From the Reifel Migratory Bird Sanctuary (British Columbia Wildlife Society) site:
"During their stay here, favourite natural foods for these birds are the intertidal marsh plants of the estuary. Marsh plants such as bulrush (Scirpus americanus) store starch reserves in their roots and rhizomes. The geese dig up these food sources using their strong bills. The soils in the Delta area are rich in iron compounds, and stain the head feathers of the geese orange when they have been digging in the marsh."