by Dr. Frank C. Rohwer and Dr. David B. Smith
There is little doubt that the population of American black ducks (Anas rubripes, hereafter black ducks) has shown a long and serious decline from when state and federal biologists first started conducting mid-winter inventories. However, the causes of that decline have been debated for decades. Traditional explanations for wildlife population declines, such as loss and degradation of critical winter habitat and overharvest, certainly have advocates. Overharvest is unlikely, but there is no doubt that there has been serious loss of wintering habitat. It is also clear that breeding habitats, especially areas in the southern segment of eastern Canada and New England have been greatly altered. One of the most interesting and likely explanations of population declines is that black ducks have suffered from interactions with their closest relatives, namely mallards (Anas platyrhynchos).
Wildlife managers often face situations where management aids one species while it has an adverse affect on other species. Sometimes these trade-offs are obvious. On a 36-square mile area in North Dakota in spring 2001 we have bumped nesting success for dabbling ducks from 18% to something above 70% with seasonal predator control. While that is remarkably good for ducks, it is obviously bad management if you like skunks, raccoons, and red fox. Habitat management may appear less controversial, but it has the same sorts of trade-offs. Managing forests so that there are openings in the canopy to promote edge provides good cover and forage plants for traditional game species like quail, rabbits, and deer, but the same canopy openings are very detrimental for many forest interior birds, such as hermit thrushes. Thus, many bird watchers favor different habitat management than do consumptive users of wildlife, most of which like white-tails more than thrushes. What is interesting about the mallard-black duck situation is that one user group, namely eastern seaboard wildfowlers, may have to decide which of the two prized species they want to favor. Clearly, we need to know if managing for mallards comes at the detriment of black ducks.
Biology students learn that two species can interact in several ways, including as predator-prey, host-parasite, competitors, or partners for hybridization. Mallards have not been accused of being predators or parasites of black ducks, but they present threats on three other fronts: as competitors for resources; sources of diseases; or undesirable mates leading to hybridization. The disease question mostly relates to captive-reared mallards, but there are scant data on this subject and the issue has been reviewed by Dr. James Pearson in another article on the SCWA web site, so we shall focus on competition and hybridization.
Do mallards usurp vital resources from black ducks?
Mallards and black ducks are genetically and ecologically very similar, so there is plenty of potential for competition. However, the evidence for competition is not particularly strong. First we will focus on events during the breeding season. The eastern expansion of breeding populations of mallards has been mirrored by an eastern retreat for breeding black ducks in Ontario and Quebec. These concomitant population changes may reflect the outcome of competition for breeding habitat, with mallards as the clear winners. We agree with that assessment, but we note that it is difficult to rule out the alternative idea that habitat changes in southern Ontario were favorable to mallards and made the habitat unsuitable for black ducks. In this scenario competition plays no role in the black duck population changes.
Assessing whether mallards or black ducks compete for limited resources where they share wintering sites is even more difficult than on breeding areas. There is essentially no data to directly assess competition in the winter. However, our research in Maryland on mallard release sites provided information on habitat use that is relevant to the issue. When individuals or clubs release captive-reared mallards on the eastern shore of Maryland they will only retain those birds on their property when they also provide habitat with abundant food, such as flooded agricultural fields. Aerial surveys we conducted showed that wild ducks make extensive use of habitats that are intensively managed for released mallards, yet wild ducks comprise only a small fraction the harvest on these intensively managed Regulated Shooting Areas. Capital investment into captive-reared mallards motivates management of habitat to provide food to hold birds in the area. That management certainly benefits wild waterfowl as well. Thus, it seems unlikely that released mallards are competing with black ducks. We did many behavioral observations of birds on these highly managed areas and almost never saw black ducks and mallards interacting; we certainly did not see exclusion of black ducks from prime feeding areas.
There is absolutely no doubt that hybridization poses a serious threat to black ducks. The fraction of black ducks that are hybrids with at least partial mallard ancestry ranges from 10% to almost 50%. The concern is that far more abundant mallards will eventually hybridize black ducks out of existence. The pressing management issue is whether releases of mallards exacerbates the hybridization problem. It seems logical that releasing captive-reared mallards in areas with black ducks would increase hybridization rates. However, data from Maryland do not readily support this idea.
Maryland has a long tradition of large scale releases of mallards, with numbers approaching 100,000 in peak years. When we initiated research in 1991 we expected, based on published work, to encounter frequent pairings between captive-reared drake mallards and wild black duck females. However, in three years we examined 492 mallard and 159 black duck pairings and saw few mixed species pairs. Three black duck females had paired with male mallards. The single drake mallard that we could positively identify was of wild origin. Three female mallards paired with males that were mallard-black duck hybrids. The only female of those three with known origin was a wild bird. Our pairing data suggest that mixed species pairings are only 1.6% for black ducks, but harvest data reveal that at least 8.4% of black ducks are actually black duck-mallard hybrids. This suggests that mixed species matings are more common than winter pairing behavior suggests. Perhaps repairing or forced copulation at northern breeding sites explain the discrepancy between the low occurrence of mixed species pairs in the winter in Maryland and the actual frequency of hybrids. Our pairing data clearly contradict the findings of Brodsky and Weatherhead from 1984, which suggest that as soon as the last female mallards had paired the remaining unmated drake mallards would then intensively court and form mixed species pair bonds with hen black ducks.
Our telemetry studies showed that released mallards rarely moved more than a few miles during the six months after release, during which time there was about 80% mortality. Pairing data revealed that captive-reared mallards preferentially mated with other captive-reared birds; likewise wild birds preferentially mated with wild mallards. This means that most captive-reared mallards in Maryland were shot or died long before the breeding season, they rarely moved far from their release site and the great majority of the time were on intensively managed Regulated Shooting Areas, and they preferentially form pairs with other released mallards. Accordingly, there is little probability that mallards released in Maryland increase the rates of hybridization with black ducks. Mallard releases in other eastern states have resulted in greater dispersal. For instance, 31% of band recoveries from mallards released in Pennsylvania were from out-of-state.
In summary, our intensive research on Maryland’s eastern shore revealed almost no information to suggest that the large-scale state and private releases of captive-reared mallards cause serious problems for black ducks by increasing hybridization rates or facilitating competition. Of course, the situation with released mallards is quite different in North and South Carolina, but it is also clear that wintering black ducks are far less abundant that in the Chesapeake Bay wetland. Concerns over the possible adverse impact of released mallards on black ducks could easily be addressed in North and South Carolina, by studying pair formation during the winter and spring months.