j. Field Ornithol., 51(4):361-364 PARASITISM OF CANVASBACK NESTS BY REDHEADS BY LAWSON G. SUGDEN Redheads(Aythya americana) lay eggsparasitically in the nestsof sev- eral other duck species(Weller 1959,Joyner 1973, 1976). Where Can- vasbacks(A. valisineria)are sympatricwith Redheads,they are the pri- mary host for Redhead nest parasitismbecause the two specieshave similar habitat requirementsand breeding seasons(Weller 1959). Al- though several studies(Hochbaum 1944, Weller 1959, Olson 1964) have consideredvarious aspectsof Redhead parasitismof Canvasback nests,our understandingof the interspecificrelationship is incomplete, particularly for pothole habitat. A study of Canvasbacksin the Saskatch- ewan parklandsfrom 1971 through 1975 afforded an opportunityto record some host-parasiterelationships between the two species. STUDY AREA AND METHODS The studyarea and methodshave been described previously (Sugden 1977, 1978). The area (52øN, 106øW), 48 km east of Saskatoon,Sas- katchewan, was a 15.5 km" block of farmland in 1971 and 1972 and 31.1 km2 in 1973, 1974, and 1975. Pond densitiesaveraged about 18 to 26 per km2. Size of ponds ranged from lessthan 0.04 ha to 8.1 ha. Breeding pair countswere made in May. Two countswere made in 1971, 10 in 1972 and 1973, and nine in 1974 and 1975. Pairs and lone drakes were used to indicate breeding pairs of Canvasbacks,and total femalesindicated pairs of Redheads.Two nest searcheswere made be- tween late May and late July. RESULTS AND DISCUSSION FactorsAffecting Rate of Parasitism Weller (1959: 352) concludedthat the incidenceof Redhead parasit- ism was "influencedby habitat preferencesof the parasite,chronology of laying by host and parasite, and especiallyby the number of host nestsper parasite." My data support the first two conclusions.Using a subjectivescale for rating nestingpond quality (Sugden 1978), I con- cluded that Canvasbacknests in preferred Redhead habitat had higher rates of parasitismthan thoseon pondsconsidered to be inferior (X" = 8.08, df = 2, P < .02). Similarly, Canvasbacknests initiated in June had a higher rate of parasitism(80%) than thosestarted before June (58%) (x "= 6.25, df = 1, P < .02). Because Redheads tended to nest later than Canvasbacks,it is reasonableto expectlate neststo receivemore parasiticintrusions. My data on Canvasbacknest densitiesand numbers of female Red- heads(Table 1) indicatethat the relationshipbetween the two in pothole habitat is complex. Data for 1973 are of particular interest because, despitethe lowesthost-nest/parasite ratio, the rate of parasitismwas also lowest. This, of course, is contrary to the expected relationship (see 361 362] L.G. Sugden J.Field Ornithol. Autumn 1980 TABLE 1. Breeding pairs, nests,and egg data for Redheads and Canvasbacks. Year 1971 1972 1973 1974 1975 Size of area (km'-') 15.5 15.5 31.1 31.1 31.1 Redhead indicated pairs 14 16 49 26 43 Canvasbackindicated pairs 18 27 74 40 54 Redhead nests 2 8 28 9 35 Canvasback nests 17 21 45 51 66 Canvasbacknests per • Redhead 1.21 1.31 0.92 1.96 1.53 Percent Canvasbacknests I parasitized 63 86 54 62 68 Eggs/• Redhead in Redhead nests" 0.64 3.37 3.65 2.35 5.44 Eggs/• Redheadin Canvasbacknests" 1.07 3.37 1.73 2.85 5.05 1 Completed nests. "Includes eggsoutside nests. Weller [1959: 351] for discussion).Egg data (Table 1) alsoindicate that parasitismwas comparatively depressed in 1973. The ratio of eggslaid parasiticallyto those laid in Redhead nestswas relatively constantfor other years,agreeing with Olson's(1964) report. However,in 1973, the ratio was lower. I suggestthat the unexpectedlow rate of parasitismin 1973 was related to the large number of non-nestingCanvasback pairs that year (Sugden1978), which gave false information to female Red- heads searchingfor host nests. On large marshesRedheads locate nests by observingthe activitiesof the host, by searchingthe cover for host nests,and probablyby com- binations of the two (Weller 1959). Canvasbacknests in the pothole habitat were so widely distributedthat cover searchingalone would be unproductive.Canvasbacks tended to neston pondssolitarily (Sugden 1978); during the five years, 200 nestswere found on 177 different ponds. On the average,only about 8% of the ponds had one or more Canvasbacknests. These includedterminated nests,so at any given time the percentageof ponds occupiedby active host nestswould have been lower. With this kind of nest distribution, the presenceof the host (fe- male Canvasback)on a nest pond must have been a major factor in determining the successof a female Redhead in locating a host nest. Seeinga female Canvasbackon a pond likely stimulatesnest searching on the pond by a Redhead.Thus, in 1973, the presenceof non-nesting Canvasbackpairs would have thwarted would-be parasitic Redheads. Effect on CanvasbackClutches The mean number of Canvasbackeggs in 74 completednests that were not parasitizedby Redheadswas 8.1 compared with 5.4 in 168 parasitizednests. The differenceof 2.7 eggsper nestwas significant (t -- 7.34, P < .001). Bellrose (1976) summarized data from other studies Vol.51, No. 4 CanvasbackNest Parasitism [363 8 I ß Y= 7.523-0.370X 7 13 R2=.25 •6 e•'- P<O.001 • 4 8•'• 3•• 3 •2' I 1I I 3I I 5I I 7I i • i 11I I 13I I 15I I 17I Number of Redhead eggs at Canvasback nest FIGUREl. Relationshipbetween number of Canvasbackeggs in the nest and total Red- head eggspresent. Numbers indicate number of nestsin sample. reporting similar clutch reductions,and Weller (1959) stated that re- duced clutchesresulted from suppressedovulation in the hostand loss of eggsfrom the nest. During the last three years,systematic searches were made for eggsoutside all nestsso I could comparetotal eggslaid in parasitizedand unparasitizednests. The mean number of Canvasback 1 Y:O.378x- O.Oll ß R2-- .42 0.001 3 ß 4 1 I 4I I 6I I 8I I 1I0 I 12I I -14I I 16I i Number of Redhead eggs at Canvasback nest FIGURE2. Relationshipbetween number of Canvasbackeggs outside the nest and total Redheadeggs present. Numbers indicate number of nestsin sample. 364] L.G. Sugden J.Field Ornithol. Autumn 1980 eggsin and around 40 completedunparasitized nests was 8.7 compared with 7.6 for 67 parasitizednests (t -- 2.38, P < .02), indicatingthat sup- pressedovulation through parasitismresulted in about one lessCan- vasbackegg per clutch. In the same sample,eggs outsideparasitized nests averaged 1.84 compared with 0.23 outside unparasitizednests. Thus, Redheadparasitism resulted in a total reductionof about 2.7 eggs per nest--l.1 due to suppressedovulation and 1.6 due to eggs lost outside the nest. The number of Canvasbackeggs found inside parasitizednests was inverselyrelated to the total number of Redheadeggs found at the nest (Fig. 1). A similar relationshiphas been shownfor CinnamonTeal (Anas cyanoptera)nests parasitizedby Ruddy Ducks (Oxyurajamaicensis) and Redheads(Joyner 1976). Conversely,the number of Canvasbackeggs outsideparasitized nests was directly related to the number of Redhead eggs(Fig. 2). Total Canvasbackeggs was unrelated to the number of Redhead eggs (P > 0.7), indicating that egg reduction due to sup- pressedovulation was unrelated to the number of parasiticintrusions. ACKNOWLEDGMENTS I thank my field assistantsand the farmers upon whose land we worked. I am grateful to Milton W. Weller and Arthur S. Hawkins for commentingon the manuscript. LITERATURE CITED BELI•ROSE,F.C. 1976. Ducks,geese & swansof North America. Harrisburg,PA, Stack- pole Books. HOCVIBAt•, H. A. 1944. The Canvasbackon a prairie marsh. Washington,Am. Wildl. Inst. JoY•ER,D. E. 1973, Interspecificnest parasitism by ducksand cootsin Utah. Auk 90: 692-693. 1976. Effectsof interspecificnest parasitismby Redheadsand Ruddy Ducks.J. Wildl. Manage. 40: 33-38. OLson, D. P. 1964. A study of Canvasbackand Redheadbreeding populations,nesting habitatsand productivity.Ph.D. dissertation,Univ. of Minnesota,Minneapolis. St•ct)•, L. G. 1977. Horned Grebe breeding habitat in Saskatchewanparklands. Can. Field-Nat. 91:372-376. 1978. Canvasbackhabitat use and productionin Saskatchewanparklands. Can. Wildl.Serv. Occas. Paper No. 34. 30 p. WELLER,M. W. 1959. Parasiticegg laying in the Redhead (Aythyaamericana) and other North American Anatidae. Ecol.Monogr. 29:333-365. CanadianWildlife Service,115 PerimeterRoad, Saskatoon,Saskatchewan, CanadaS7N OX4. Received20 November 1979, accepted11 March 1980. .