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Factors Affecting The Abundance Of Ducks

The best aquatic habitat for ducks and other waterfowl are those with relatively shallow water, with very productive vegetation and large populations of invertebrates. Those habitats with a large ratio of shoreline to surface area, favors the availability of secluded nesting sites. These sorts of habitat occur to some degree in most regions, and are primarily associated with wetlands, especially marshes, swamps, and shallow, open water. In North America and elsewhere during the past century, extensive areas of these types of wetlands have been lost or degraded, mostly because they have been drained or filled in for agricultural, urban, or industrial use. Wetlands have also been degraded by eutrophication caused by excessive nutrient inputs, and by pollution by toxic chemicals and organic materials. These losses of habitat, in combination with overhunting, have caused large decreases in the populations of ducks throughout North America, and in most other places where these birds occur. Consequently, there are now substantial efforts to preserve or restore the wetlands required as habitat by ducks and other wildlife, and to regulate hunting of these animals.

The most important breeding habitats for ducks in North America occur in the fringing marshes and shallow open-water wetlands of small ponds in the prairies, known as "potholes." The marshy borders of potholes provide important breeding habitat for various species of dabbling ducks such as mallard, pintail, widgeon, and blue-winged teal, while deeper waters are important to lesser scaup, canvasbacks (Aythya valisneria), redheads (Aythya americana), and ruddy ducks. Unfortunately, most of the original prairie potholes have been filled in or drained to provide more land for agriculture. This extensive conversion of prairie wetlands has increased the importance of the remaining potholes as breeding habitat for North America's declining populations of ducks, and for other wildlife. As a result, further conversions of potholes are resisted by the conservation community, although agricultural interests still encourage the drainage of these important wetlands.

In years when the prairies are subject to severe drought, many of the smaller potholes are too dry to allow ducks to breed successfully, and ponds and wetlands farther to the north in Canada become relatively important for breeding ducks. Another important source of natural mortality of ducks and other waterfowl are infectious disease, such as avian cholera, which can sweep through dense staging or wintering populations, and kill tens of thousands of birds in a short period of time. When an epidemic of avian cholera occurs, wildlife managers attempt to manage the problem by collecting and burning or burying as many carcasses as possible, in order to decrease the exposure of living birds to the pathogen.

Lead shot is an important type of toxic pollution that kills large numbers of ducks and other birds each year. Lead shot from spent shotgun pellets on the surface mud and sediment of wetlands where ducks feed, may be ingested during feeding and retained in the duck's gizzard. There the shot is abraded, dissolved by acidic stomach fluids, absorbed into the blood, and then transported to sensitive organs, causing toxicity. An estimated 2-3% of the autumn and winter duck population of North America (some 2-3 million birds) dies each year from lead toxicity. As few as one or two pellets retained in the gizzard can be enough to kill a duck. Fortunately, steel shot is rapidly replacing lead shot, in order to reduce this unintended, toxic hazard to ducks and other wildlife.

Ducks and other aquatic birds may also be at some risk from acidification of surface waters as a result of acid rain. Although it is unlikely that acidification would have direct, toxic effects on aquatic birds, important changes could be caused to their habitat, which might indirectly affect the ducks. For example, fish are very sensitive to acidification, and losses of fish populations would be detrimental to fish-eating ducks such as mergansers. However, in the absence of the predation pressure exerted by fish in acidic lakes, aquatic invertebrates would become more abundant, possibly benefitting other species of ducks such as common goldeneye, ring-necked duck, and black duck. These scenarios are inevitably speculative, for not much is known about the effects of acid rain on ducks.

Ducks can also be affected by eutrophication in aquatic habitats, a condition characterized by large increases in productivity caused by large nutrient loads from sewage dumping or from the runoff of agricultural fertilizers. Moderate eutrophication often improves duck habitat by stimulating plant growth and their invertebrate grazers. However, intense eutrophication kills fish and severely degrades the quality of aquatic habitats for ducks and other wildlife.

Some species of ducks nest in cavities in trees, a niche that has become increasingly uncommon because of forestry and losses of woodlands to agriculture and urbanization. Together with overhunting, the loss of natural cavities was an important cause of the decline of the wood duck and hooded merganser (Lophodytes cucullatus) in North America. Fortunately, these species will nest in artificial cavities provided by humans, and these ducks have recovered somewhat, thanks in part to widespread programs of nest box erection in wetland habitats.

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Science EncyclopediaScience & Philosophy: Direct Variation to DysplasiaDucks - Dabbling Ducks, Bay And Sea Ducks, Economic Importance Of Ducks, Factors Affecting The Abundance Of Ducks - Tree or whistling ducks, Stiff-tailed ducks, Mergansers