Limiting factors for the Trumpeter Swan include
Shortage of Winter Habitat, Shortage of Breeding
Habitat, Disturbance in Breeding Habitat,
as well as Hunting.
Shortage of Winter
Habitat: A critical shortage of winter habitat is widely considered to be the primary limiting factor for the
Rocky Mountain population, which includes birds breeding in Alberta. This population of Trumpeter Swans faces a unique challenge, because over 90% of the birds winter in a very limited region of the
tri-state area. As a result, because of migration tradition, birds that breed successfully in Canada and the United States, return to the same crowded wintering area, limiting their ability to discover other more favourable wintering areas. The concentration of such a large portion of the population also means that any significant loss of habitat could cause a large decrease in the population. The overcrowded conditions have been the cause of large winter die-offs; in 1984 and 1989 severe winter conditions caused the Snake River to freeze resulting in the death of at least 50 and 100 swans, respectively.
Competition for food in overcrowded wintering areas causes at least three possible problems. First, because they have not fed properly, migratory females start the breeding season in poor condition. Second, females from the
tri-state subpopulation may make the short flight from their wintering areas to their breeding areas too early, when the breeding areas are still frozen, giving them little chance to improve their nutrient reserves. Finally, overcrowding and competition for food increases the prevalence of parasitic nasal leeches in food-deprived birds which can further weaken adult birds and make them more susceptible to severe winter weather, or can kill cygnets outright.
Trumpeter Swan relocation and hazing (harassing swans to encourage them to go elsewhere) programs in the
tri-state area are showing some degree of success in dispersing Trumpeter Swans to other wintering areas. However, as more cygnets are produced each year on the breeding grounds, the pressure on the wintering areas will only increase.
Shortage of Breeding
Habitat: It is not clear whether lack of breeding habitat is a limiting factor for Trumpeter Swans. Breeding habitat, primarily in the Aspen
Parkland region, has been lost through drainage and water manipulation projects that accompany agriculture, industrial development, and urbanization. However, availability of breeding habitat or the ability of swans to colonize new areas is not believed to be a limiting factor by some. In the Grande Prairie area, lakes that have successfully produced cygnets in the past, are not used every year, suggesting that swans are not limited by the number of suitable lakes. Disturbance of breeding habitat, rather than lack of breeding habitat, may in fact be limiting Trumpeter Swans during the breeding season.
Disturbance in Breeding
Habitat: Few studies have investigated the effects of disturbance on Trumpeter Swan breeding and behaviour. Trumpeter Swans can become accustomed to air traffic and small amounts of automobile traffic, even when the disturbance is relatively close to nesting areas. However, Trumpeter Swans are very sensitive to extremely loud traffic
such as large gravel trucks and motorbikes, boats, floatplanes, pedestrian traffic, and human intrusion on a breeding lake. These disturbances may cause nest failures or cygnet loss by disrupting feeding behaviour or causing females to take extended absences from the nest. Because Trumpeter Swans will not re-nest, clutch failure results in the loss of the breeding production for an entire year. Furthermore, in the case of severe disturbances, a pair may permanently abandon a breeding lake.
Habitat degradation and disturbance are major problems in the Grande Prairie area. As land use intensifies, previously productive lakes become unsuitable for Trumpeter Swans. Three previously productive lakes in this area are no longer used for breeding, probably because of both recreational use and adjacent intensive land use. The disturbance of Trumpeter Swans on lakes in Alberta can be controlled effectively through land use permits, if those lakes are surrounded by Crown land.
Once access to lakes is created, secondary human access and recreational activity can have a profound impact on swan breeding.
Biologists generally agree that the dramatic reduction in Trumpeter Swan numbers that occurred in the 18th, and 19th,
centuries was the result of overexploitation through commercial hunting. Hunting of Trumpeter Swans has been illegal since the inception of the Migratory Bird Treaty Act in 1918. However, because of the similarity between Tundra and Trumpeter Swans, hunters may mistakenly shoot Trumpeter Swans where legal seasons exist for Tundra Swans. In 1992, wildlife departments in Montana, Nevada and Utah (where there are legal seasons to hunt Tundra Swans), requested that hunters report bill length of harvested swans to detect Trumpeter Swans that were shot accidentally. From these reports, they found that 11 of 295 reported harvested swans were likely Trumpeter Swans.
Collisions with Power Lines. Electrocution resulting from collisions with power lines is thought to be a significant source of mortality for Trumpeter Swans. Several studies report high mortality from power lines and wire fences. In the Grande Prairie area, 6- 10 swan electrocutions are reported annually, but the actual number of deaths from electrocution is likely much higher.
Reprinted from Alberta Wildlife Status Report No. 26
with permission from Alberta
Sustainable Resource Development.