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Home , Anas, Anatinae, Diving duck, Northern shoveler, Shoveler

The Auk 113(2):505-511, 1996

SecondarySex Ratio in

PETER BLUMS • AND AlVARS MEDNIS Instituteof Biology,Latvian Academy of Sciences,Miera 3, LV-2169, Salaspils,Latvia

Most waterfowl have highly skewedtertiary (adult) (Howe 1977, Fiala 1981, Davies and Payne 1982, sex ratios in favor of males (Bellrose et al. 1961, Al- Weatherhead 1985), parental age (Blank and Nolan drich 1973, Bellrose 1980, Owen and Dix 1986). Sex 1983), size (Howe 1976, Fiala 1981, Mead et al. ratios of diving generally are more distorted 1987), or season(Howe 1977, Patterson et al. 1980, than thoseof dabbling ducks(Mendall 1958,Aldrich Fiala 1981, Ligon and Ligon 1990).A potential bias 1973, Bellrose 1980). Perhaps the greatest disparity in studiesof variation in sex ratios is that only those has been recorded for ( valisi- showing significanteffects are likely to be reported neria),and Common Pochards(Aythya ferina), with (Leblanc 1987). two to three or more malesfor every female in some Here we presentdata on secondarysex ratios at the majorwintering areas(Haramis et al. 1985,Owen and populationand individual levels, as well as investi- Dix 1986). The skewed sex ratio among Temperate gate the relationshipbetween sex ratiosat hatching Zone ducks is of particular interest becausemost of and date of hatching,clutch size, duckling mass,and thesespecies show seasonalmonogamy (reviewed by the age and massof female parent for three Rohwer and Anderson 1988, Oring and Sayler 1992). of wild ground-nestingand over-water-nestingducks. Thus, many maleshave little opportunityfor repro- Further, we examined if parental allocation of re- duction, especiallybecause the secondaryreproduc- sources,as determined by newly hatched duckling tive tacticof forcedcopulation appears to be confined mass, was similar in the two sexes. In addition, we to successfullypaired individuals (McKinney et al. review the published information on secondarysex 1983,McKinney 1985,Gauthier 1988).However, it is ratios in Anatinae and other taxa. important to realize that male-biasedsex ratios in Methods.--Wedetermined the sexfor newly hatched adult ducksare the product,not the cause,of seasonal ducklings of Northern ( clypeata), monogamy(Oring and Sayler 1992). The strong and Common Pochards,and Tufted Ducks (Aythya fuli- consistentskew in tertiary sex ratios raisesthe ques- gula)by cloacalexamination (Hochbaum 1942) as part tion of when and how skewed sexratios develop, and of a long-termpopulation study conducted on Engure suggeststhat femalesmay manipulatesex ratios.If Marsh, Latvia (57ø15'N, 23ø07'E)over a 16-year period females can control the sex ratio of her progeny to from 1978 through 1993. The accuracyof sex deter- increasesurvival of one sex,parental fitnesswill in- mination was enhanced because about 95% of all crease too (Leblanc 1987). ducklingswere sexedby the junior author.Incubating Surprisinglylittle informationhas been published females were captured on the nestsusing drop-door on secondary(at hatching) sex ratios of ducks.Most traps (Blumset al. 1983)or dip nets during the last data come from ducklings hatched in an incubator week of incubation and fitted with conventional leg from eggscollected from wild flee-ranging females bands. and provide no evidence that sex ratios at hatching We obtaineda sampleof known-agefemales using differ from unity (SowIs1955, Mendall 1958,Bellrose two methods. First, more than 65,000 day-old - et al. 1961, Swennen et al. 1979). Dubovsky (1990) lingswere individuallymarked using plasticine-filled hatched captive wild-strain and game-farm female leg bands(Blums et al. 1994). Subsequentrecaptures (Anasplatyrhynchos) in an incubatorand of thesebirds as breeding females allowed us to assign found no evidence that sex ratio in this speciesvaries them an exactage. Second,unmarked incubating fe- with laying order, egg mass,and clutch sequence maleswere agedas either yearlings(one year old) or within a breeding season. adults (two or more years old) using wing- More complete information is available for other characteristics(Blums et al. 1996). There were rela- bird taxa, but the evidence for adaptive control of tively few unmarked (no plasticine-filledor offspringsex is scantand controversial(reviewed by conventionalbands) in the sampleof incubating fe- Clutton-Brock1986, Ligon and Ligon 1990). For ex- males whose progeny were sexed (21, 24, and 33% of ample, some studies report variation in sex ratios at Tufted Ducks, Northern Shovelers, and Common Po- hatchingas a functionof laying or hatching sequence chards,respectively). Conspecificbrood parasitism was common in the diving ducks,but exceptionallyrare in the Northern • Presentaddress: Gaylord Memorial Laboratory, . Widely differing initiation dates, differ- Schoolof Natural Resources,University of Missouri- encesin egg colorationand shape,appearance of two Columbia, Puxico, Missouri 63960, USA. or more eggs per day, recordsof one or more eggs

5O5 506 ShortCommunications and Commentaries [Auk, Vol. 113

TABLE1. Sexratios at hatchingof TABLE2. Sex ratio of one-day-oldducklings in re- ducklingsin 117 clutchesin which all eggshatched lation to their hatching time and mass,and in re- and sexwas determinedfor all ducklings(Engure lation to clutchsize, mass and ageof femaleparent. Marsh, Latvia, 1978-1993). Sex ratio within clutches Data from Engure Marsh, Latvia, 1978-1993. For did not deviate from expected binomial distribu- individual species,the sexratio in eachsubdivision tion (P > 0.05). of independent variable did not differ from 50:50 distribution (z-test,all P > 0.11); all combined G-tests 8:• No. clutches for sex ratio differences between two subdivisions also were not significant(all P > 0.05). 9 eggs 2:7 2 Northern Common Tufted 3:6 8 Shoveler Pochard Duck 4:5 7 Type of 5:4 3 partition M F M F M F 6:3 6 Hatching timea (G = 0.11, 0.80, 0.50) 7:2 2 8:1 1 Early 779 777 276 289 830 842 Late 439 426 229 214 639 683 10 eggs Mass of ducklingsb (G = 0.04, 3.74, 1.66) 2:8 2 3:7 7 Light 586 575 278 246 823 817 4:6 13 Heavy 617 615 188 215 584 639 5:5 16 Clutch size c (G = 0.03, 0.10, 1.02) 6:4 6 Small 622 615 149 151 558 612 7:3 4 Large 574 576 164 158 541 545 8:2 3 Mass of female parentb (G = 0.03, 1.41, 0.26) 11 eggs Light 567 540 160 149 515 522 3:8 2 Heavy 556 537 125 142 525 556 4:7 5 5:6 7 Age of female parent (G = 0.01,1.11, 0.26) 6:5 9 Yearling 486 484 102 89 192 193 7:4 10 Adult 705 696 206 216 896 954 8:3 4 ß(Early) hatchedbefore the yearly œhatching date; (late) hatched after the yearly œhatching date. b(Light) < yearly œ;(heavy) > yearly œ. ' Smalland large clutches were negativeand positive deviations from yearly predicteddecline in clutch size (seeMethods). beingfound outsidethe nestbowl, and excessivesizes of clutches(>13 eggs for Northern Shoveler and CommonPochard; >14 eggsfor ) pro- vided evidenceof brood parasitism.Hatching date z-test; Snedecor and Cochran 1980); and (3) the sex was defined as the date when at least several eggsin ratio of ducklingsdoes dot differ from a 50:50distri- a clutch were externally double-starpipping. We bution during the lifetime of an individual female weighedducklings (+1 g) and incubatingfemales (chi-squaretest of homogeneity;Sokal and Rohlf 1995). (+ 10 g) using Pesolaspring scales. We usedSAS (SAS Institute 1987) for other analyses: Our initial analyseswere based on numerous ar- G-testfor independenceto test the null hypothesis bitrarilyselected subdivisions of the continuousvari- that sex ratios do not differ between two subdivisions ables(i.e. femaleage and mass,clutch size, etc.) in of the independent variable; and Student's t-test to order to maximize possibledifferences in the sex ra- comparethe hatchingmass of the two sexes.We based tios.After not finding significantdeviations from 50: our clutch-sizeanalyses on deviationsfrom predicted 50 sex ratiosin any of thesecomparisons, we based seasonaldeclines in clutch size. Beforeapplication of most of our analysespresented here on positive or the G-test,we regressedclutch size on timeof nesting negativedeviations from yearly meansof indepen- (nest initiation date) and then estimated sex ratios for dent variable. The two subdivisions were approxi- positive and negative residuals.All positive devia- mately equal as suggestedby Clutton-Brock(1986), tions from the yearly predictedvalues were defined and thisprocedure allowed also to controlfor possible as large clutchesand negative deviationsas small annual variation. clutches. We used several statistical tests to evaluate the null Results.--We determined the sex of 2,425 Northern hypothesesthat: (1) ducklingsex frequencies within Shovelers, 3,035 Tufted Ducks, and 1,035 Common clutches follow a binomial distribution (chi-square Pochards immediately after hatching. Data from goodness-of-fittest; Sokal and Rohlf 1995);(2) the sex clutcheswith and without egg mortality were com- ratio in eachsubdivision of the independentvariable bined after we determined that there were no sig- does not differ from a 50:50 distribution (two-tailed nificant differences in sex ratios for these two classes April 1996] ShortCommunications andCommentaries 507

TABI•E3. Sex ratio of one-day-oldducklings for an 0.33). In addition, we found no difference in mass individual female a Northern Shoveler over five between one-day-oldfemales and males (Table 4). yearsof breedinghistory at EngureMarsh, Latvia. Discussion.--Publishedinformation on secondary There was no evidence (chi-squaretest of homo- sex ratios in ducks is scarceand deals mainly with geneity, P > 0.75) againstconsistency in 50:50 sex ratio. incubator-hatchedducklings (Table 5). Although eggs hatched in an incubator were collected from free-

Corn- rangingfemales, an incubatoreffect cannot be dis- missed(Hochbaum 1959:51). This possibilityhas not Femaleage Offspring sex Duck-lings clutchplete yet been testedformally. Analysesof our data and Year (years) Male Female sexed size evaluation of published sourcesdo not indicate the 1982 2 7 4 11 12 presenceof any nonrandomprocess operating on sex 1983 3 5 6 11 11 distribution in 10 Anatinae species (Table 5). Sur- 1984 4 4 7 11 11 prisingly,however, a significant(z = 2.41, P = 0.02) 1985 5 6 5 11 11 femalebias occurred at the populationlevel in a large 1987 7 5 4 9 10 sampleof CommonEider (Somateriamollissima) duck- Total -- 27 26 53 -- lings at KandalakshaNature Reserve,White Sea,Rus- ßNo signsof conspecificbrood parasitism recorded for this female. sia (Shklarevitch and Nikulin 1979). In that study, 3,470 ducklings were sexed by cloacal exami- nation under natural conditions, but authors did not provideany evidenceregarding the accuracyof their of broods(G-test, P > 0.27).Sex ratios of ducklings sexdetermination. We suspectRussian biologists may from 216 nests of Northern Shovelers and Tufted have had a systematicbias toward femalesat the be- Ducks,in whichall ducklingshatched and were sexed, ginning of their five-yearstudy. The percentageof did not differ from the expectedbinomial distribution femaleswere muchhigher in the firsttwo years(52.5 (seeexample for Northern Shovelerin Table 1). For and 56.0%)of the studyand decreasedthereafter (51.4, the , too few data were available 50.0, and 48.6%).In contrast,duckling sex ratios of for statisticalanalysis. Common in The Netherlands (Swennen et al. We did not find significantdifferences in the sex 1979),approximately 2,200 km southwestof the Rus- ratio at hatchingamong broods produced by females sianstudy site, did not differ from a 50:50distribution of differentages, or amongthose hatched at different (Table 5). times during the breeding season(Table 2). The sex We found no seasonal variation in sex ratio with ratio also was independentof clutch size, duckling hatchingdate. This isconsistent with resultsobtained mass,and female parent mass(Table 2). None of the by Sayceand Hunt (1987)on the WesternGulls (Larus sexratios in Table2 were significantlydifferent from occidentalis),but contradictsstudies on severalspecies unity (P > 0.11).Although there were no significant of passetines(Howe 1977,Patterson et al. 1980,Fiala differencesin the sex ratio of ducklingsbetween 1981,Weatherhead 1983, Ligon and Ligon 1990).Our broodswith and without signsof conspecificbrood resultsprovide no evidencethat femalesinvest more parasitism(G-test, P > 0.56), we excluded nestswith energy in eggs that will produce a particular sex mixedclutches from someanalyses (female age, mass, (judgedby newly hatchedduckling mass). Similarly, and clutch size). Few females were monitored for no correlationbetween sex and hatching masswas more than four breedingseasons and none of these found in two speciesof geese(Harmsen and Cooke successfullyhatched all eggsthat were produceddur- 1983, Leblanc 1987) or the Common Eider (Swennen ing their lifetimes.There was no evidence(chi-square et al. 1979).These data suggestthat for most,if not test of homogeneity,X 2 = 1.91, df = 4, P > 0.75) all, speciesof ducksand geesethere may not be an againstconsistency in 50:50sex ratio of ducklingsfor advantageto allocatingmore resourcesto one sex. a highly successfulindividual female during five It is not known how offspringsex ratios vary over breedingseasons (Table 3). Wedid not find significant the lifespan of an individual female, and it will be variationin the sexratios among years over the entire very difficult to obtain this informationfor any spe- studyperiod for all three species(G-test, df = 15, P > cies,especially for free-rangingducks. The maximum

TAI•LE4, Mean + SE (with n in parentheses)of ducklingmass (g) by sexfor three duck speciesat Engure Marsh, Latvia, 1978-1993.

Species Male Female ta Northern Shoveler 27.2 + 0.06 (1,198) 27.3 + 0.06 (1,188) 1.48 Common Pochard 43.6 + 0.16 (478) 43.9 + 0.16 (472) 1.27 Tufted Duck 38.3 + 0.07 (1,425) 38.4 + 0.07 (1,468) 1.17 Student's t-test (two-tailed). All P > 0.05. 508 ShortCommunications andCommentaries [Auk,Vol. 113

longevity of breeding femalesrecorded in this study was at least 14 years (Common Pochard and Tufted Duck); however, few birds were monitored over more than four breeding seasons.A highly successfulfe- male Northern Shovelerwas monitored for five years and hatched53 ducklings,which accountedfor 96.4% of eggs laid during this period. Offspring sex ratio for this individual did not deviate from a 50:50 dis- tribution (Table 3). The causesof disparatesex ratios in adult ducksare not well understood.Hunting mortality rates differ betweenthe sexesin many speciesand may influence the sex ratios among adults (Johnsonand Sargeant 1977);however, there is no consensusconcerning the importanceof hunting effectson sexratios (e.g. Bell- rose et al. 1961, Olson 1965, Aldrich 1973, Owen and Dix 1986, Haramis et al. 1994). Intersexual competition, more specifically male dominance, may contribute to higher overwinter mortality of adult femalesin severalduck species(e.g. Nichols and Haramis 1980, Owen and Dix 1986, Al- exander 1987). Studies on social dominance in win- tering waterfowl haveshown that pairedfemales were more dominant than unpaired females, and males dominatedfemales when their pair statuswas iden- tical (Hepp and Hair 1984, Heitmeyer 1985). More- over, females of late-pairing speciesremained sub- ordinate for longer periods and were more likely to be excludedfrom preferredfeeding sites,particularly during timesof limited resources.Consequently, these speciesappear to have more disparatesex ratios dur- ing the nonbreeding period than speciesthat pair early (Hepp and Hair 1984). Indeed, diving ducks generallydo pair later (Weller 1965,Paulus 1983, Hepp and Hair 1984) and have more distorted sex ratios (Mendall 1958, Aldrich 1973, Bellrose 1980) than dab- bling ducks. The Common Pochardsprobably pair latestamong the temperateduck species (Bezzel 1969) and have one of the most disparateadult sex ratios both in wintering and breeding areas(Bezzel 1969, Owen and Dix 1986, Blums et al. 1990:60). Researchon the breeding ecology of waterfowl during the last three decadeshas led to a belief that high mortalityof femalesduring the breedingseason is a major causeof disparatesex ratiosamong adult ducks,at least in (e.g. Sargeantet al. 1984,Johnson et al. 1992,Sargeant and Raveling 1992, Greenwood et al. 1995, Reynolds et al. 1995; but see Owen and Black 1990:128). The question of adaptive control of offspring sex in wild birds is controversial.Many studies,including thosebased on relatively large samplesizes, indicate that there is no convincingevidence of parentalcon- trol over sexdetermination (e.g. Zwickel and Bendell 1967,Newton and Marquiss1979, Harmsen and Cooke 1983, Leblanc 1987, Ryder and Termaat 1987, Sayce and Hunt 1987; see also Table 5). Evidence of some nonrandompatterns of sexallocation (e.g. Howe 1977, Pattersonet al. 1980, Blank and Nolan 1983, Gowaty April 1996] ShortCommunications andCommentaries 509 and Lennartz 1985, Weatherhead 1985, Breitwisch (D. S. Farner,Ed.). National Academy of Sciences, 1989,Ligon andLigon 1990)may represent an artifact ,D.C. of annualvariations caused by factorsother than nat- ALEXANDER,W.C. 1987. Aggressivebehavior of win- ural selection (Weatherhead 1985, Leblanc 1987), or tering diving ducks(Aythyini). Wilson Bull. 99: caseswhere the null hypothesishas been wrongly 38-49. rejectedby chance(Clutton-Brock 1986). We areaware ANKNEY,C. D. 1982. Sex ratio varies with egg se- of at least two studies (Lesser Snow , Chen ca- quencein LesserSnow Geese.Auk 99:662-666. erulescens[Ankney 1982];Ring-billed Gull, Larusde- BELLROSE,F. C. 1980. Ducks, geese and of lawarensis[Ryder 1983])that may have sufferedfrom North America, 3rd ed. StackpoleBooks, Harris- a typeI errorwhen smallsample sizes gave statistical burg, Pennsylvania. significanceto a nonexistentphenomenon (Ryder and BELLROSE,F. C., T. G. SCOTT,A. S. HAWKINS,AND J. B. Termaat 1987). Collection of additional data in each LOW. 1961. Sex ratios and age ratios in North case(Cooke and Harmsen1983, Meathrel and Ryder American ducks. Ill. Nat. Hist. Surv. Bull. 27:391- 1987) provided no evidencefor laying-ordereffects 474. on sexratios at hatching. BEZZEL,E. 1969. Die Tafelente. Neue-Brehm Buech- Ligon and Ligon (1990)have suggested that adap- erei. A. ZiemsenVerlag, Wittenberg-Lutherstadt. tive manipulationof the hatchingsex ratio possibly BLANK,J. L., ANDV. NOLAN,JR. 1983. Offspring sex mayoccur under certain ecological conditions in some ratiosin Red-wingedBlackbirds is dependenton sexually size-dimorphic altricial speciesthat share maternalage. Proc. Natl. Acad.Sci. USA 80:6141- certainimportant features (e.g. exhibit strongsexual 6145. dimorphismat nestlingstage, experience seasonal or BLUMS,P., J. BAUMANIS,AND P. LEJA. 1990. Natal and temporal variation in food abundance, and charac- breeding philopatry of migratory teristicallyvary in the amountof provisioningassis- males in Latvia. Pages58-64 in Proc. Int. Orni- tancethe female breeder will receive).However, most thol. Conf. Baltic Birds 5, vol. 1 (J. Viksne, and I. studiesthat suggestsome kind of secondarysex-ratio Viiks, Eds.).Riga, 1987.Zinatne Publishers,Riga, adjustmentin birds have been short-term (two to five Latvia. years)and provide little or no evidencethat sexratio BLUMS,P., A. MEDNIS, I. BAUGA,J. D. NICHOLS,AND J. is biased at the population level. The results of our E. HINES. 1996. Age-specificsurvival and phil- study do not supporttheoretical predictions of the opatryin threespecies of Europeanducks: A long- Trivets-Willard hypothesison how selectionfavors term study.Condor 98:(in press). facultativemanipulation of sex ratios among verte- BLUMS,P., A. MEDNIS, AND J. D. NICHOLS. 1994. Re- bratesof polygynousspecies (most ducks are season- tentionof web tagsand plasticine-filledleg bands ally monogamous)under varying environmental cir- applied to day-old ducklings.J. Wildl. Manage. cumstances (Trivets and Willard 1973). We believe 58:76-81. more long-term studieson secondarysex ratios in BLUMS,P., V. REDERS,A. MEDNIS, AND J. BAUMANIS. birds,including waterfowl, are neededand, aspoint- 1983. Automatic drop-door traps for ducks. J. ed out by Cooke and Harmsen (1983), large sample Wildl. Manage. 47:199-203. sizesare necessaryand samplingtechniques that ex- BREITWISCH,R. 1989. Mortality patterns, sex ratios, clude possiblebiases should be used. and parent investment in monogamousbirds. Acknowledgments.--J.D. Nichols first suggested that Curt. Ornithol. 6:1-50. we write this paper. We thank: P. Leja and I. Bauga CLUTTON-BROCK,T. H. 1986. Sex ratio variation in for field assistance;L. H. Fredrickson, J. D. Nichols, birds. Ibis 128:317-329. R. D. Drobney, J.P. Ryder, P. A. Magee, B. D. Dugget, COOKE, F., AND g. HARMSEN. 1983. Does sex ratio an anonymousreviewer, and especiallyF. C. Rohwer vary with egg sequencein LesserSnow Geese? and G. D. Schnell for suggestionsand commentson Auk 100:215-217. earlier drafts of the manuscript;and J. D. Nichols and DAVIES,D.C., AND J. K. PAYNE. 1982. Variation in G. F. Krause for statisticaladvice. P. B. was supported chick sex ratios during hatching.Anita. Behav. by GaylordMemorial Laboratory(School of Natural 30:931-932. Resources,University of Missouri-Columbia,and DUBOVSKY,J. A. 1990. Reproductiveconsequences Missouri Department of Conservationcooperating) of reduced winter-food quality and availability during the dataanalysis and preparation of the manu- in captive, wild-strain and game-farmMallards. script. This is Missouri Agricultural ExperimentSta- Ph.D. dissertation,Mississippi State Univ., Mis- tion project 183, Journal Series#12,333. sissippiState. F•ALA,K. L. 1981. Sex ratio constancyin the Red- winged Blackbird. 35:898-901. LITERATURE CITED GAUTHIER, G. 1988. Territorial behaviour, forced copulationsand mixed reproductivestrategy in ALDRICH,J. W. 1973. Disparatesex ratios in water- ducks. Wildfowl 39:102-114. . Pages482-489 in Breedingbiology of birds GOwATY,P. A., ANDM. L. LENNARTZ.1985. Progeny 510 ShortCommunications and Commentaries [Auk, Vol. 113

sex ratios of Red-cockadedWoodpeckers favors Mountain White-crownedSparrows. Condor 89: males. Am. Nat. 126:347-353. 798-803. GREENWOOD,R. J., A. B. SARGEANT,D. H. JOHNSON,L. MEATHREL,C. E., AND J.P. RYDER. 1987. Sex ratios g. COWARDIN,AND T. L. SHAFFER. 1995. Factors of Ring-billed Gulls in relation to egg size, egg associated with duck nest success in the Prairie sequenceand femalebody condition. Colon. Wa- PotholeRegion of Canada.Wildl. Monogr. 128. terbirds 10:72-77. HARAMIS,G. M., E. L. DERLETH,AND W. A. LINK. 1994. MENDALL,H.L. 1958. The Ring-neckedDuck in the Flock size and sex ratios of Canvasbacks in Ches- North-east. Univ. Maine Bull. 60:1-317. apeakeBay and North Carolina.J. Wildl. Manage. NEWTON,I., ANDg. MARQUISS.1979. Sexratio among 58:123-131. nestlingsof the EuropeanSparrowhawk. Am. Nat. HARAMIS,G. M., J. R. GOLDSBERRY,D. G. MCAULEY, 113:309-315. ANDE. L. DERLETH.1985. An aerialphotographic NICHOLS,J. D., AND G. M. HARAMIS. 1980. Inferences censusof ChesapeakeBay and North Carolina regarding survival and recoveryrates of winter- Canvasbacks.J. Wildl. Manage. 49:449-454. banded Canvasbacks.J. Wildl. Manage. 44:164- HARMSEN,R., AND F. COOKE. 1983. Binomial sex ratio 173. distribution in the Lesser Snow Goose: A theo- OLSON,D.P. 1965. Differential vulnerability of male retical enigma. Am. Nat. 121:1-8. and female Canvasbacksto hunting. Trans. N. HEITMEYER,M. E. 1985. Wintering strategiesof fe- Am. Wildl. Nat. Resour. Conf. 30:121-135. male related to dynamicsof lowland ORING,L. W., ANDR. D. SAYLER.1992. The mating hardwoodwetlands in the upperMississippi Del- systemsof waterfowl. Pages190-213 in Ecology ta. Ph.D. dissertation, Univ. Missouri, Columbia. and managementof breeding waterfowl (B. D. J. HEPP, G. R., AND J. D. HAIR. 1984. Dominance in Batt et al., Eds.). Univ. Minnesota Press, Minne- winteringwaterfowl (Anatini): Effects on distri- apolis. bution of sexes. Condor 86:251-257. OWEN, M., AND J. M. BLACK. 1990. Waterfowl ecol- HOCHBAUM,H.A. 1942. Sex and age determination ogy. Chapmanand Hall, New York. of waterfowl by cloacalexamination. Trans. N. OWEN, M., AND M. DIX. 1986. Sex ratios in some Am. Wildl. Conf. 7:299-307. commonBritish wintering ducks.Wildfowl 37: HOCHBAUM,H.A. 1959. The Canvasbackon a prairie 104-112. marsh, 2nd ed. Stackpole Books, Harrisburg, PATTERSON,C. B., W. J. ERCKMANN,AND G. H. ORIANS. Pennsylvania. 1980. An experimentalstudy of parentalinvest- HOWE,H. F. 1976. Egg size, hatching asynchrony, ment and polygynyin male blackbirds.Am. Nat. sex, and brood reduction in the Common Grack- 116:757-769. le. Ecology57:1195-1207. PAULUS, S. L. 1983. Dominance relations, resource HoWE,H.F. 1977. Sex ratio adjustmentin the Com- use,and pairing chronologyof Gadwallsin win- mon Grackle. Science 198:744-746. ter. Auk:947-952. JOHNSON,D. H., J. D. NICHOLS,AND M. D. SCI-IWARTZ. REYNOLDS,R. E., R. J. BLOHM, J. D. NICHOLS, AND J. E. 1992. Populationdynamics of breeding water- HINES. 1995. Spring-summersurvival rates of fowl. Pages446-485 in Ecologyand management yearling versusadult Mallard females.J. Wildl. of breedingwaterfowl (B. D. J. Batt et al., Eds.). Manage. 59:691-696. Univ. MinnesotaPress, Minneapolis. ROHWER,F. C., AND M. C. ANDERSON. 1988. Female- JOHNSON,D. H., ANDA. B. SARGEANT.1977. Impact biasedphilopatry, monogamy, and the timing of of predation on the sex ratio of prairie pair formationin migratorywaterfowl. Curr. Or- Mallards.U.S. Fish Wildl. Serv.Wildl. Res.Rep. 6. nithol. 5:187-221. LEBLANC,Y. 1987. Relationshipsbetween sex of gos- RYDER,J.P. 1983. Sex ratio and egg sequencein ling and position in the laying sequence,egg Ring-billed Gulls. Auk 100:726-728. mass,hatchling size, and fledglingsize. Auk 104: RYDER,J.P., AND B. M. TERMAAT.1987. Secondary 73-76. sex ratios and egg sequencein Herring Gulls. LIGON,J. D., AND S. H. LIGON. 1990. Female-biased Auk 104:526-527. sexratio at hatchingin the Green Woodhoopoe. SARGEANT,A. g., S. H. ALLEN, AND g. T. EBERHARDT. Auk 107:765-771. 1984. Red fox predationon breeding ducksin McKINNE¾,F. 1985. Primary and secondarymale midcontinentNorth America.Wildl. Monogr.89. reproductivestrategies of dabbling ducks. Or- SARGEANT,A. B., AND D. G. RAVELING. 1992. Mor- nithol. Monogr. 37:68-82. tality during the breeding season.Pages 396-422 McKINNEY, F., S. R. DERRICKSON, AND P. MINEAU. in Ecologyand managementof breeding water- 1983. Førcedcøpulatiøn in waterføwl' Behaviøur fowl (B. D. J. Batt et al., Eds.). Univ. Minnesota 86:250-294. Press,Minneapolis. MEAD, P.S., M. L. MORTON, AND B. E. FISH. 1987. SAS INSTITUTE.1987. SAS/STAT guide for personal Sexualdimorphism in egg size and implications computers,6th ed. SASInstitute, Inc., Cary,North regarding facultative manipulation of sex in Carolina. April 1996] ShortCommunications andCommentaries 511

SAYCE,J. R., AND G. L. HUNT, JR. 1987. Sex ratios of Notes on the sex ratio in the Common Eider, prefiedgingWestern Gulls. Auk 104:33-37. Somateriamollissirna (L). Ardea 67:54-61. SHKLAREVITCH, F. N., AND V. F. NIKULIN. 1979. Sex TRIVERS,R. L., AND D. E. WILLARD. 1973. Natural determinationof day-old ducklingsin the field selectionof parental ability to vary the sex ratio and sex ratios of broods in the Common Eider. of offspring. Science179:90-92. Pages106-112 in The ecologyand morphology WEATHERHEAD,P. ]. 1983. Secondary sex ratio ad- of eider ducksin the USSR(A. A. Kischinski,Ed.). justment in the Red-winged Blackbird (Agelaius Publ. House Nauka, Moscow [in Russian]. phoeniceus).Behav. Ecol. Sociobiol. 12:57-61. SNEDECOR,G. W., AN• W. G. COCHRAN. 1980. Statis- WEATHERHEAD,P.J. 1985. Sex ratios of Red-winged tical methods, 7th ed. Iowa State Univ. Press, Blackbirdsby egg size and laying sequence.Auk Ames. 102:298-304. SOKAL,R. R., AND F. J. ROHLF. 1995. Biometry:The WELLER,M.W. 1965. Chronology of pair formation principlesand practiceof statisticsin biological in someNearctic Aythya (). Auk 82:227- research, 3rd ed. W. H. Freeman and Co., New 235. York. ZWICKEL,F. C., AND J. F. BENDELL.1967. Early mor- SOWLS,L. K. 1955. Prairie ducks:A study of their tality and the regulation of numbers in Blue behavior, ecologyand management.Stackpole . Can. J. Zool. 45:817-851. Books,Harrisburg, Pennsylvania. SWENNEN, C., P. DUIVEN, AND L. A. REYRINK. 1979. Received19 January1995, accepted25 April 1995

The Auk 113(2):511-516, 1996

DNA Fingerprinting RevealsMonogamy in the Bushtit, a Cooperatively Breeding Species

JEFFREYP. BRUCE•, JAMESS. QUINN •, SARAH A. SLOANE2'3, AND BRADLEYN. WHITE • • Departmentof Biology,McMaster University, Hamilton, Ontario L8S 4K1, Canada;and 2T.H. MorganSchool of BiologicalSciences, University of ,Lexington, 40506, USA

Cooperative breeding systems, which are mated males, or birds of both sexes that have characterizedby individuals contributing pa- failed in earlier breedingattempts (Sloane 1992). rental care to offspring that are not their own In approximately19% of nests,helpers have direct descendants, have received much atten- been observedto join prior to or during the tion in the past three decades(Hamilton 1964, egg-layingstage, thereby providing the oppor- Brown 1978, 1987, Emlen 1982).The aid-givers tunity for genetic contributions via extrapair may be nonbreedingadults, in which casethey fertilizations or intraspecificbrood parasitism are usually called "helpers," or they may be (Sloanein press).In addition,the relatively high cobfeedersthat share reproduction with the incidence of double brooding in these birds other group membersof the samesex. (Sloane unpubl. data) may give additional re- The Bushtit(Psaltriparus minimus) is one of the productiveoptions to helpers,if thosejoining first speciesof cooperativebreeders ever de- a nest after the first clutch later become breeders scribed(Skutch 1935). Bushtitsbreeding in the or cobreeders for the second brood. Molecular- ChiricahuaMountains of Arizona display no- genetic studiesare required to investigate the table variation with respectto breeding-group parentagecontributions made by helpers. composition.On averageone-third of the nests The development of DNA probes (e.g. Jef- have more than two attending adults (Sloane freys et al. 1985) that detect high levels of ge- in press). The helpers are predominantly un- netic variation hasgreatly simplified parentage determinationsand also permitted the assign- ment of pairs of to relatednesscate- 3 Current address:Department of Biology,Franklin gories(e.g. Wetton et al. 1987,Burke et al. 1989, and Marshall College,Lancaster, Pennsylvania, 17604, Westneat1990, Piper and Rabenold1992, Quinn USA. et al. 1994, Jamiesonet al. 1994). Overall, studies