Duckling Mortality in Barrow's Goldeneye and Bufflehead Broods

DUCKLING MORTALITY IN BARROW'S GOLDENEYE AND BUFFLEHEAD BROODS

JEAN-PIERREL. SAVARD,• G. E. JOHN SMITH,• AND J. N.M. SMITH2 •CanadianWildlife Service, Pacific and YukonRegion, P.O. Box340, Delta,British Columbia V4K 3Y3, Canada,and 2Departmentof Zoology,University of BritishColumbia, 6270 UniversityBoulevard, Vancouver, British Columbia V6T 2A9, Canada

ABSTRACT.--Wecompared duckling mortality patternsin Barrow'SGoldeneyes (Bucephala islandica)and Buffleheads(B. albeola)in the aspen parkland of central British Columbia, Canada.Hatching phenology of both speciesvaried between years in relation to spring temperatures.Buffieheads hatched on average a few days later than Barrow's Goldeneyes eachyear. Mortality estimatesfor ducklingsdiffered by as much as 55%,and dependedon the techniqueused. Estimatesof mortality rates per brood were higher and more variable than those calculatedper duckling. The Mayfield estimate of mortality per duckling day, adjustedfor duckling age,was the mostaccurate. Mortality ratesof Barrow'sGoldeneyes and Buffieheadsvaried similarly between years. Mortality patterns in relation to duckling age, however, varied between speciesand between years.The highest mortality usually occurred in the week after hatching.Hatching date influencedmortality ratesin 3 of 5 yr, but there was no consistentpattern. Barrow's Goldeneye ducklings suffered higher mortality than Buffieheadducklings in 4 of 5 yr. This patternwas reversed in 1984,when the largestnumber of Barrow'sGoldeneye broods occurred. Duckling mortality washigher on pondswith several broodsthan on ponds with single broods,which implies density-dependentmortality. This may have been due to intra- and interspecificaggression. The high mortality of Barrow's Goldeneyeducklings in 1983 was followed by lowered female recruitment in 1985. Because aggressivenessby Barrow'sGoldeneyes has more impact on conspecifics,Barrow's Goldeneyes and Buffieheadscan coexistat low to moderatedensities of goldeneyes.At high goldeneye densities,Buffieheads may be excluded from some ponds. ReceivedI June1990, accepted 23 December 1990.

COMPETITIONOCCURS when two speciesexploit feted greatermortality at higher densitiesand commonresources in short supply or when they that high densitiesof goldeneyeswere alsocor- interfere with one another, even if resources related with increasedmortality of ducklings are sufficient(Birch 1957).For two populations in other sympatricwaterfowl species.Bengtson to coexist stably, each must inhibit its own (1972) and Einarsson(1985) quantified mortal- growth more than that of the other species ity in broodsof Icelandic Barrow'sGoldeneye, (Pianka 1978). In areas of sympatry, interfer- and Erskine (1972) and Gauthier (1987) did so ence competitionbetween Barrow'sGoldeneye for Buffieheads in British Columbia. However, (Bucephalaislandica) and the Bufflehead (B. al- mortality of Barrow'sGoldeneye and Buffie- beola)is strong (Savard 1982, 1984), and Bar- headducklings has not been compareddirectly row's Goldeneyesdominate Buffieheadsin ag- in areasof sympatry. gressiveinteractions (Savard and Smith 1987). Our aim wasto comparehatching chronology In someregions of central British Columbia, and duckling mortality of the two species.We densities of Barrow's Goldeneyes are almost also compare the effectivenessof six methods twice as high as those of Buffieheads(Savard to estimatemortality. 1982), when the reverse pattern might be expected from relative body size and availability METHODS of nest sites(Gauthier 1985, Gauthier and Smith Barrow'sGoldeneye and Buffieheadbroods stay in 1987).Female Barrow's Goldeneyes with young open water and rarely seek cover. This, and the lack do not tolerate Buffieheads within their terri- of emergent vegetationon most of our study ponds, tory and are particularly aggressivetoward Buf- greatly facilitatedbrood checks.In 1980and 1981,we fiehead broods (Savard 1982, 1988a; Gauthier countedbroods on I00 pondsat intervals of 14 days. 1985;Savard and Smith 1987). Bengtson(1972) In 1982, 1983, and 1984, we counted every 3 days. found that Barrow'sGoldeneye ducklings suf- During each visit, we recorded the location of the

568 The Auk 108:568-577. July 1991 July1991] DucklingMortality in Goldeneyes 569 brood,the ageof the youngand the numberof young the nest or brood is under observation.Ringelman in each brood. Most broods were identified from cen- and Longcore(1982) and Gauthier (1987) have mod- susto censusby a combinationof the mother'snasal ified the method to estimate survival rates of duck- disk,age, number of young,and locationof the brood. lings. For eachperiod, an exposureindex is calculat- We determinedages of ducklingsby the criteriaof ed. Exposureis defined as the number of ducklings Gollop and Marshall (1954)and Taber (1971).We used multiplied by the length of period (i.e. 3 ducklings age classesfor LesserScaup (Aythya affinis) as an ap- alive for 2 daysrepresent 6 duckling-days).Mortality proximation of Buffieheadage classes(Taber 1971). is expressedas lossesper exposure,where lossesare Buffieheadand LesserScaup have similar growth pat- the number of ducklings that disappearedin each terns (Erskine 1972).As data on plumage growth and period. When the exact time of duckling losswas ageare not availablefor Barrow'sGoldeneye, we as- unknown, we assumedthat the mortality rate was sumedtheir growth to be similarto that of Common constant, and we estimated lossesand exposure for Goldeneye (Gibbs 1961). eachday of the period (Johnson1979). We alsoas- Many Barrow'sGoldeneye females were marked sumedthat losseswere constantduring the period, with nasal disks (26% in 1982, 47% in 1983, 56% in or that lossesoccurred mid-way through the period. 1984) (Lokemoenand Sharp 1979, Savard and Eadie Resultswere almost identical for all three assump- 1989). Femaleswere captured in mirror traps (Savard tionsbecause of our frequentsurveys. Thus, we use 1985)and on the nest during the last week of incu- only the assumptionof constantmortality rate here. bation. We assumed that reduction in brood size be- We derived two estimatesof daily mortality from tween consecutivecensuses indicated duckling mor- the Mayfield method:a weighted estimate,where ex- tality. On some ponds, broodsamalgamated (Savard posuresand losseswere summed for all broods and 1987). In most cases,we could identify such occur- the daily mortality rate was calculated,and an un- rencesfrom duckling age and numbers.We assigned weighted estimate,where mortality rates were cal- theyoung to theirrespective broods. Broods that could culatedfor each brood individually and then aver- not be followedindividually were excludedfrom some aged. When mortality varies with age, these two analyses. estimatescan be further refinedby the product-mo- Major nest predators were the black bear (Ursus ment method (Klett and Johnson 1982). Survival rates americanus),Douglas squirrel (Tamiusciurusdouglasii), are then calculatedas the productof age-classsurvival and marten (Martes americana).Potential duckling rates. Thus four estimates (3-6) were obtained from predators were Raven (Corvuscorax), Great Horned the Mayfield method:(3) a weighted estimate,(4) the Owl (Bubovirginianus), Northern Harrier (Circuscy- weighted estimate refined by the product-moment aneus),coyote (Canis latrans), and red fox (Vulpesvulpes). method, (5) an unweighted estimate,and (6) the un- Eachyear, we divided broodsinto three equal-sized weighted estimaterefined by the product-moment groups(early, middle, late) basedon their hatching method.Depending on samplesize and type of anal- date. We estimatedmortality in 6 ways: 2 direct es- yses,mortality is expressedas either daily mortality timatesand 4 obtainedfrom the Mayfield method.In rate,mortality over a 4-weekperiod, or mortalityover Method 1 we recordedthe number of young present a 6-week period. when a brood was first sighted and used that value We usedthe jackknifetechnique (Cochran 1977) to to calculatethe numberof youngthat hatchedon the estimatethe variability of the weighted daily mor- studyarea. We comparedthis value with the number tality rates.To test for differencesamong several of youngpresent on the studyarea just before fledg- means,multiple comparisontests cannot be usedhere ing. All first sightingswere of broods< 1 week old. becausethe jackknifemethod was usedto compute The last count for each brood was done at least 4 standarderrors. To make such comparisons,we used weeks after the first sighting. This method allowed pairedz-tests and adjusted x (probabilitylevel) to x/c, us to estimateminimum duckling mortality on lakes where c is the number of comparisonsmade. where broodscould not be followed individually becauseof mixing. RESULTS The second direct estimate (Method 2) assumedan average initial brood size based on values reported Hatchingchronology.--During the 5 yr of the in the literature.Over 4 yr, the averagenumber of study,mean hatching dates of Barrow'sGold- young that hatchedfrom successfulBarrow's Gold- eneyebroods ranged from 14-24 June and those eneyenests in the studyarea was 10 (Savard1988b). of Buffiehead broods from 16-25 June (Table 1). An averageof 8 younghatched from successfulBuf- Mean hatching date varied significantly be- fiehead nests (Erskine 1972). This allowed us to ac- tween years(two-way ANOVA P < 0.001),and countfor the mortalitythat occurredbefore the first sighting of a brood. Buffieheadshatched slightly--but significantly The Mayfield method (Mayfield 1961, 1975) ac- (P < 0.001)--later than Barrow's Goldeneye. counts for bias related to the time when a nest or There were no interactions between year and broodis firstfound by consideringthe periodthat species.For both species,1983 was an early 570 $AVARD,SMITH, AND SMITH [Auk, Vol. 108

TABLE1. Mean hatchingdate (+SD) of Barrow'sGoldeneyes and Buffieheadsin relation to averagetemper- atures(øC) in April and May.

Temperature Barrow'sGoldeneye Buffiehead Year April May ns Hatchingdate n Hatchingdate 1980 9.4 14.3 84 19 June + 7 A b 67 22 June + 8 AC 1981 6.8 13.1 84 19 June + 7 A 73 20 June + 8 A 1982 5.3 11.5 85 24 June + 7 C 78 25 June + 7 C 1983 8.6 14.5 95 14 June + 8 B 73 16 June + 9 B 1984 7.5 10.1 108 19 June + 10A 78 20 June + 9 A

a Number of broods. bStudent-Newman-Keuls test was used to testfor differencesin hatchingdates within speciesbetween years. Dates followed by differentletters differ significantly(P < 0.05) betweenyears. hatching year and 1982 a late one. In all years, age differences(Method 3). Although estimates >70% of the broods hatched within 3 weeks. were slightly different, both methodstracked The earliest brood hatched in the first week of yearly differencesin mortality rates similarly. June in 1983, and the latest hatched in the sec- Analysesof mortality on a broodbasis permit ond week of July in 1982. the useof standardstatistical techniques. A four- Estimationof ducklingmortality.--Mortality es- way analysisof varianceof the effectof species, timatesvaried with the techniquesused to cal- year, age, and hatching date on brood daily culate them (Tables 2 and 3), and Method 1 mortality rates indicated that daily mortality yielded the lowest estimates.The six estimates ratesvaried significantlywith speciesand year, ranged from 37% to 69% for Barrow'sGolden- decreasedwith increasingduckling age (P < eye, and from 10%to 65% for Buffiehead.Ad- 0.001), but did not vary with hatching date (P justments for age differences influenced esti- < 0.199). Interaction terms indicated that age- mates based on broods more than those based specificmortality differed between years (P < on ducklings. The estimateswere affected in 0.001). The absenceof significantinteractions someyears but not in others. Differenceswere between speciesand year (P = 0.067), and be- high between estimateswith broods as units tween speciesand hatching dates (P = 0.547), (2-37%) especiallyin 1983-1984,and relatively indicatesthat goldeneyesand Buffieheadsre- low for thosewith ducklings (2-8%). Estimates sponded similarly to factors that influenced madeby assumingan averageinitial broodsize mortality between years and periods. (Method2) wereusually higher than those based Weighted mortality estimates(on a duckling on the weightedMayfield methodadjusted for basis) were less variable and permitted a more

TABLE2. Estimatesof mortalityof Barrow'sGoldeneye ducklings (+ SE).All estimatesare for 43 daysfollowing hatching.Mortality is expressedin %.

Mayfield estimate Weighted estimate Unweighted estimate Direct estimate (on a duckling basis) (on a brood basis) Not Not %dead adjusted adjusted Adjustedfor for age Adjusted for for age Year na A b Bc age difference difference na age difference difference 1980 84(584) 35 55 49 ñ 4 A e 52 ñ I 55 63 ñ 7 74 ñ 5 1981 84 (612) 21 43 31 ñ 4B 33 ñ I 54 49 ñ 10 76 ñ 16 1982 87 (621) 21 44 28 ñ 3 B 29 ñ I 67 38 ñ 8 58 ñ 10 1983 95(779) 46 56 62 ñ 2A 70 ñ I 69 79 ñ 5 98 ñ 2 1984 110(922) 21 34 30 ñ 3 B 32 ñ 1 93 55 + 9 90 ñ 7

Number of broodson the study area (maximumnumber of young seen). Estimatederived from the number of young seen. Estimateassuming an initial brood size of 10 young (Savard1988b). Number of broodsused for the Mayfield estimates. Similar lettersindicate that the estimatedid not differ significantlyat the P = 0.05 level. July1991] DucklingMortality inGoldeneyes 571

TAnLE3. Estimatesof mortalityof Buffieheadducklings (+SE). All estimatesare for 43 daysafter hatching. Mortality is expressedin %.

Mayfield estimate Weighted Direct estimate (on a ducklingbasis) Unweighted Not (ona broodbasis) %dead adjusted AdjustedNot adjusted Adjusted for for age for age for age Year na Ab Bc age difference difference na difference difference 1980 68(433) 32 46 38 ñ 4A e 44 ñ 1 48 43 ñ 8 48 ñ 6 1981 75(514) 12 25 17 ñ 4B 15 ñ 1 36 21 ñ 10 22 ñ 5 1982 78(541) 12 24 20 ñ 4BC 22 ñ 1 49 24 ñ 10 34 ñ 11 1983 73(475) 36 48 45 ñ 4A 50 ñ 1 47 59 ñ 9 77 ñ 7 1984 74 (520) 26 35 32 ñ 3AC 37 ñ 1 70 54 ñ 13 91 ñ 8

Number of broods(maximum number of young seen). Estimatebased on the maximum number of young seen. Estimateassuming an initial brood size of 8 young (Erskine 1972). Number of broodsused for the Mayfield estimates. Similar lettersindicate that the estimatedid not differ significantlyat the 0.05 level.

detailed analysis of the factors affecting mor- marked females. We found no differences in the tality ratesof both species.All subsequentanal- patterns of mortality between this subsample yses here are on a duckling basisunless oth- and the total sample.The sampleof marked erwise specified. females, however, allowed us to estimate the Effectof ducklingage on mortalityrates.--Daily loss of eggsbefore hatching. Egg losseswere mortality ratesof Barrow'sGoldeneye and Buf- 6%(n = 234 eggs)in 1982,11% (n = 519)in 1983, fiehead ducklings generally declined with and 15% (n = 631) in 1984. duckling age (Figs. I and 2), but the pattern of We believe the mostcrucial time for duckling variation differed between years. In 4 of 5 yr survival was the first week post-hatch. Even (1980 was the exception),daily mortality rates within the first week, we observed declines in of Barrow'sGoldeneye ducklings were highest mortalityrates in eachof the 3 yr (Fig. 3). in the first week after hatching, and declined Effectof hatchingdate on mortality rates.--Mor- subsequently.This decline was gradual in 1983, tality ratesof Barrow'sGoldeneye during the but in 1981, 1982, and 1984, mortality rates first 4 weeksof life varied significantlyin some dropped sharply in the secondweek and re- yearsin relationto hatchingdate (Fig. 4). The mained similar thereafter. In 1980, fluctuations patternof variation,however, differed between in daily mortality rateswith duckling age were years.In 1980,mortality was significantly lower smallerthan in other years,although the rates for broods that hatched in midseason, whereas themselves were often higher. Patterns ob- in 1983it waslower for early hatchingbroods. servedin 1980and 1981are lessprecise than in In 1982, late-hatchingbroods suffered higher otheryears because surveys were lessf•equent. mortalitythan midhatchingbroods (P = 0.004); As with Barrow'sGoldeneye, daily mortality differenceswith early-hatchingbroods were not rates of Buffieheadducklings were highest in significant(P = 0.03)at theadjusted Bonferroni the first week post-hatchand decreasedsubse- significancelevel (0.02). quently. Again, fluctuationsin mortality rates Mortality ratesof Buffieheadducklings did in 1980 were lessdramatic than in other years, not vary significantlyin relationto hatching and there was an unusually high mortality in datein any year (Fig. 5), and no clearpatterns the fifth week of 1983.Variability between years were apparent.Yearly effects tended to domi- in daily mortality rates for both specieswas nate seasonaleffects on mortality ratesfor both highestfor the first week post-hatchthan for species.In 1983,mortality of ducklingswas any other duckling age classes. higherfor all hatchingdates, but the pattern Because female Barrow's Goldeneyes were reversed in 1981. markedfor only someof the broodswe studied, Mortalityrates of Barrow'sGoldeneye and Buf- we repeatedthe analysisfor the sampleof 120 fieheadducklings.--Seasonal mortality rates of 572 SAVARD,SMITH, AND SMITH [Auk,Vol. 108

.088

.080

.024

Fig. 1. Daily mortality rates of Barrow'sGolden-

eye ducklingsin relation to duckling age. o

Barrow'sGoldeneye ducklings were higher than Fig. 3. Daily mortality rates of Barrow'sGolden- those of Buffiehead ducklings except in 1984 eye ducklingsduring the first week post-hatch(cal- (Tables 2 and 3). Significant differencesbe- culatedfrom broodsof marked femalesonly; • + 2 tween speciesoccurred in 1980 (P = 0.03), 1981 SE). (P = 0.01), and 1983 (P < 0.01). We compareddaily mortalityrates of the two speciesfor various duckling ages. There were lyzedeach year individually. This reduced sam- no clear effectsof duckling age on the relative ple sizes and thus our ability to detect mortality rates of Barrow'sGoldeneye and the differences.We compareddaily mortality rates Buffiehead. In all five comparisonswhere mor- of ducklingsfor the first week post-hatchonly, tality differed significantly, Barrow'sGolden- becausemost mortality occurred at that time, eye had a higher mortality rate than Buffiehead. and ducklingsof this age are most susceptible Mortality rates of Barrow's Goldeneye and to intraspecificaggression. In 1984,mortality of Buffieheadducklings did not differ consistently Barrow'sGoldeneye ducklings was significant- in relation to hatchingperiods over 5 yr (Table ly higher on lakes with severalbroods than on 4). Differencesin mortality rates between the lakeswith singlebroods (Table 5). Although no specieswere slightly more common for early- other comparisondiffered significantly, the hatching broods than for late ones. mortality values found were all higher on lakes Mortality ratesin relationto brooddensity.--To with several broods and higher for the second assess if interactions between broods affected brood to hatch on a given lake than for the first duckling mortality, we compared duckling mortality on lakes with single broods and on lakes with two or more broods. Because duckling mortality varied between years, we ana-

0.020 -

O.OLO 1980 1981 1982 1983 1984

YEAR

o Fig. 4. Mortality estimates (_+SE) of Barrow's Goldeneyeducklings in relation to hatchingdate for the first4 weekspost-hatch. (Comparison within years Fig. 2. Daily mortality rates of Buffieheadduck- only. Similar lettersindicate no significantdifference lings in relation to duckling age. at P = 0.05.) July1991] DucklingMortality in Goldeneyes 573

TABLE4. Differencesin mortality ratesa of Barrow's Goldeneyeand Buffieheadducklings in relation to t hatching period.

Hatching group Year Early Mid Late 1980 0.26 ***b -0.11 0.08 1981 0.25*** 0.17' -0.03 1982 0.06 0.02 0.06 1983 0.13' 0.17'** 0.33*** 1984 0.02 -0.09 0.06

1900 1981 1982 1•)83 1904 ' Mortality rateswere calculatedby the product method over 4 age classes(1-8, 8-15, 15-22, and 22-29 days old). YEAR bBarrow's Goldeneye mortality rate-Buffiehead mortality rate: * = P Fig. 5. Mortality estimates(+SE) of Buffiehead < 0.10; ** = P < 0.05; *** = P < 0.017. ducklings in relation to hatching date for the first 4 weekspost-hatch. (Comparison within yearsonly. No ble 1). Spring weather may determine the onset significantdifference at P = 0.05 in any of the years.) of laying in severalspecies of prairie waterfowl (Hammond and Johnson 1984). one. In 1984,there was alsoa tendencyfor high- Estimationof ducklingmortality.--Mortality es- er daily mortality ratesof ducklingsfor Buffie- timates varied with the technique used to cal- head broodsthat shareda lake with goldeneyes culate them. Estimatesexpressed on a brood ba- (0.057 + 0.02) compared with solitary broods sis were higher and more variable than those (0.003 + 0.002; P < 0.05). Samples were too expressedon a duckling basisas expectedfrom small for robust comparisonsin other years. statistical considerations. When the brood is the Effectof ducklingmortality on recruitment.--We unit, a brood of 1 duckling has the sameweight looked at relationshipsbetween recruitment and as a brood of 15 ducklings, whereas the brood mortality ratesin Barrow'sGoldeneye. From the of 15 has15 timesmore weight when the duck- proportion of marked adult females returning ling is taken as a unit. Thus, both estimates (Savard and Eadie 1989), we estimated female sometimesdiffered considerably.Biologically, mortality and calculated the expected number both estimates are useful, but each has limita- of unmarked females in the population (Table tions.Mortality sometimesacts on whole broods 6). We assumedthat mortality rateswere similar (e.g. during overland movements or in terri- for marked and unmarked females. Then, the torial interactionsamong broods; Savard 1987), difference between the expected number of fe- but more often mortality affects individual males in the population and the number actu- ducklings(e.g. bad weather,partial predation). ally presentprovides an estimateof the number That the Method 1 basedon duckling counts of new females recruited into the population. underestimatesmortality is not surprising, be- In 1983,duckling mortality was nearly twice as causeof the high mortality during the first few high asin the three other years(Table 2), which daysof life. Peak mortality often occursbefore corresponded with an unusually low recruit- the brood is sightedfor the first time. However, ment in 1985 (Table 6). when an averageinitial brood size is assumed, the direct method (2) provides estimatessimilar to those derived from the Mayfield method. DISCUSSION Becausebroods of Barrow'sGoldeneyes and Buf- fieheadsare easyto surveyas they frequent open Hatching chronology.--Hatchingchronology water, direct method (2) can be used. The meth- varied among yearsdue to the timing of spring 'od providesa good estimateof the total number thaw, which was related to April temperatures. of young producedin a given area. By estimat- In 1980, 1981, and 1982, somebreeding ponds ing the initial number of broods in the area, were still frozen in late April. In 1983 and 1984, one can then estimateduckling mortality for a all ponds were free of ice by the third week of given area.Because of significantdifferences in April. More broods hatched late in 1984 than mortality ratesof ducklingswith age, we rec- in 1983, perhaps becausecold weather in early ommend the product-moment method for es- May 1984delayed laying by somefemales (Ta- timating mortality with the Mayfield method. 574 SAVARD,SMITH, AND SMITH [Auk, Vol. 108

TABLE5. Comparisonof daily mortality rates(+SE) of Barrow'sGoldeneye ducklings in the first week after hatchingin relationto the presenceof otherBarrow's Goldeneye broods. Number of broodsis in parentheses.

Year Solitary brood 1st brood to hatch 2nd brood to hatch _>2Barrow's broods 1982 0.018 _+ 0.010 (14)• 0.000 (4) 0.036 + 0.030 (4) 0.023 + 0.015 (7) 1983 0.045 + 0.015 (14)a 0.044 + 0.021 (21) 0.066 + 0.031 (14) 0.088 + 0.022 (17) a 1984 0.01 + 0.008 (12) b 0.024 + 0.016(14) 0.067 _+ 0.028(14) 0.057 + 0.018 (17) b

Differencebetween solitary and >-2 broodsare statisticallysignificant only at P < 0.10. Differencesbetween solitary and >2 broodsare statisticallysignificant at P < 0.05.

Unweighted estimates,which were particularly tistically significant.Other factors,unrelated to affected by age difference in mortality rates, hatching date had greater effectson duckling provided overestimatesof mortality. mortality. Our definitions of early and late Mortality of both species' ducklings was hatching (first third and last third of the season) higher in 1983 than in the four other years.We may not have reflectedecological divisions. Data suspectthat the cold temperaturecoupled with on the effectsof intra- and interspecificaggres- constantrain during the peak hatching period sion on mortality suggestan advantageof early in 1983 contributed.Bad weather can depress hatching. This advantage, however, could be the survival rate of precocialbirds markedly negated by other factorsin some years. Some either directly (Hilden 1964, Mendenhall and studiessuggest that hatching date affectsduck- Milne 1985) or through food reduction (Erik- ling survival (Grice and Rogers 1965, Ringel- stad and Andersen 1983). man and Longcore 1982),while other studiesdo Mortality was highest in the first week post- not (e.g. Dzubin and Gollop 1972, McAuley hatch for both speciesas reportedin other wa- 1986). Fluctuating directional selection based terfowl (Patterson 1982, Hill and Ellis 1984, on hatching date has been demonstratedin the Mendenhall and Milne 1985). At hatching, Great Tit (Parusmajor). ducklings are vulnerable to predators, bad Comparisonof mortalityrates of Barrow'sGold- weather, and intraspecific aggression(Make- eneyeand Buffieheadducklings.--Barrow's Gold- peace and Patterson 1980). Bengtson (1972), eneye ducklingssuffered higher mortality than Koskimies and Lahti (1964), and Hilden (1964), Buffieheadducklings in 4 of 5 yr. Both species found that mortality of young ducklings in- frequent similar habitat and have similar breed- creased in bad weather. Sj6berg and Danell ing behavior and ecology (Savard 1982, 1984; (1982) found that aquaticinsect availability was Gauthier 1985).Several factors might contribute reduced in adverse weather, and Hill and Ellis to the higher mortality of goldeneyeducklings: (1984) showedthat ducklingsare inefficient di- vers at hatching and depend on small prey near 1. Barrow'sGoldeneyes often nest farther from the surface. Within the first week of life, we brood-rearing ponds than Buffieheads do, found especially high mortality in the first 2 becauseof lower nest-site availability (Er- dayspost-hatch. Travel from the nest site to the skine 1972, Peterson and Gauthier 1985, Gau- water (up to 2 km away) may contribute. Also, thier and Smith 1987). The longer overland territorial behavior among females may cause journeyto water by Barrow'sGoldeneyes may duckling losses(Petrie 1984, Savard 1987). As cause mortality directly or may weaken ducklings get older, they are better equipped ducklingsand increasetheir vulnerability to escapeattacks by territorial females.The high once they reach the pond. daily mortality rate (0.04-0.09) of ducklings in 2. Goldeneye ducklings are twice the size of the first 2 days has important implicationsfor Buffieheadducklings and thus have greater studiesof mortality. daily energy requirements(Kendeigh 1970, We found no consistentpattern of an effect King 1973). of hatchingdate on mortalityrates among years. 3. Aggressiveneglect (Ripley 1961) could con- Bengtson (1972) found a decreasein survival tribute to this higher mortality. Barrow's for late-hatching Barrow's Goldeneye in Ice- Goldeneyefemales spend on average12% of land. In someyears, mortality rateswere higher their time in aggressiveintra- and interspe- later in the seasonfor both Barrow'sGoldeneye cific interactions,leaving their young un- and the Buffiehead, but this trend was not sta- protectedduring theseevents (Savard 1988a). July1991] DucklingMortality inGoldeneyes 575

TABLE6. Relationshipbetween Barrow's Goldeneye duckling mortality (%) and recruitment into the breeding population.

Expected No. of no. of No. of unmarked unmarked unmarked % of marked females in females in females in New females Mortality in Yearx birdsreturning a yearx - 1 yearx yearx in yearx year(x - 2)b 1983 89 (36) 165 147 233 86 31 1984 71(82) 233 164 264 100 28 1985 66 (105) 264 172 184 12 62 1986 66 (119) 184 121 200 79 30 • Number of birds markedthe previousyear (x - 1) is in parentheses(Savard and Eadie1989). • Weightedestimate adjusted for agedifferential in mortality(over a 6-weekperiod). Mortality rate 2 yr before,as female goldeneye recruit into populationwhen 2 yr old (Eadieand Gauthier1985).

Comparatively,female Buffieheadsspend on ducks(Williams 1974,Makepeace and Patterson averageonly 5% of their time in aggressive 1980, Pienkowski and Evans 1982), and else- interactions (Savard 1986). where in Barrow'sGoldeneyes (Einarson 1988) 4. Intraspecificaggression causes more duck- and in Buffieheads(Gauthier 1987). Established ling deathsin Barrow'sGoldeneye than in territorial females should have an advantage the Buffiehead. Buffiehead ducklings are over arrivals with newly hatched broods (Sa- rarely killed by conspecifics(Erskine 1972; vard 1987,Petrie 1984,Figler and Einhorn 1983), Donaghey 1975; Gauthier 1985, 1987), but becausethey regainedsome of the weight lost such deaths are common in Barrow's Gold- during incubation.Newly arrived broodscould eneye (Bengtson1972, Savard 1988a). alsobelong to lessexperienced first-time breed- ers, who tend to breed later (Afton 1984, Gau- In 1984, levels of mortality were similar in thief 1989) and may be less efficient in pro- ducklingsof both species.We suggestthat this tecting their young. Goldeneye females are was due to interspecificaggression by Barrow's highly philopatric (Dow and Fredga 1983, Sa- Goldeneye,which were at their highestdensity vard and Eadie 1989, Gauthier 1990), thus the (Savard 1988b). Survival of Buffiehead duck- population size of an area should be influenced lings was significantlylower in 1984 on lakes by the number of young producedin that area. with Barrow'sGoldeneye broods. The interspe- This low dispersalhas important management cific aggressivenessof Barrow's Goldeneye is applications. well known and often resultsin ducklingdeath The methodologicaldifficulty of measuring (Sugden 1960, Robertson and Stelfox 1969, productivity in waterfowl populations is ap- Bengtson 1972, Einarsson1985, Savard and parent in our results.We believe that territorial Smith 1987,Savard 1988a).Inter- and intraspe- aggressionshould be consideredin the man- cificaggression within the genusBucephala may agementof Barrow'sGoldeneye and Buffiehead be a responseto limited food resources(Savard populations. Efforts to reduce nest predation 1982, Savard and Smith 1987). Barrow's Gold- and increasereproductive successcould be ne- eneyeand Buffieheadducklings overlap in size gated in years when density-dependentmor- due to hatching asynchronybetween broods; tality factors may operate. It appears that ag- older Buffieheadsare similar in size to younger gressivenessin Barrow'sGoldeneye has a greater goldeneyes,and potentially compete for the same food resources. Food limitation restricts intraspecificthan interspecificeffect, which al- lows Buffieheadsand Barrow'sGoldeneye to co- reproductive successof some speciesof water- exist (Pianka 1978). fowl (Hunter et al. 1984, Pehrsson 1973). There was higher mortality of Barrow'sGold- eneye ducklings on ponds with severalbroods ACKNOWLEDGMENTS than on thosewith singlebroods. Also, newly arrived broodson a lake tendedto sufferhigher This studywas supported by the CanadianWildlife mortality than previously establishedbroods. Service,Ducks Unlimited, and a grant from NSERC Similar patterns have been observed in Shel- (to J. N.M. Smith). We thank A. Breault, B. Emery, 576 SAVARD,SMITH, AND SMITH [Auk,Vol. 108

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