A Change of Feathers: The Molt

Feathers are one of the defining characteristics of birds. They produce the brilliant red of the Northern Cardinal, and the shocking blue of an Indigo Bunting. Though one purpose of feathers is to be flashy and attractive, especially to a potential mate, feathers have a host of other functions. From helping a bird keep warm, to protecting it from UV rays, to masking the sounds of flight, to the act of flying itself, feathers allow birds to survive in nearly every ecosystem on Earth. And yet, they are not permanent fixtures on a bird’s body.

Northfield, the Broad-winged Hawk, growing in some new tail feathers. Contrast the 2 white-striped on the right with the duller brown on the left.

The process of losing and replacing feathers is called the molt, and it is a uniquely bird event. Though snakes shed their skin, dogs their fur, and insects their exoskeletons, a bird’s molt is timed, ordered, and an astounding physiological process.

The function of the molt is to replace last year’s feathers that have been damaged by wear and exposure, to ensure the thermoregulatory and aerodynamic properties remain reliable. In North America, most birds go through a molt of all of their flight and body feathers each summer, a process that can take weeks or months. Most wouldn’t want to drop all of their feathers at once, then spend a week grounded while regrowing them all! Instead, the pattern of molting in each species is unique, but ordered. Some, like the hawks, lose their innermost primary flight feather first, and proceed outward. Some start in the middle and proceed in both directions, like the falcons. Some owls molt synchronously, losing all the feathers on their tails in the space of a few days, but it does not appear to impede their mobility.

Molting body feathers: the reddish bars on Northfield’s flanks are replacing the brown, tear-drop splotches.

Our ambassador Broad-winged Hawk, Northfield, is an excellent molting educator, and this summer is helping visitors visualize the molt in action. As a second-year bird, Northfield came to VINS in June 2016, still with a coat of down. By late August he had grown in the full set of feathers that was his “juvenal plumage”, characterized by mottled brown on his back and wings, a brownish tail with multiple thin stripes, and most noticeable of all, heart-shaped brown splotches on his flanks and legs. This summer, Northfield is undergoing what ornithologists call the “prebasic molt” into his adult or “basic plumage”. His brown mottling is becoming darker and more reddish as new body feathers grow in. His tail feathers are being replaced by wide, starkly black-and-white stripes, and sadly, his “heart-pants” are giving way to russet horizontal bars on his chest.

Broad-winged Hawks are long-distance migratory birds, and so Northfield’s wild counterparts often have trouble fitting the entire molt in before September, when they must soar down the Appalachians to Central America. As such, some Broad-wings suspend the molt mid-summer, and finish it on their wintering grounds. 

Molting birds can be hard for the casual observer to identify–a strange mix of two radically different birds in one—but understanding molting patterns can help scientists learn much about a particular bird’s age and lifestyle. Within each feather is the story of when and where it was grown.

What should you do if you find a molted feather? Enjoy looking at it, then leave it be.