Salt Tolerance in American Black Ducks, Mallards, and Their F1-Hybrids

SALT TOLERANCE IN AMERICAN BLACK DUCKS, MALLARDS, AND THEIR F1-HYBRIDS

GREGORY G. BARNES AND THOMAS D. NUDDS Departmentof Zoology,University of Guelph,Guelph, Ontario NIG 2W1,Canada

A13STRACT.--Weperformed experiments on pure, wild-strainMallards (Anas platyrynchos), AmericanBlack Ducks (A. rubripes),and first filial (F1) Mallard-BlackDuck hybridsto in- vestigatewhether ducklingsurvival and growth variedwith salinity,and whether hybrids acquiredsalt tolerance. Ducklings were assignedto treatmentsin a 4 x 3 factorialexperiment that involved a salinity gradient(0, 0.5, 1.0, 1.5%NaCI) and duckling age (7, 14, 21 days).An additionalexperiment subjected ducklings to 3% NaC1at 48 h post-hatch.Black Duck ducklings had higher survival and growth ratesthan did Mallards with salt concentrationincreasing up to !.5%.Hybrids were moresimilar to BlackDucks than to Mallardsin that regard.Salinities >2% were uniformly fatal to all 48-h-oldducklings. Salt glandsobtained from fully grown individuals from each treatment were heaviest in Black Ducks and hybrids, and glands increasedin size with increasingage and salinity. Salt glandshypertrophied to a maximum size at 1%NaCI, which indicatesthat even BlackDuck ducklingspossessed salt glandscapable of osmoregulationonly at low levelsof salinity.Interspecific differences with respectto salt toleranceare probablyinsufficient to serveas a postmatingreproductive isolating mechanism betweenthese species in estuarinehabitats. Received 24 January1990, accepted 18 July1990.

GEOGRAPHICisolation may oncehave been the On the other hand, a stable coexistence be- most important pre-mating reproductive iso- tween the two speciesmight develop where lating mechanism between breeding popula- new reproductiveisolating mechanismscan be tions of American Black Ducks (Anas rubripes; established. Black Ducks principally occupy hereafter Black Ducks) and of Mallards (A. plat- brackish, coastal marshes and may have sec- yrhynchos).Historically, land clearingin eastern ondarily expanded their breeding range to in- North America altered habitat and may have clude wooded freshwater ponds (Heusmann contributed to the eastwardrange expansionby 1974). Black Ducks may continue to decline in Mallards into areas formerly occupied primar- freshwater habitats where they are sympatric ily by BlackDucks (Heusmann 1974, Dennis et with Mallards. Ultimately, Black Ducks might al. 1984, Conroy et al. 1989). Concomitantly, reside only in some form of habitat refuge in Black Ducks declined (Collins 1974, Rogersand coastalwetlands isolated from gene introgres- Patterson 1984). Becausethe decline in Ontario sion with Mallards. (where Mallards have increased)has been par- Little information existsabout the suitability ticularly marked (Collins 1974, Johnsgardand of brackish habitats for duckling growth and DiSilvestro 1976, Dennis et al. 1984), introgres- survival and, specifically, about the ability of sive hybridization, competitive exclusion, or ducklings to regulate the salt concentrationof both, have been proposed to be major, proxi- body fluids (Serie and Swanson 1976, Swanson mate agents of the decline of Black Ducks et al. 1984). Many aquatic birds and reptiles (Johnsgard 1967, Brodsky and Weatherhead possessnasal salt glands, which constitute an 1984, Ankney et al. 1987). extrarenal excretory pathway for the elimina- The continued eastward expansion of the tion of excesselectrolytes accrued in a saline Mallard may eventually causethe loss of the environment (Schmidt-Nielsen 1960, Bradley Black Duck phenotype through introgressive and Holmes 1972, Holmes and Phillips 1985, hybridization (Ankney et al. 1987) in much the Grigg 1981).These supraorbital glands also exist sameway that indigenous speciesof duckshave in nonmarine waterfowl (Scothorne 1959a, b) lost specific integrity through hybridization and secretefluids containing NaC1 at concen- with introduced populations of the Mallard trationsequal to or greaterthan that of seawater elsewhere (Braithwaite and Miller 1975, Had- (Schmidt-Nielsen and Fange 1958, Schmidt- don 1984, Gillespie 1985). Nielsen 1960). Size and, therefore, functional

89 The Auk 108: 89-98. January 1991 90 BARNESANI> NUl>IVS [Auk,Vol. 108 capacityof the saltgland dependson both ge- 1984). Birds were allowed to pair naturally through netic and environmental determinants (Holmes the winter months. et al. 1961, Cooch 1964, Cornelius 1982). In April 1986, pairs were transported to Lake St. Mallard ducklingswere intolerant of salt (El- Clair National Wildlife Area (NWA). Each pair was lis et al. 1963, Schmidt-Nielsen and Kim 1964, placed in a 4 x 3 x 2 m pen isolated visually from the others and supplied with ad libitum commercial Cooch 1964, Anonymous1983, Swansonet al. waterfowl breedingdiet (19%protein) and oystershell, 1984) at concentrations as low as one third of a 44-1tub for bathing, and a nest box. All birds were that found in the coastal marshes where Black returned to the aviary for the winter months. Ducks breed and winter. Solutions of 1-1.5% In March 1987, identified pairs remained in five NaC1(10-15 ppt) suppliedas drinking water to large pens (15 x 7 x 3 m) at the Guelph aviary because captiveMallard ducklingsretarded growth rates the transportation of pairs to Lake St. Clair NWA in and causedsome mortality; 2-3% NaC1 caused the year previous seemedto delay breeding. No more 100%mortality in 2-5 days (Anonymous1983, than 4 females and 4 males bred in each pen. Birds Swansonet al. 1984).If wild Mallard ducklings were supplied with a 3-m diameterwading pool, nest are affectedin much the sameway, then breed- boxes,and ad libitumfood and oyster shell. In both years,all clutcheswere removed on day 5 ing successof Mallards in brackish environ- of incubation to induce renesting. Ducklings were ments may be impeded. hatched in incubatorsat the ResearchStation, Poultry We conductedtests to determine whether (1) Division of the Ontario Ministry of Agriculture and BlackDuck ducklingspossessed functional salt Food (OMAF), Arkell, Ontario, according to proce- glands at hatch and (2) introgressivehybrid- dures in Ward and Batt (1973). At hatch, ducklings ization might lead to the Mallard's acquisition were sexed and web-tagged. of an efficientextrarenal excretory mechanism. Experiments.--Thefirst experimentwas designedto If Mallard ducklingscannot acquire functional examine the ability of Black Ducks, Mallards, and salt glands through hybridization with Black their Fl-hybrids to maintain water and electrolyte homeostasis when forced to drink water with a salin- Ducks, a new postmatingisolating mechanism ity of 3% NaC1, 48 h post-hatch.Effects of salinity on might be established(through impaired repro- growth and survival are known for Mallards and oth- ductive successof Mallards and hybrids), and er prairie-nesting species(Cooch 1964, Holmes et al. Mallards should not have the potential to ex- 1968, Anonymous 1983), but not for Black Ducks. pand their range to include the coastalregions In 1986, ducklings of the same age were kept in presently occupiedby Black Ducks. groups of <20 in brooder pens that consistedof a cardboard ring 3 m in diameter and 1 m high. The METHODS floor wascovered with wood shavings,and ducklings were reared accordingto proceduresoutlined in Ward In June 1985,39 eggsfrom 7 clutchesof wild Mal- and Batt (1973). Forty-eight hours after hatch, duck- lardswere collectedin central Saskatchewan,approx. lings of each sex were randomly assignedto either 80 km NW of Saskatoon,and 34 eggsfrom 6 clutches fresh water or a 3% NaC1 solution as their sole source of wild Black Ducks were collected from Prince Ed- of drinking water. ward Island, Nova Scotia, and New Brunswick. Col- We designed a second experiment in 1987 to de- lecting in these regions maximized the probability termine whether more subtle differences exist in the that eggs obtained were a product of pure strain par- developmentof the salt gland that may influence the ents of each species(Johnsgard and DiSilvestro 1976). developmentof a functional salt gland found in adult All ducklings were hatched and reared at an aviary waterfowl (Scothorne 1959a, Cooch 1964) and that located on the University of Guelph campus. Exper- might facilitate habitat segregationin these species. iments were conductedunder the auspicesof the Uni- Ducklingswere assignedto 1 of 10 treatmentsinvolv- versity Animal Care Committee. ing a salinity gradient (0, 0.5, 1.0, 1.5% NaCI) and Birds used for captive breeding were wing-clipped duckling age (7, 14, 21 days),subject to the constraint and were maintained on an ad libitum commercial wa- that siblings were distributed acrossgroups. Duck- terfowl maintenancediet during the months of Au- lings were housed in (2 x 1 x 1 m) metal cagesand gust through March in each of 1985/1986 and 1986/ supplied with fresh water before treatment. Room 1987. During these months, groups of birds were temperature was maintained at 25øCuntil birds were housedin 15 x 7 x 3 m outdoorflight pens.We used 14 days old, when the temperature was reduced to pure Black Ducks (7 female, 7 male), pure Mallards 20øC.Pens were cleaned every 2-4 days, and water (8 female, 7 male), and Mallard male-Black Duck fe- containerswere washedand refilled daily. Ducklings male (6 female, 7 male). Natural hybridization ap- were fed ad libitum a commercial starter diet (21% parentlyoccurs most frequently between female Black protein) to age 21 days, after which they received a Ducksand male Mallards (Brodskyand Weatherhead commercial grower diet (19% protein). January1991] HybridDuck Salt Tolerance 91

T^I•I•œ1. Means(+ SE)and univariatestatistics for male and femaledifferences in growth rates(k) of American Black Ducks, hybrids, and Mallards raised on fresh water.

Group Sex n k F P Black Duck male 14 1.0214 + 0.0007 1.67 0.21 female 12 1.0208 _+ 0.0005 Mallard-Black hybrid male 11 1.0255+ 0.0004 2.48 0.14 female 16 1.0244 _+ 0.0007 Mallard male 19 1.0253 + 0.0006 2.22 0.15 female 13 1.0247 _+ 0.0005

In both experiments,we weighedducklings at hatch urnal changesin weight (Blake 1956). At the time of (day 0) and every other day until day 14, after which death, either as a result of the treatment or euthanasia masswas recordedevery 4 days.Mass was measured after attaining asymptoticmass, the salt glands were to the nearest 0.1 g with an electronic balanceuntil excisedand weighed to the nearest0.001 g on a digital ducklingsattained 400 g, and thereafter,to the nearest pan balance.Skull length was measuredwith calipers 10 g with a Pesolaspring scaleuntil the individual to the nearest 0.01 mm. reachedan asymptoticbody weight. Ducklings were Statisticalanalyses.--Increments to body massof each placedin a nylon stockingto restrictmovement dur- individual in eachof three groups(Black Ducks, Mal- ing weighing, and they were weighed at the same lards,and hybrids)were plotted and curvesfit by eye. time each day to minimize variability causedby di- To determinewhether curveswere bestdescribed by

(a) (b) 1200 1200 -

IOO0 IOO0

2OO 2OO

IOO0

400

2OO 2OO

3 5 • 9 1•1 AGE (weeks) AGE (weeks) Fig. 1. Growth curvesfor body mass(g) of BlackDucks (&), hybrids(•), and Mallards (0) raisedon fresh water in 1986:(a-c) Males (solid symbols)and females(open symbols)plotted separately;(d) Males and femalescombined. Curves were bestfit by the Gompertzequation. For clarity, only weekly valuesare plotted. 92 BARNESAND NUDDS [Auk,Vol. 108

1200- length amongBlack Ducks, Mallards, and hybridsby analysisof variance (ANOVA) and, where appropri- ate, we analyzeddifferences between means with the 1000- leastsignificant difference (LSD) multiple range test (SAS Institute Inc. 1985). Log transformationswere usedto normalizedata (Ostleand Mensing 1975).In 800 - 1987, we analyzed growth rates and gland massby 3-way factorialANOVA, testingfor interaction(Sne- decor and Cochran 1980). If ANOVA indicated sig- 600 - nificant interactions,data were separatedby one of the interactingeffects and reanalyzed.

400 -

RESULTS

200 In 1986, 62 ducklings were hatched from 9 Black Duck clutches,54 from 7 hybrid clutches, and 47 from 10 Mallard clutches. Mean duck-

1 3 5 7 9 11 ling massdid not differ among groups at hatch (F = 1.30, P = 0.30) or at 48 h post-hatch (F = AGE (weeks) 2.98,P = 0.18).Mean duckling mass at 48 h was Fig. 2. Growth curvesfor body mass(g) of Black 51 + 5.3 grams. Ducks (&), hybrids (I), and Mallards (0) raised on There were no differencesin growth between freshwater in 1987.Curves were bestfit by the Gom- males and females in any group (Table 1), so pertz equation. For clarity, only weekly values are we pooled sexesfor all subsequentanalyses. On plotted. fresh water in 1986, Black Ducks grew slower (F = 37.62, P < 0.0001) than hybrids and Mal- logistic,Gompertz, or Von Bertalanffyequations, each lards (Fig. 1). Hybrids and Mallards did not dif- curve was transformedto a straight line following fer in growth. No mortality occurred in duck- Ricklefs (1967). The slope of the transformed curve lings raised on fresh water, but all ducklings is proportionalto the overall growth rate. A growth given 3% NaC1 at 48 h post-hatch died within rate constant (k), which takes into account the form 50 h. Postmortem examinations were not done, of the curve,was calculated from the growth equation (Ricklefs 1967). "Adult" mass was considered to be but ducklings exhibited signs associatedwith the asymptoteof the juvenile growth curve even salt intoxication. The earliest signswere a dull, though this massmay underestimatetrue body mass, listless appearance, followed by progressive whichincludes more fat (Lightbodyand Ankney 1984). muscular weakness characterized by stagger- Becauserepeated measureswere taken on each in- ing. Eventually, ducklings were too weak to dividual through time, we usedthe meansfor single stand and remained close to or directly in their measurementsthrough time to calculatea growth rate water containers and drank constantly until for each individual rather than calculate a single death. At death, mean mass(29 + 8.1 g) of all growth rate for each species.This allowed for a more ducklings combined was 43% lower than the conservative test of differences between Mallards, body massof 48-h-old ducklings. BlackDucks, and hybrids. Forboth experiments,contingency analysis was used In 1987, 68 ducklings hatched from 6 Black to test the hypothesisthat the probability of survival Duck clutches, 72 from 9 hybrid clutches, and was the samein all treatments.In 1986,we compared 69 from 7 Mallard clutches. Limited numbers differencesin growth rates,salt-gland size, and skull of ducklings prevented equal numbers of males

TABLE2. Univariate statisticsand multiple comparisonsfor differencesin growth rates,gland weights, and skull lengthsof AmericanBlack Ducks (B), Mallards (M), and hybrids (H) raisedon fresh water.

Variable Source df F P Multiple comparisona Growth rate species 2 37.62 0.0001 B < H < M Gland weight species 2 7.01 0.0001 B > H > M Skull length species 2 1.09 0.3393 B < H < M Speciesunderscored by the sameline are not significantlydifferent at P < 0.05. January1991] HybridDuck Salt Tolerance 93

1.5% NaCI

5a 7a 8a 7a

10a

5a 5a 5a 6b i i I

7 14 21

1.0% NaCl

10a 7a 6a 7a 8a 11a 9a

12b

7 14 21

0.5% NaCl

8a 5a 6a 7a 9a 7a 5a 5a 5a

7 14 21 AGE (days) Fig. 3. Proportionsof BlackDucks (solid bars),hybrids (cross-hatchedbars), and Mallards (open bars)that attained adult body massin each age-salinity treatment. Groups sharing similar lower-caseletters do not differ (P < 0.05) by contingencyanalysis. Numbers above histograms are samplesizes. and females in each treatment, and therefore 2). Although skull length did not differ among data could not be analyzed by sex. As in 1986, groups,Black Ducks and hybrids on fresh water Black Ducks on fresh water grew slower (P < had heavier salt glandsthan Mallards (Table 2). 0.05) than hybrids and Mallards (Fig. 2, Table Survivorship,measured as the proportion of 94 BARNES AND NUDDS [Auk, Vol. 108

1.0222,

8 10 1.0218 5 5 5 I [ i 7 14 21

1.0% NaCl

1.0226

1.0222, 10 11 8 12 9

o 1.0218 9

7 14 21

0.5% NaCI

5 5 5 1.0225

1.0218

7 14 21

AGE (days) Fig. 4. Mean growth rate (k) for BlackDucks (solid bars),hybrids (cross-hatchedbars), and Mallards (open bars) attaining adult body massin eachage-salinity treatment. Numbers above histograms are samplesizes. individuals that attained adult body mass in Ducklings that received salt concentrations eachage-salinity treatment, increased with age >0.5% NaC1 regularly had nasal fluid coming and decreasedsalinities for all groups (Fig. 3). from their nostrils,indicating a functioningsalt No mortality occurred in ducklings raised on gland. 0.5%NaC1, but 1.5%NaC1 was fatal to all 7-day- Generally, growth was more retarded the old ducklings(Fig. 3). In general,Mallard duck- greater the salt concentrationin drinking wa- lings were lesstolerant of salt than BlackDucks ter and the younger the ageat which ducklings and hybrids when ducklings <21 days were were administered the treatment (Fig. 4). We administered salinities >0.5% NaC1 (Fig. 3). found significantinteractions among group, sa- January1991] HybridDuck Salt Tolerance 95

T^BI.I•3. Effectsof group (Mallards, Black Ducks, and hybrids) and salinity on growth rate (k) by age (A), and the effectsof speciesand age on growth rate (k) by salinity (B).

(A) Age (days) 7 14 21

Source df F P F P F P Species 2 999.99 0.0001 999.99 0.0001 2.90 0.0670 Salinity 2 999.99 0.0001 999.99 0.0001 123.39 0.0001 Speciesx salinity 4 999.99 0.0001 999.99 0.0001 14.66 0.0001 (B) Salinity (% NaC1) 0.5 1.0 1.5

F P F P F P Species 2 25.23 0.0001 999.99 0.0001 999.99 0.0001 Age 2 2.15 0.1307 999.99 0.0001 999.99 0.0001 Speciesx age 4 1.18 0.1353 999.99 0.0001 999.99 0.0001

linity, and age. When growth rates were rean- hatchedBlack Duck ducklingsdied after drink- alyzed by age, effects of speciesand salinity ing water containing > 1% NaC1. decreasedwith age (Table 3). When data were Identification of salt intoxication in wild reanalyzedby salinity, effect of speciesand age ducklingsis difficult, and this may be why salt increased with increasing salinities (Table 3). Although group differenceswere apparent at 1.5% NaCI low salinities, they were similar to differences found for the freshwatercontrols (Fig. 2). Black

Ducksand hybridswere lessaffected by changes 0.30 in salt concentrations than were Mallards. Mass of salt glands from ducklings that at- 0.10 tained adult body massdiffered among groups and was affected by age and salinity (Fig. 5, 7 14 21 Table 4). Salt glandswere heavier in BlackDucks 1.0% NaCl and hybrids than in Mallards and generallyin- '• lO creasedin massfaster than in Mallards as age and salinityincreased. Salt glands of BlackDucks and hybrids reacheda maximum size when 1% NaC1 was administered at 14 days of age, but not until 1.5%NaC1 and 21 daysof age for Mal- lards (Fig. 5). Group differencestended to de- 7 14 21 creasewith age and increase with increasing 0.5% NaCl salinity (Table 4).

DISCUSSION 0.30 5 6

Few studieshave documentedthe reproduc- 0.10 tive output of Black Ducks breeding in estuarine or coastalenvironments (Gross 1945, Wright 7 14 21 1954, Bartlett 1963, Reed and Moisan 1971, Reed 1975). Thus, it is unknown if Black Duck broods AGE (days) inhabit theseareas regularly. BlackDuck nests Fig. 5. Mean salt-glandmass (g) for BlackDucks have been found on coastal islands in Maine, (closedbars), hybrids (cross-hatched),and Mallards many of which lacked obvious sourcesof fresh (open bars)that attainedadult body massin eachage- water (Gross 1945), but brood survival in these salinity treatment. Numbers above histogramsare habitatsis unknown.In our experiments,newly sample sizes. 96 BARNESAND NUDDS [Auk,Vol. 108

TABLE4. Effectsof group (Mallards, Black Ducks, and hybrids) and salinity on salt gland massby age (A), and the effectsof speciesand age on salt gland massby salinity (B).

(A) Age (days) 7 14 21

Source df F P F P F P Species 2 108.85 0.0001 151.27 0.0001 33.57 0.0670 Salinity 2 126.84 0.0001 18.89 0.0001 39.01 0.0001 Speciesx salinity 4 24.85 0.0001 11.22 0.0001 00.50 0.7390 (B) Salinity (% NaC1) 0.5 1.0 1.5

F P F P F P Species 2 39.43 0.001 147.98 0.0001 165.75 0.0001 Age 2 1.38 0.1307 35.27 0.0001 540.84 0.0001 Speciesx age 4 0.82 0.5203 8.54 0.0001 72.19 0.0001

tolerance in precocial hatchlings has been (Reed and Moisan 1971). We demonstrated that largely assumed.For example,Reed (1975) re- duckling mortality decreasedwith age and de- ported that Black Ducks on the St. Lawrence creased salinity, and salt-gland weights in- estuarynested on mainlandareas and on islands creasedwith age and salinity, consistentwith 5.6 km offshore. Broods were reared in tidal Reed and Moisan's (1971) observations that ac- marshesalong the mainland. Of 47 broods climation to salt water is age-dependent.In an hatched on the islands, only 42% migrated suc- east-coastriver, distance upstream was nega- cessfullyto the tidal marshes,and only 36%of tively related to agesof BlackDuck broods,but these survived. Intensive observations revealed not broodsof Blue-winged Teal (A. discors),a that avian predatorscaused a small fraction of typically freshwaterbreeder whose broodsre- the observed mortality; the remainder was mained upstreamregardless of age (Barnesand thought to be due to chilling and exhaustion. Martin unpubl. data). However, duck broods routinely travel long BlackDuck ducklingsappear to survive and distances(Ringleman and Longcore 1982), so grow slightly better than Mallard ducklingsin exhaustion may be an unsatisfactoryexplana- brackishenvironments, but theinterspecific dif- tion for the high mortality. Reed (1975) re- ferencesappear insufficient to affectestablish- ported that ducklings that arrived at the tidal ment of a postmating reproduction isolating marsh did not "display the typical vigor of mechanism between Black Ducks and Mallards ducklingsof that agewhen feeding and follow- for two reasons. First, salinities at which we ing the hen" (p. 252),and they often died with- found differencesin survival and growth be- in 2 days. Perhaps these ducklings were salt tween Mallard and Black Duck ducklings were intoxicated. low and similar to salinities along tidal river There are at least two ways that precocial systems.Black Ducks appear unable to maintain young that lack functional salt glandsat hatch water and electrolytehomeostasis in their body (like Black Ducks) can fledge in brackishhab- fluidsat salinitylevels great enough to facilitate itats. First, enough "metabolic water" may be habitat segregationthrough reducedsurvival produced from digestion. Second, ducklings of hybrids and Mallards on coastalbreeding may respondbehaviorally to a salinity gradient areas.Second, the functionalcapacity of the salt in an age-dependentfashion (Bartlett 1963, Reed gland was partially dependentupon a genetic 1975).For instance,newly hatchedBlack Duck component and, although Mallard ducklings broods in the St. Lawrence estuary restricted were less tolerant of salt than were Black Ducks, their use of tidal marshes to areas close to shore hybrid ducklingsapparently inherited salt tol- where salinitylevels were typicallylower (Reed erance more similar to Black Ducks. Our data and Moisan 1971). With increased age, how- are inconsistentwith the "habitat refugia" hy- ever, broods extended their range "seaward" pothesis,so it appearsthat hybrids have the January1991] HybridDuck Salt Tolerance 97 potential to inhabit coastal regions presently BRODSKY,L. M., & P. J. WEATHERHEAD.1984. Beha- occupied by Black Ducks, and to further back- vorial and ecological factors contributing to cross with either Black Ducks or Mallards. Mal- American Black Duck-Mallard hybridization. J. lards may continue, in the absenceof any sig- Wildl. Manage. 48: 846-852. COLLINS,J.M. 1974. The relative abundance of ducks nificant counterselection,to expandtheir range into estuarine habitats where the Black Duck is breeding in southern Ontario in 1951 and 1971. Pp. 32-44 in Canadian Wildlife Servicewaterfowl the dominant species. studies in eastern Canadaß1969-1973 (H. Boyd, Ed.). Can. Wildl. Serv. Rep. No. 29. ACKNOWLEDGMENTS CONROY, M. G., G. G. BARNES,R. W. BETHKE,& T. D. NUDDS. 1989. Increasing Mallards, decreasing We wish to thank the Canadian Wildlife Service American Black Ducks: no evidence for cause and (CWS) for their financial and logistical support effect. A comment. J. Wildl. Manage. 53: 1065- throughout this study. In particular,we thank J. Car- 1071. reiro, G. McCulloughßand D. Dennisßwhose help in COOCH,F. G. 1964. A preliminary study of the sur- the initial stages made this study possible. We also vival value of a functional salt gland in prairie thank R. Dibblee, E. Paceß A. Smithß L. Beattieß P. Anatidae. Auk 81: 380-393. Madore, D. Gow, M. Kasserra, and J. Hanselman for CORNELIUS,S.E. 1982. Wetlandsalinity and saltgland their help in variousaspects of this research.G. Hur- size in the Redhead,Aythya americana.Auk 99: nik permitted useof facilitiesat the ResearchStation, 774-778. Poultry Division of the Ontario Ministry of Agricul- DENNIS, D. G., K. L. FISCHER, & G. B. MCCULLOUGH. ture and Foodß Arkell, Ontarioß and F. Black at the 1984. The changein statusof Mallards and Black Lab Animal BuildingßUniversity of Guelph. Special Ducks in southwesternOntario. Pp. 27-31 in Wa- thanks to K. Barnesand C. Earley for help in raising terfowl studies in Ontarioß 1973-81 (S. G. Curtisß the ducklings and for all aspectsof data collection. D. G. Dennisß& H. Boyd, Eds.).Can. Wildl. Serv. A. Middletonß V. ThomasßJ. Ballßand R. Bethke, G. Occas. Paper No. 54. Hepp, T. Quinney, and an anonymous reviewer pro- ELLIS, R. A., C. C. GOERTMILLER,P. A. DELELLIS,& Y. vided valuable comments on earlier drafts of this H. KABLOTSKY. 1963. The effect of salt water manuscript. This researchwas supported by grants regimen on the development of salt glands of and contractsfrom the Natural Sciencesand Engi- domestic ducklings. Dev. Biol. 8: 286-308. neering ResearchCouncil (NSERC;Grant No. A7757), GILLESPIE,G. D. 1985. Hybridizationßintrogression the CWS (Ontario Region)ßand the University Re- and morphometric differentiation between the search Support Fund of CWS to Nudds, and schol- Mallard (Anasplatyrhynchos) and Grey Duck (Anas arships from the Ontario Federationof Anglers and superciliosa)in Otago, New Zealand. Auk 102:459- Hunters and NSERC to Barnes. 469. GPaGG, G. C. 1981. Plasma homeostasis and cloacal

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