some Nearcftc Aythya (Anatidae). Auk 82: 227-235.

Departmentof Biology,Qucen's University, Kingston, Ontario, Canada, and Department of Zoology, University of Western Ontario, London, Ontario, Canada. Presentaddress of secondauthor: Fish and Wildlife Research Branch, Ontario Ministry of Natural Resources,Maple, On- tario, Canada. Present address of third author: District Biologist, Ministry of Natural Resources,Moosonee, Ontario, Canada. Accepted 3 June 1974.