Population status and productivity of Tundra Swans Cygnus columbianus in North America
Population indices from Midwinter Waterfowl Surveys conducted in the Atlantic and Pacific Flyways show that the Eastern Population, Western Population and the total North American Tundra Swan population have increased significantly (P<0.001) at annual rates of 2.4%, 1.8%, and 2.1%, respectively, during the period 1955-89. During 1980-89, the EP continued to increase and the WP may have possibly declined resulting in stable numbers for the total North American population; data are most reliable as an index to population trends when viewed over the long term. In the short term, considerable variability exist between periods because of inexplicable erratic changes in population indices. The EP and WP comprise 58% and 42% of North American’s Tundra Swans, respectively. North Carolina and California winter nearly 70% of the continent’s population, and these two subpopulations continue to increase both numerically and proportionately. Productivity estimates determined from observations of age-ratios and number of young per family in the EP and WP during fall and early winter show little change over the long or short term. These estimates were not well correlated with population changes, probably due to erratic population indices as well as possible biases in productivity assessments. However, since productivity indices have not changed significantly over the period when numbers of Tundra Swans have increased, it may be valid to assume that present levels of productivity are adequate to sustain growth.
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