The winter distributions of North American ducks also vary from year to year, depending on prevailing conditions. Many species move further south in cold winters than in mild ones or further when low rainfall reduces the numbers of wetlands available. This pattern in Mallard Anas platyrhynchos was confirmed by winter ring recoveries which were centred further north during winters when December-January temperatures were high than during winters when they were low (Nichols et al. 1983). Birds also tended to concentrate in areas that experienced greater autumn-winter rainfall, and hence good wetland conditions. Similarly, in the Wood Duck Aix sponsa, both adults and young migrated further in years of low summer rainfall and the young also migrated further south when autumn temperatures were lower than normal (Hepp & Hines 1991). Both rainfall and temperature affect the availability of wetland habitat and food, competition for which may well stimulate further migration, with young ducks more affected than older ones.
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