This is the time of year we begin to see birds migrating back to their southerly winter homes, but many of them look very different than they did when they arrived here in the spring ready to breed. Most birds have two outfits in their wardrobe: a non-breeding basic plumage that may be drab but serviceable for all-around activities, like migration and over-wintering; and a brightly colored (in the case of males) alternate plumage that is meant just to show off their stuff in the breeding season. In some cases, bills, skin around the eyes, feet, etc. may also be brightly colored, only during the breeding season.
Here’s a look at how this works in a small diving “duck” (not really a duck) called the Pied-billed Grebe, whose basic, non-breeding plumage gives no trace of the pied bill.
Pied-billed Grebes were diving among the lily pads looking for small fish or crayfish lurking there.
No trace of that characteristic marker of the broad black stripe on the bill though. The one at the bottom of the image still has the faint head stripes of juvenile plumage.
Typical juvenile plumage in the Pied-billed Grebe
A few months earlier adults looked like this, with a more definite black stripe through the pale, silver bill.
Earlier in the breeding season, both adults and youngsters looked quite a bit different than they will during the non-breeding season. Photo from Audubon field guide.
Of course seasonal changes in the grebes are far more subtle than those in some of the warbler species that take on completely different colors and color combinations between basic and alternate (breeding) plumages. For example, gorgeous red Scarlet Tanagers molt to a green-gold plumage in the non-breeding season, making them look like a completely different species.