It’s late October and the birds we watch on the Mississippi Flyway are pouring south while local plant and animal populations dwindle in response to diminishing daylight length, colder temperatures, and reduced food availability. How do migrating birds find enough to eat in the diverse, rapidly changing habitats they travel through? Read these four foraging stories to learn more about how birds cope with the challenge of finding food on migration!
Sandhill CranesTrumpeter SwansBald EaglesAmerican Golden Plovers
Sandhill Cranes: Hammer Time!
Sandhill cranes are omnivorous feeders who adapt their diets to the habitat around them. They use their long, straight beaks to probe for food up to a foot below the water’s surface, hammering to dislodge soil and break vegetation, and flipping their heads to remove soil particles. A sandhill crane can forage deeper in the water, probe farther beneath the water’s surface, and eat larger items than short little waders like dunlin, godwits, and sandpipers. Unlike some waders, sandhill cranes aren’t restricted to shorelines. They are more than happy to eat domestic grains when migrating through agricultural areas.
How deep will a sandhill crane wade? A lot of papers mentioned ‘shallow water’, but I couldn’t find a definition. So: two subspecies found in Florida stand erect at three to three-and-a-half feet high and have legs about 13.8 to 14.5 inches long, which averages about 36% of their total height. According to the Minnesota DNR, the average sandhill crane in our region stands erect at right around five feet high, which would give them legs about 21.6 inches long. We’ve seen them forage up to their 11-12 inch high ‘ankles’ in still water: habitat that is literally over the heads of most wading birds.
Seasonably social
Long legs, long beaks, and flexible palettes aren’t the only things that help migrating sandhill cranes find food. Adult sandhill cranes are defensive loners until the end of the breeding season. But once they leave their territories, they join huge sandhill crane flocks that stage, migrate, and winter together. While more cranes might mean less food for any one crane, flocking reduces the likelihood of predation and gives each crane a better chance of finding food for a lower investment cost. When one crane finds food, all cranes find food – even unexperienced juveniles on unfamiliar territory.
Trumpeter Swan: Longed-necked Herbivores
Tundra swans front, Trumpeter swans rear
How do trumpeter swans compete with migrating puddle ducks and tundra swans? Like most shallow foragers, a trumpeter swan tips up: pivoting forward from its legs and stretching its neck to reach submerged vegetation. Their long bodies and necks allow them to forage at depths of up to four feet: about six times deeper than a mallard duck, around four times deeper than a sandhill crane, and roughly a foot deeper than a tundra swan.
Is this important? It is! Tundra and trumpeter swans eat similar foods and migrate at about the same time. If emergent and floating vegetation is plentiful, the two species might feed along the same shorelines and littoral zones. But if above-water foodstuffs are scarce, trumpeter swans can forage for tubers and roots in deeper water. Just one vertical foot of foraging access gives both species room to thrive.
A small group of trumpeter swans floats deeper in the water, loosely mingling with a larger flock of tundra swans. Vegetation is scarce, but roots and tubers are plentiful enough to support both species. Why compete when you can forage in deeper water? With enough habitat and support, tens of thousands of generations will stop here to revive on their way south. Long may they fly!
Bald Eagle: Incredible Adaptor
Although migratory bald eagle populations begin increasing on Lake Onalaska in September, they don’t peak until late October or early November. In the fall, northern boreal forests and arctic tundras empty out as species burrow, die, hibernate, disappear underneath the ice, or migrate. Bald eagles are among the last raptors to leave the north woods. Why do so many of them stop here on the way to their wintering grounds? Lake Onalaska is brimming with food, and its islands, shallows, and riverbanks have an abundance of trees, snags, and sandbars for resting, hunting, and replenishing.
Individual bald eagles can behave predictably, but the species as a whole is remarkably adaptable. Within the context of carnivory, bald eagles eat almost everything. Nesting bald eagles are defensive loners, but non-territorial bald eagles winter in large flocks. While bald eagles usually hunt for food by themselves, some individuals hunt cooperatively for what looks like sport and/or practice. Migratory bald eagles might fly a direct route or wander so widely that it can be hard to determine when migration ends. Bald eagle behavior is influenced by size, age, territoriality, and breeding status, but eagles find food on migration – and at every other time of the year – by adapting to whatever the circumstances require. Few raptors make me say what? as often as bald eagles do.
Incredible adaptors
Watch the Flyway to see eagle adaptability in action! Bald eagles congregate in loose groups that perch, chase, break up, and form again around food, perches, flowage, and other prime river real estate. Any given eagle might hunt for fresh prey by itself; steal food, hunt and perch with, or tackle another eagle; or rummage along a sandbar or ice flow for carrion. If a migratory eagle can’t catch food or keeps losing it to other eagles, it can change its foraging behavior, join a new eagle group, or move on downriver to the next eagle hotspot.
American Golden Plovers: New season, new friends, new food!
American Golden Plovers
The Mississippi River delivers an almost constant stream of food to the shores of Lake Onalaska. An American golden plover can’t uproot tubers or turn stones, but its short, slender beak and run-stop-peck foraging style – run along the shoreline and in the shallows, stop to look for food, and peck it up quickly when you see it! – is perfectly suited to catching invertebrates on the river’s sandbars and mudflats.
Plovers sometimes migrate with other wading birds like knots, dunlin, and godwits. Waders forage for similar foodstuffs in slightly different ways, which reduces competition and improves each bird’s chance of finding food. Do deep waders create more food for American golden plovers? When flock mates disturb mud and sand in deeper water offshore, they could uncover food that drifts to hungry plovers wading near the shoreline.
New season, new diet, new friends
What do we know about American golden plovers? Not much. But we think that these largely insectivorous birds change their diets in response to migratory food availability and caloric demands. Their summer diet consists primarily of terrestrial invertebrates, but their staging and migratory diet includes more aquatic invertebrates, berries, and fat-and-protein heavy seeds. Dietary and foraging flexibility helps plovers build fat stores for extended flight and find food in a variety of migratory habitats. Mixed species flocking makes it easier to find food.
River Dreaming
Picture a Mississippi River lake: a lake filled with islands, channels, snags, and shallow water. A small family of trumpeter swans floats fifteen feet from a crowded sandbar. They tip forward, duck the front half of their bodies underwater, and stretch their long necks to feast on tubers and roots buried four feet beneath them. They bob and dip serenely, well away from noisy puddle ducks and stalking sandhill cranes. A large group of tundra swans swims nearby. While tundra swans can’t forage as deeply as trumpeter swans, they can reach vegetation three feet beneath them – food that mallards, shovelers, teal, and pintails can’t get to. Like the trumpeter swans, they keep an eye on the bustling sandbar.
Let’s move in closer. Sandhill cranes are foraging between the sandbar and a foot of water, probing and hammering with their long, strong beaks as they look for food. Water, silt, and small invertebrates swirl around their legs, released from the sand and mud underfoot. They step around puddle ducks and loafing geese, stopping to investigate a juvenile eagle. Something catches the eagle’s attention and it flies away to join a large group of eagles on another sandbar.
What’s going on? A subadult eagle has dragged a large fish on shore. Adult, juvenile, and subadult eagles fly in. The eagle mantles over its prize, but another eagle snatches it away! Soon the eagles are fighting over fish, perches, and position. Our juvenile darts in under the scrum and grabs a scrap of fish. It flies away with other eagles in hot pursuit.
Up on the sandbar, American Golden Plovers look for food. Waves and current have carried in bits of vegetation and small invertebrates released from deeper water by the sandhill cranes. The plovers run, stop, peck, and feed. They might be hundreds of miles south by this time tomorrow, but for now, they rest and feed in the chilly October sunlight.
Resources and References
The Raptor Resource Project