JOHNNY KAHLERT', MALCOLM COUPE2 and FRED COOKP

WINTER SEGREGATION AND TIMING OF

PAIR FORMATION IN RED-BREASTED MERGANSER

MERGUS SERRATOR

JOHNNY KAHLERT', MALCOLM COUPE2 and FRED COOKP

I National Environmental Research InstitUte. Dept. of Coastal Zone Ecology. Grenavej 12. DK-841 0 Ronde.Denmark

'Simon Fraser University, Dept. Biological Sciences, 8888 University Drive, Burnaby. B.C.. VSA 1S6 Canada

During the late winter and spring of 1997, sex-ratios and pairing chronology of Red-breasted Mergansers were studied in the Fraser River Estuary. south west Canada.Until late March at one site, English Bay, there was a strong male-bias. whereas a femalebias was evident at the nearby Boundary Bay. This pattern was interpreted as sexualwinter segregation. At the male-biased site. antagonistic and kleptoparasitic behaviourinitiated by males towards females could not account for the winter segregation. In lateMarch and April, the sex ratios at the two study sites converged to similar values. Thistrait coincided with an increase in the number of courtship displays and percentage ofpaired birds. Although the majority of mergansers exhibited this pairing chronology.pairs were observed as early as mid-February. suggesting a gradient in the timing ofpair formation. Pairs tended to segregate from flocks. If pairs are able to segregatethroughout the winter, early pairing may be beneficial as food resources could bemonopolised and interference with con specifics avoided. However, the frequency ofinteractions with con specifics tends to be unpredictable, since local population sizefluctuated considerably.

Keywords: Red-breasted Merganser, Sex-ratio, Winter Segregation, Pair Formation, Courtship,Antagonistic Behaviour

Introduction

In populations of unpaired Anatidae, males tendto be dominant (Paulus 1983. Hepp & Hair1984, Alexander 1987) which enables them toexclude females and immatures from the higherquality habitats (Choudhury & Black 1991). Theconventional outlook regarding timing of pairformation in the waterfowl of the northernhemisphere has developed from the idea thatpairing benefits the female by increasing herdominance rank. which provides enhancedforaging opportunities (Afton & Sayler 1982,Paulus 1983, Hcpp & Hair 1984). By contrast,

©The Wildfowl & Wctlands Trust

pairing may lead to energetic costs to males,which must perform a strenuous courtshipbehaviour (Afton & Sayler 1982) andsubsequently guard their females in thepresence of unpaired males, restricting themobility of the paired male and interruptingfeeding activities (Spurr & Milne 1976, Paulus1983,Wishart 1983).

Assuming that females benefit from pairingand suffer a cost by the use of male-biasedhabitats while unpaired, either in increasedwinter mortality or decreased reproductiveoutput, pair formation should occur shortlyafter unpaired birds arrive at their wintering

WILDFOWL (1998) 49: 161-172

162 WINTER SEGREGATION AND PAIR FORMATION IN MERGANSERS

grounds, in autumn and early winter (pairformation strategy one). However, if it is in theinterest of individual males to participate in theexclusion of females from higher qualityhabitats, while it is costly to males to form pairbonds, the sexes should become spatiallysegregated during the winter, and pairformation should occur in late winter or earlyspring (pair formation strategy two). Theassumptions behind the prediction of pairformation strategies suggest a conflict, if eithersex try to maximise their reproductiveinterests (Trivers 1972). By early pairing underthe above assumptions males are expected tocompromise their interests.

In many diving ducks, pair formation occursshortly before migration to the breedinggrounds (Rohwer & Anderson 1988), which isin contrast to many species of dabbling duckswhose pair formation occurs in autumn orearly winter. Among the late-pairing divingducks, there are several examples of habitats,which are dominated by females during thewinter, for example for Goldeneye Bucephalaclangu/a and Goosander Mergus merganser(Nilsson 1970, Anderson & Timken 1972,Duncan & Marquiss 1993) or at least divergingfrom the usual pattern of strong male bias inthe winter quarters, for example forCanvasback Aythya valisineria, Pochard Aythyaferina, Bufflehead Buchaphala a/beo/a, LesserScaup Aythya affinis, Ring-necked Duck Aythyacol/aris and Ruddy Duck Oxyura jamaicensis(Nichols & Haramis 1980, Bergan & Smith 1989,Carbone & Owen 1995). Habitats with a biastowards females, as well as habitats with strongmale bias which cannot be accounted for by themale biased sex ratio of the population, areindications of sex class segregation.

If sex class segregation during the winter is aresult of males excluding females from goodforaging habitats (Nichols & Haramis 1980), wewould predict that females in male-biasedhabitats would be subject to antagonistic andkleptoparasitic behaviour rather than courtshipdisplays. We would further predict that adultmales would initiate more antagonistic andkleptoparasitic interactions towards femalesthan females would towards males. We would

also expect males to be more successful inthese interactions, either by chasing femalesaway or by stealing prey items from them.

In this study, sex ratios during the non-breeding season and timing of pair formationwere studied in an unmarked population ofwintering Red-breasted Merganser Mergusserrator. The aim was to determine whetherone of the proposed pair formation strategiesis an accurate description of its behaviour. Inorder to test whether adult male dominance isthe cause of sex class segregation, we recordedincidences of antagonistic behaviour andkleptoparasitism.

Methods

Counts and sex ratio estimation

The study was conducted at two sites in theFraser River Estuary of British Columbia,Canada. English Bay and Boundary Bay areapproximately 30 km apart (Figure I).Boundary Bay comprises relatively shallowwater « 4 m), whereas the deeper parts ofEnglish Bay extend down to 20 m, areas whichwere frequently visited by the mergansers.

In 1997, English Bay was visited 28 timesduring mornings from sunrise and three hoursahead in the period 17 February to 24 April.Boundary Bay was visited nine times in the latemorning between 22 February and 23 April.Observations were conducted as a series ofpoint counts from strategic positions along thecoastline (points, bridges etc.). The watersurface was scanned slowly in order to give fullobservational coverage to the entire pointcount area. All Red-breasted Mergansers werecounted, and if possible aged and sexed using a20-60 x telescope. Double counts wereminimised by noting the movements ofmergansers.

The Red-breasted Merganser acquiresbreeding plumage in November-December (egPalmer 1962), yet adult females and immaturebirds ('brown heads') may be difficult todistinguish at a distance and in poor light. Oneidentification mark which appears to be reliablefor adult females is a blackish spot around the

WINTER SEGREGATION AND PAIR FORMATION IN MERGANSERS 163

North

10 Km

t-----

,------------------\

\

\

Canada

U.SA

Figure I. Map showing the position of the two study areas near Vancouver, BritishColumbia.

eye which sometimes extends along theforehead. Examination of museum specimenscollected I January to 30 April in BritishColumbia, Greenland and Denmark showedthat this trait was likely to be diagnostic. Onlyone out of 24 examined adult females had noblack spot and this was a Danish bird frombefore 14 February, ie before the study period.

A proportion of 'brown-headed' birds couldnot be sexed and aged due to poor observationconditions. In order to estimate the number offemales present, a model was used which

incorporated the number of females andimmatures observed, and the number ofundetermined adult females or immatures(hereafter referred to as IF's). Assuming thatthe probability of identifying an IF as animmature or an adult female is a linear functionof their respective frequencies in the countpopulation, we predicted:


Where Fw,= total number of adult females, Fobs=number of observed adult females, lobs= numberof observed immatures and IFunk= number ofbirds which could not be aged or sexed.

The sex ratios were calculated thus:

Where Mw, represents the total number ofadult males.

Pairing chronology

A flock was defined as a unit of mergansers,that stayed close together « 50 bird lengths)for a period of at least two and a half minutesduring a five-minute focal sampling period.Within a flock, a pair was defined as an adultmale and female which exihibited synchronisedbehaviour during the entire focal samplingperiod (five minutes). Observations ofcopulation events, and adult males consistentlyguarding females against males were alsointerpreted as being indicative of a pair bond.Mate-guarding was exhibited either as swim-guarding, in which the male consistently placedhimself between the female and another male,or as aggressive behaviour such as charging andbill threats.

The pairing chronologies were prepared byplotting the proportion of females in pair bondsfor each date of observation. It was consideredthat the difficulty of distinguishing adult femalesfrom immatures had the potential to lead to anunderestimation of the number of pairs. Forexample, flocks of IF's with no adult malescannot be paired, but a flock consisting of anadult male and an IF may have been a pair.Therefore, the number of pairs in each pointcount had to be modelled. All observedcombinations of IF's and adult males wereincluded in the model, but groups of one maleand one IF were the only compositions tocontribute to an increase in the suggested totalnumber of pairs. Therefore, the model could bewritten:

Where P'o'= total number of pairs, Pobs=observed number of pairs, MlFseg= number offlocks with one adult male and one IF P =, seg

number of flocks consisting of a pair, and Mlseg=number of flocks consisting of an adult maleand an immature.

Interactions between unpaired males and unpairedfemales

In order to determine the effect of thepresence of each sex on the behaviour ofunpaired individuals before and during thepairing process, the response of unpairedindividuals was assessed for individualmergansers sitting in the same flock asconspecifics of the opposite sex. Threeresponse types, namely antagonistic behaviour,courtship and no response, were noted duringfocal observations for periods of five minutes.At English Bay the response types of 105unpaired males and 15 females were sampled.

Antagonistic behaviour was defined asswimming raids with bill charging towardsconspecifics. Kleptoparasitism was noted whenan individual brought prey to the water surfaceand other mergansers tried to steal the prey. Inall incidents of antagonistism andkleptoparasitism, the initiator of the behaviour,the recipient and the outcome (win/defeat) wasnoted. A win was defined as an initiator, whichstimulated the recipient to escape, or as aninitiator, which succeeded in stealing a preyfrom the recipient. A defeat was assigned toevents where the initiator escaped or did notsucceed in stealing prey.

Courtship behaviour included head jerks andnods, air-pointing bill and head withdrawal,water kicking and display-skating Uohnsgard1965). Courtship behaviour tended to occur inlarge flocks. Thus, the sex and age classcomposition of these flocks was noted in orderto test whether the presence of a particularsex or age class stimulated unpaired males toexhibit courtship displays. The response


variable (presence or absence of courtship)was tested in a logit model with differentcategories of mergansers as independentvariables.

Results

Seasonal count trends and sex ratio

Total numbers of Red-breasted Mergansersfluctuated throughout the study period in bothstudy areas (Figure 2). There were otheroccupied merganser habitats within shortdistances of English Bay and Boundary Bay,hence a great potential for local movement.Furthermore, the latter part of the studyperiod coincided with spring migration frommore southerly habitats to the breedinggrounds in the arctic and subarctic (Camp bell etal. 1990). Thus, migrants may also havecontributed to the observed fluctuation innumbers.

Despite these fluctuations in numbers andpossible turnover of flocks, the sex ratioshowed a rather consistent pattern in bothstudy areas. In February and early March,English Bay was dominated by males, whereasBoundary Bay held a majority of females(Figure 3). From late March onwards, the sexratios converged to similar values (0.6: 1.0). AtEnglish Bay, this change was facilitated by anincrease in the number of females whereas thenumber of males remained relatively constantthroughout the study period (see Figure 2).At Boundary Bay, the number of adult males

increased while the number of females wasrelatively constant (Figure 2).

Pairing chronology

At English Bay, pairs were present in smallnumbers during February (Figure 4). The pairswere likely formed between flight feather moult(in early autumn) and late winter. However, therate of pair formation increased from lateMarch onwards. A comparison of the pairingchronology with the seasonal trends in sexratio (Figure 3) showed that the highest rateof increase in the percentage of pairs occuredsimultaneously with the convergence of the sexratios at English Bay and Boundary Bay.

At English Bay the percentage of pairedindividuals was significantly lower for malesthan for females (comparison of interceptsbetween cubic regression lines on arcsine-transformed data: t7,l147=7.14,P<O.OOO I, Figure4). This may indicate that some males wereunable to pair due to a general male bias in thepopulation (see Figure 3). At Boundary Bay,such a difference was not detected (comparisonof intercepts t7,122=1,16, P<0.25; all slopes wereequal: P-values between 0,05 and 0.07).

Pairs segregated from larger flocks; 61%(n= 114) of all pairs were seen in groups of onemale and one female. For comparison, 'flocks'with solitary males constituted ~O% (n=303) ofall flocks with unpaired males (x =35.20, DF= I,P<O.OOO I), and solitary females constituted 33%(n=70) of all flocks with unpaired females(x'= 14.14, DF= I, P<0.0002).

Table I. Maximum likelihood AN OVA of presence or absence of courtship behaviour forunpaired males in the presence of other unpaired males (male UPD), unpaired females(female UPD) or pairs.

Presence of x2 P

Male UPD 0.D2 0.88

Female UPD 12.05 0.0005

Pair 0.11 0.74

Likelihood ratio 1.07 0.90


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Figure 2. Total number of Red-breasted Mergansers in English Bay (upper) andBoundary Bay (lower) from February to April 1997 broken down by males and females.Estimated number of females were based on equation (I). For clarity numbers were presented on alog scale.

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Figure 3. Sex ratio in Red-breasted Mergansers in English .Bay (upper) and BoundaryBay (lower), February to April 1997. Cubic regression lines weighted by the denominator.

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Figure 4. Percentage of merganser paired males to total number of males, andpercentage paired females to total number of females in English Bay (upper) andBoundary Bay (lower), February to April 1997. Cubic regression lines for males (solid) andfemales (dashed) weighted by the denominator.

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Unpaired males initiated antagonistic behaviouragainst unpaired females twice during the focalbehaviour sampling. This translates to 0.89events/hour, n=27 focal samples (Figure 5).One attack was considered unsuccessful as thefemale responded by attacking the male. Inanother incident, an unpaired female initiatedantagonistic behaviour towards an unpairedmale and was considered to have won thecontest (Figure 5).

Antagonistic,behaviour

kleptoparasitic and courtshiP Kleptoparasitism initiated by unpaired malestowards unpaired females was not observed.However, two incidents (0.47 events/hour, n=51focal samples) were noted in which unpairedmales kleptOparasitised IF's (immature/female).In one of these cases, the male succeeded bystealing the prey item. Unpaired females neverinitiated kleptoparasitic behaviour.

Courtship behaviour among unpaired malesoccurred significantly more in the presence ofunpaired females (5.78 events/hour, n=27 focalsamples) compared to the presence of otherconspecifics (v. 1.54 events/hour, n=78 focal

21-MarDate

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Figure 5. Cumulative percentage number of courtship events exhibited by unpairedmales compared to the cumulative percentage number of flocks with unpaired females,February to April I 997.Arrows denote events of antagonistic (solid) and kleptOparasitic (dashed)behaviour iniated by unpaired males (d') and unpaired females (<;?).The outcome is denoted by 'W'or 'D' to define win or defeat outcomes.


samples; Table I), and we suggest that unpairedfemales acted as a stimulus to initiate thisbehaviour. The presence of courtshipbehaviour at the male-biased habitat at EnglishBay during February suggests that unpairedmales were ready to pair when unpairedfemales appeared. In fact, the presence ofcourtship behaviour showed no seasonal trendother than, was expected from the presence offemales (x =0.37, DF=2, P=0.83, Kolmogorov-Smirnov Test; Figure 5).

Discussion

This study confirmed that the Red-breastedMerganser is a late pairing species (Cramp &Simmons 1977, Johnsgard 1978). The mainperiod for pair formation was initiated by theend of March. Before pair formation, sexualsegregation was observed, and therefore thebehaviour for the majority of Red-breastedMergansers complied with pair formationstrategy two. The convergence of sex ratios atthe two habitats occurred simultaneously withthe commencement of pair formation. This iscompatible with both sexes seeking mates atthe same time. However, a study involvingmarked birds will be necessary to show theextent and direction of mate-seekingmovement by each sex.

The suggestion that sex class segregationshould be a result of antagonistic orkleptoparasitic behaviour from unpaired malesimposed on females was not supported.Assuming that courtship behaviour is theprelude to pairing, the unpaired males showedwillingness to pair when encountered by anunpaired female rather than being aggressive,even in February (see also Johnsgard 1978).A recent study by Platteuw & van Eerden (1997)

suggested that sex class segregation may resultfrom a sexual dimorphism among mergansers,which enables the males to perform deeper divesand exploit more profitable food resources.Although based on restricted data this may applyto the present study, as males constituted themajority in the deeper English Bay,whereas in theshallower Boundary Bay showed a female biasbefore the main period of pairing.

Other alternative explanations of sex classsegregation related to climatic conditions(Carbone & Owen 1995) or male advantages ofbeing closer to the breeding grounds (Anderson& Timken 1972) are unlikely to be applicable inthe present scenario, as the male and female-biased habitats were only 30 km apart.

As already emphasised the majority ofmergansers complied with a strategy of sex classsegregation during the winter and late pairing.However, variation in the timing of pairformation was observed, suggesting that timingof pair formation should rather be considered asa gradient. A few paired birds were alreadyapparent in mid-February, one month before themain pair formation period. This, together withobservations of unpaired females showing up atthe male-biased habitat at English Bay, andunpaired males appearing at the female-biasedBoundary Bay,suggests that some individuals donot comply with the conventional pair formationstrategy for Red-breasted Merganser.

There are potential advantages of earlypairing. Weller (1967) suggested that earlypairing prolongs the period of mate testing, andPaulus (1983) suggested that early pairing inGadwall allows the male to defend high qualityfood resources on the female's behalf. The firstof these hypotheses remains rather speculativein this context, whereas resource defence maybe an option in early paired mergansers. Thus,61% of the pairs were segregated from otherflocks of mergansers. This could be interpretedas territoriality, as was also suggested forBarrow's Goldeneyes (Savard 1988), and thepotential that food resources could bemonopolised.

However, pair segregation and aggressivebehaviour may also be interpreted as avoidanceof interference with conspecifics. For pairs in amale-biased habitat, with high density ofunpaired males, the best solution may be tosegregate from conspecifics at the flock level. Ifpair segregation is possible, then early pairingmay be less costly to the male, because thefrequency of interactions with unpaired maleswill be reduced. However, the frequency ofinteractions may be quite unpredictable, as localpopulation size fluctuated. For example, in late


February the numbers trebled in four days. Thisunpredictability may explain why pair formationamong mergansers in the Fraser River Estuary ishighly synchronised to a period close to thebreeding period and why early pairing is such arare event.

J K was funded by grants from The DanishResearch Council and the Danish Hunters'Association. Thanks to Christine Adkins at theVertebrate Museum of the University of the BritishColumbia, Mike McNall at the Royal BritishColumbia Museum and Jon Fjeldsd at theZoological Museum of University of Copenhagenfor providing dead specimens to verify plumagepatterns of the birds. Finally, special thanks toThomas K. Christensen, Magella Guillemette,

Jesper Kyed Larsen and Leif Nilsson forcommenting on earlier drafts of the manuscriPt.

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JOHNNY KAHLERT', MALCOLM COUPE2 and FRED COOKP

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