Our understanding of the distribution and movement patterns of the more obvious irruptive migrants has been pieced together over many years from local observations and ring recoveries. For North American waterfowl, however, detailed continent-wide information on breeding and wintering distributions has been obtained over many years by aerial survey. This has enabled their wide-scale distribution patterns to be examined in relation to prevailing wetland conditions, in both breeding and wintering areas. From the results of aerial surveys conducted annually in May over 27 years, Johnson & Grier (1988) examined duck numbers and wetland conditions across a large part of western North America. Much of this region is arid and subject to large annual fluctuations in precipitation. Johnson & Grier (1988) envisaged three possible patterns of spring settlement in this vast region, where wetlands varied greatly in numbers and extent from year to year, the shallower ones disappearing altogether in dry years.
1. Homing, with adults returning to breeding areas used in the previous year, and yearlings returning to near their natal areas. This pattern was expected in predictable environments where habitat conditions remained fairly stable from year to year. On this system, the large-scale distribution of particular species would be expected to be fairly consistent from year to year.
2. Opportunistic settling, in which birds occupy the first suitable sites encountered on their migration routes, providing that such sites are not already taken by other birds. This pattern was expected where habitat conditions in particular localities were unpredictable from year to year. Such opportunistic settling could minimise migration costs because it ensures that individuals migrate no further than necessary, but it could result in large-scale changes in the distribution of populations from year to year, depending on the distribution of suitable wetlands. It could also result in some long-distance shifts of individuals between their natal and breeding sites, and between their breeding sites in different years. Local abundance levels would be influenced not just by local conditions, but also by conditions elsewhere in the range.
3. Flexible settling, in which birds home to the area used in the previous year, but move on if conditions there are not suitable. This pattern could be viewed as a compromise between the first two (homing and opportunistic settling), and could affect yearlings more than adults. If birds simply maintained the same migration direction when they moved on, this would result in a general onward displacement of breeding birds when conditions in the usual breeding areas were unsuitable (although birds might not breed as well in these more distant areas as in their usual areas). If yearlings that had not bred previously were more willing to move on than were site-faithful adults, yearlings would be largely responsible for the year-to-year changes in distribution, again expected to be large.
Was this article helpful?