Biologists like Brian Harrington, of the Manomet Center for Conservation Sciences, have long recognized that some red knots (a migratory shorebird) make a long-distance flights to South America while others fly a shorter distance to winter in the southeast US and the Caribbean.
We always knew that south-bound stopovers provide long-distance birds with abundant, energy-rich food necessary to build weight before they jump off on journeys of up to 5,000 miles. But now we are learning that the stopovers are crucial for short-distance birds as well.
This map shows the southbound flight of a red knot with a geolocator. The device tracked the birds flying south from the Arctic to the NJ Coast where it stopped before flying to the lessor Antilles. On the way it encountered Tropical Storm Danny, flew around it spending 4 days at sea in constant flight
Flight feather molt tells us why. One of the most important tasks for a bird is to molt (or shed) last years feather and grow new ones — replacement of primary flight feathers is most important and physically demanding. This is especially true for shorebirds that make long migratory flights, some lasting 6 days of non-stop flight covering thousands of miles. Growing feathers requires a great amount of energy which often keeps red knots at fat free weight for months when they should be buidling fat reserves to fuel migration or to provide a fat “reserve” when food becomes scarce. The loss of flight feathers also makes them more vulnerable to avian predators like the peregrine falcon.
Long- and short-distance knots molt differently — the former on their wintering areas in South America, the latter while enroute to wintering sites. The short-distance fliers will stopover in places like Cape Cod, Massachussetts, and the southern Atlantic Coast of New Jersey for up to 70 days to completely molt their flight feather before moving south to wintering areas. The needs of migrating and molting shorebirds suggest the need for new conservation policy and management because southbound stopovers are vital to long-term preservation of shorebird populations. I pointed out why in my previous post on our disturbance experiment in Brigantine. These important stopovers are also crowded with people.
A red knot with outer primaries nearly full grown except the outer-most (10th) primary. 
A Red knot just starting primary molt (from inner-most primary outward) by shedding worn (pale-brown) flight feathers and starting growth of new, blood-filled pin feathers
In late July and August, when the long distance birds move through, the beaches are crowded with beachgoers. In September to November, when the short distance birds are molting, the beaches are filled with 4-wheel drive SUV’s and pickups, some used for fishing, many just riding the beach. Recently, I saw a person driving his SUV along the shoreline, flushing feeding and roosting shorebirds, with his leashed dog running along side. Surely we can make better use of our valuable coastal habitat.
Last week we learned more. Much of our work in the fall focuses on red knots, but last week we caught sanderlings, and they too were molting their primary flight feathers. This might seem like a small thing but these feathers tell us that stopovers like Avalon/Stone Harbor are important to many species and not only because they provide good places to rest, but to fuel this important stage in thier lives.. 
Sanderling with nearly complete primary molt (outer-most primary nearly complete).
