Population ecology and harvest of the American black duck: a review

Abstract

1. The purpose of our review was to examine available data on population trends and current status of black ducks and trends in natality and survival and to relate these, where possible, to changes in habitat, predation, disease, contaminants, harvest, and hybridization with mallards. 2. The number of black ducks tallied in the winter survey has declined steadily over the past 30 years at an average rate of about 3%/ year. Reliability and precision of the survey are uncertain; it may not provide an adequate index to the continental population of black ducks. Breeding surveys are incomplete and sporadic, but black ducks have decreased in Ontario and increased in the Maritime Provinces and Quebec. 3. Recent declines in numbers of black ducks tallied in the winter survey are not unusual in magnitude or much different from those that have occurred among several other species of waterfowl. 4. At present, black ducks are not especially scarce relative to numbers of several other ducks in eastern North America. 5. There is no solid evidence of major decreases in quality or quantity of breeding habitat for black ducks in recent years; in some areas, habitat has improved. 6. Natural mortality of black ducks has not been well studied, but does not seem unusually high compared to other dabbling ducks. 7. Harvest rates of black ducks are similar to those of sympatric mallards as determined by banding analyses. 8. There is no strong evidence for direct effects of contaminants on black ducks, but some indirect effects through invertebrate food resources have been detected. 9. Age ratios in black ducks show no trend in the past 18 years. 10. The quality and quantity of wintering habitat for black ducks have decreased substantially in some areas. 11. Disease and other natural mortality that affect black ducks do .not occur in unusually high frequency. 12. A decline in harvest of black ducks has occurred; most of the decline has been in the United States, especially since restrictive regulations were implemented in 1983. 13. Recovery rates of black ducks have declined recently in the U.S., but not in Canada. 14. Survival rates of black ducks are 56-63% for adults and 43% for young. These rates of survival are similar to comparable estimates in sympatric mallards. 15. Long hunting seasons may depress survival in some sex-age classes of black ducks, buteffects of small reductions in survival on population trends are unknown. 16. Available evidence does not support the contention that hunting is either the sole or most important cause of the decline in the winter tally of black duck numbers. 17. Surveys and banding of black ducks should be thoroughly reviewed and maintained or improved as warranted. Obtaining or maintaining a reasonable index to numbers of black ducks is the top priority among survey needs. 18. Experimental manipulation of hunting seasons should be considered to elucidate relationships among regulations, harvest, survival, and population trends. 19. Black ducks and mallards are genetically similar; there is as much genetic differentiation within the 2 species as there is between them. 20. Black duck x mallard hybrids are fertile. Hybrids are difficult to detect by plumage and thus published frequencies (0-13%) of hybrids may be low. 21. Hybridization could be a result of concomitant mallard increases and black duck decreases, or changes in black duck-to-mallard ratios could be from hybridization and genetic swamping of black ducks.