By early May, the large number of colony nesters, including ibis, egrets and herons, have reoccupied their colonies. In plentiful water years, Franklin’s Gulls, Forster’s and Black Terns will also nest. By late summer, large numbers of American White Pelicans congregate throughout the valley wherever fish are available. Meanwhile, the marshes are alive with the sights and sounds of many other species, including American Bitterns, Sora and Virginia Rails, Marsh Wrens, and Yellow-headed Blackbirds.
Although some fall migrants already begin showing up in early July, it is in August when migration really gets underway with thousands of phalaropes, dowitchers, avocets and other shorebirds. Landbird migrants, including flycatchers, vireos, and warblers start moving from mid-August through late September. Beginning in September, wave after wave of waterfowl will arrive. October brings large numbers of White-crowned Sparrows, and pushes Red-breasted Nuthatches, Mountain Chickadees and other montane species down into the valley.
The first winter freezes push all but the hardiest migrants out as the winter residents, including Bald Eagles, Rough-legged Hawks and Northern Shrikes arrive. During mild winters the valley will continue to harbor fair numbers of ibis, egrets, herons and shorebirds.
Waterfowl begin returning in February and early March. Shorebirds begin arriving in small numbers in March, with migration peaking in the third week of April with thousands of avocets, stilts, sandpipers, dowitchers and other species. One or more Peregrine Falcons will take advantage of the growing concentration of shorebirds to visit the valley. Landbirds mostly begin arriving in April, peaking in early May, when residents such as House and Bewick’s Wrens, Lazuli Buntings, Black-headed Grosbeaks, and Bullock’s Orioles begin breeding. By early June, migration is coming to an end.