Data from FeederWatch show that invasions are not random events. Figure 3a shows how the abundance of some invading winter finches has changed from year to year. All species in Figure 3a are considered invasive winter migrants, and all are seen by many FeederWatchers roughly every second year. Even more interesting, these species usually travel south from their northern nesting areas during the same years. The simultaneous appearance of several species suggests that a single event is probably the underlying cause of all of the species' departures from their northern nesting areas in alternate years.
Researchers believe that low food supplies farther north cause invading species to travel south.
But the invasive species in Figure 3 do not all eat the same foods. Redpolls depend on birch seeds in winter, Red-breasted Nuthatches eat seeds from spruce and fir trees, Evening Grosbeaks feed on seeds from trees including maples and elms, and Pine Grosbeaks prefer to eat soft fruits in winter. Thus, data collected by FeederWatchers suggest that many different types of plants in the boreal forest synchronously produce low numbers of seeds and do so every other year. Figure 3a Figure 3b Figure 3. Unfortunately, seed production is not measured as systematically as bird abundance.