Spotting these flocks of migrants in the fall, even when they are hiding in plain sight, can be difficult. Many of the birds, which are vibrantly colored during spring migration, now appear washed out or as if they are in yellow-green camouflage to match the autumn leaves. Paul Sweet, Collection Manager of the Ornithology Department at the American Natural History Museum, explains: ‘In spring the males especially are in their brightest plumage to help them attract a mate and fend off competition.’ But breeding done, there is no more need for bright feathers and enticing songs.  Most birds molt their feathers before the fall migration. And some undergo striking changes in appearance. The brilliant red of the male Scarlet Tanager turns yellowish-green. The Blackpoll Warbler, with its distinctive black cap and bright white cheeks, turns olive green with a yellowish head. Even distinctive details like the Canada Warbler’s pretty black necklace can vanish almost entirely.  And a lot of the immature birds now passing through complicate the picture, as their plumage can be uncharacteristic.