Populations returning to normal levels from "zero" after only 2 years
would seem to argue against total mortality.
This actually seems quite plausible. These are wintering birds. If one stretch of marshes in the wintering range is emptied out of wintering birds, new birds *should* repopulate it the next winter if it’s good habitat (assuming that there isn’t huge amounts of surplus wintering habitat). Even though banding studies have shown pretty ridiculous site-faithfulness in winter, there are new young birds being hatched every year looking for places to winter.