REPRODUCTIVE OUTPUT OF BLACK DUCKS IN THE

The breeding biology of black ducks (Anas rubripes) was studied in an estuarine habitat over the period 1963-73. Nests were located on islands and on the mainland in the vicinity of tidal marshes. Of 590 nests, 248 (42 percent) reached the hatching stage; the mainland-nesting portion of the popu- lation showed a nest success of 28 percent (N = 83 nests), those using the main nesting island, Ile- aux-Pommes, 44 percent (N = 478 nests), and those using other islands 52 percent (N = 29 nests). On Ile-aux-Pommes annual nest success varied between 30 and 71 percent. Unsuccessful clutches were lost to predation (mainly gull (Larus sp.) and crow (Corvus brachyrhynchos) on islands, red fox (Vulpes fulva) and crow on mainland) or desertion. Fifty-two nests (9 percent of total) were judged to have been abandoned as a direct result of disturbance by the investigator. Annual fluctuations in nesting success could not be clearly attributed to weather factors, the quantity or quality of concealing cover, or to predator or competitor density. Nesting success did not vary according to laying date, but did vary in relation to the stage of nesting; the probability of loss was much greater during egg-laying than during incubation. Through renesting, nest losses were compensated for in some years but not in others. On the average, 65 percent of island nesting pairs brought off broods. Using capture-recapture tech- niques and visual observation, I estimated that 55 percent of all broods produced flying young, and 34 percent of ducklings that hatched reached flight age. On the basis of these estimates, 100 breeding pairs would produce 183 fledged young, a rate much lower than reported for this species in other studies; the discrepancy of results is attributed mainly to losses of entire broods which were unaccounted for in other studies. J. WILDL. MANAGE. 39(2):243-255 To manage animal populations with the aim of assuring a harvestable surplus, knowledge of the species' reproductive out- put (rate of production of fledged young) is essential. Attempts to modify waterfowl habitat to favor increased production of young should be aimed at minimizing the effects of those factors which cause most prefledge losses. Detailed knowledge of the stages at which losses occur and the circumstances in which they occur should form the basis for attempts to preserve or modify habitat and to manipulate predator or competitor populations. Both nesting and brood-rearing phases of the breeding cycle must be considered. This paper, based on the results of 11 years' study of the black duck in the St. Lawrence estuary, Quebec, attempts to evaluate the rate of production of fledged young per nesting female. Intensive hunting of the relatively small population of black ducks in eastern North

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