As a rule, birds undergo a complete molt after the breeding season. Prior to the next breeding season, many birds undergo a partial molt (typically, head and body feathers) that dresses them for courtship. Male Indigo Buntings, Rose-breasted Grosbeaks, and American Goldfinches are examples. Some birds, however, acquire their breeding plumages without replacing a single feather.
European Starling, pictured above, is one. They are dark with light speckles in the fall and winter, and black with iridescent purple and green in the spring and summer. When do they molt their speckled garb for iridescent black? Well, they don’t.
Rather than acquire their breeding plumage by molting, starlings gain it by losing the speckles from their winter plumage. The speckles are actually the cream-colored tips of the head and body feathers, and they are lost by abrasion. The reason the feathers don’t continue to wear is that dark feathers contain melanin, which is resistant to abrasion.
Producing new feathers requires energy. Fed by blood vessels, each new feather develops in a follicle under an existing feather, which the new feather pushes out as it grows. The process is called molting. The energy savings from plumage change by wear is significant.
House Sparrow is another bird that obtains its breeding plumage by wear. During the breeding season, male sparrows sport a large black bib that is only faintly visible during fall and winter because the black feathers are tipped with light gray, an adaptation against predation. As spring and summer approach, the gray tips wear off, revealing the intense black bib. Seemingly new feathers are also revealed on the head and other parts of the body.
Scientists in Denmark observed that female House Sparrows prefer males with large black bibs. They also observed that male sparrows with large bibs copulate with females other than their mates. Why would a female select an unfaithful male that would take time from her to be with others?
Producing eggs is a high-energy investment for females. They want to select a male that offers the best chance of fertilization and success. Studies have shown a correlation between large testis size and increased chances of fertilization. But how could the females know? Females have learned that males with large bibs tend to have large testes.
A plumage character such as the black bib that indicates male quality is called a badge. The size of the black-and-white crown stripes of White-throated and White crowned Sparrows and the V on the breasts of Eastern and Western Meadowlarks are also badges.
Other birds that attain their breeding plumage by wear include meadowlarks, Snow Bunting, Red-winged Blackbird, and McCown’s, Lapland, and Chestnut-collared Longspurs.
Bobolink, the only songbird that is entirely black underneath, utilizes feather wear in a unique way. Each year, it migrates from North America to southern South America, a round-trip distance of more than 12,000 miles, and it undergoes two complete molts. After breeding, both sexes molt into their female-like winter, or basic, plumage, which is yellowish and buff. Then it’s off to South America, where between late January and April they molt into their breeding, or alternate, plumage. Surprisingly, it too is female-like, but a little darker than the winter plumage.
But the male Bobolinks you know from the breeding grounds aren’t yellowish and buff. They look like they’re wearing reversed tuxedoes; they’re black below and white above, with buff collars. So what’s going on with male Bobolinks?
Well, their black head and body feathers are there, but they’re concealed beneath the yellowish and buff feather tips. By the time the birds arrive on the breeding ground in May, the tips will have worn off, revealing the Bobolink’s familiar black-and-white pattern.
Most birds that achieve their breeding plumage by wearing off feather tips do so by modifying their winter plumage months after they develop that plumage. So why do Bobolinks molt into their breeding plumage in the spring (in South America), conceal it, and then begin to wear off their feather tips? No one seems to know.
The practice might be related to the factors that cause Bobolinks to have two complete molts and plumages per year, an unusual trait shared with Marsh Wrens. The habitats of both species — grasslands for Bobolink and emergent marsh vegetation for the wren — are very abrasive. In addition, Bobolink spends most of its time in open areas, where the feather-damaging effects of ultra-violet radiation would be great.
While many birds molt into breeding plumage to attract a mate, others allow feather tips from a previous molt simply to wear away, revealing a hidden beauty that functions like a breeding plumage. These adaptations are yet other examples of those amazing birds. — Eldon Greij
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. Subscribe. This article appeared in the December 2016 issue, which will go on sale at Barnes & Noble and other newsstands in early November. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founder of Birder’s World magazine.