Physiological Changes in Migrating Birds

Northern Parula in Fall

Matt Tillett/Flickr/CC by 2.0

Migration is a strenuous journey and birds face many threats along the way, but they don't undertake this adventure on a whim. While instinct and practice guide much of birds' migration year after year, there are certain physiological changes birds undergo that helps prepare them to survive.


Birds undergo several physical changes that increase their chances of surviving the rigors of migration. While different species may change in different ways, these changes are widespread throughout the avian world and many birds exhibit several of these physical changes before every migration.

  • Molting: Fresh, new feathers are more aerodynamic and make flight easier, and most birds molt just prior to beginning their seasonal migrations. This molt is especially critical for dimorphic birds in late summer when their fall plumage will be more camouflaged and less noticeable to predators along their migration route.
  • Weight gain: As the time for migration nears, many birds increase their production of fat-metabolizing proteins and their digestive tract swells so they can more easily gain weight. Birds may seem to be overeating—a period called hyperphagia—and can double their weight before leaving on migration, storing fat that will become fuel as they travel.
  • Gonad shrinkage: Birds' testes and ovaries will shrink almost to nothingness as they prepare for migration in the fall when those organs are no longer necessary for breeding. This decreases the weight of the internal organs so the birds can fly more easily without needing additional energy.
  • Hemoglobin increase: Detailed analysis of birds' blood samples have shown that just prior to migration, birds begin to produce greater quantities of hemoglobin. This enables more oxygen to be delivered to birds' muscles, helping them sustain flight more easily without any soreness or disorientation.
  • Flock formation: Before migrating, many birds, such as swallows and waterfowl, will begin gathering in large flocks that can number hundreds or thousands of individuals. These flocks are often in areas with rich food sources, and are more common in fall than in spring, depending on the species.
  • Restlessness: As the time to depart on migration nears—triggered by light levels, daylight times and sun angles—many birds exhibit restlessness and may wander in short flights that will strengthen their wings and sharpen their senses before they begin the full migration journey.

How to Help

While birds have their own unique ways their bodies and behaviors change just before migration, there are many ways birders can help birds prepare for such an exhausting journey. Easy and effective options include:

  • Offer high-fat foods: Suet, peanuts, peanut butter, black oil sunflower seeds, Nyjer and other high-fat foods will help birds easily gain weight as they prepare for migration. Keep feeders full, and consider adding extra feeders during spring and fall when birds need these foods the most.
  • Offer natural foods: Migrating birds may not be accustomed to feeders, but they will easily recognize native plants offering natural foods such as seeds, nuts, and nectar. A bird-friendly landscape should include seed-bearing flowers, fruit trees, and other natural food sources.
  • Discourage predators: Birds molting before migration are more vulnerable to predators, and a savvy backyard birder will take steps to discourage feral cats and provide extra shelter in the backyard, such as a dense brush pile birds can use to retreat if necessary.
  • Preserve local habitat: Local habitat is essential for migrating birds to make use of as they gather in seasonal flocks or pass through along their migration flyways. Supporting nature preserves and preserving habitat will ensure all birds have the space they need.
  • Provide water: While most backyard birders focus on food for migrants, water can be just as critical as birds prepare for migration. Bathing will keep birds' plumage in top condition for the long flight, and the sight and sound of water can attract passing migrants so they can find shelter and food.

Migration can be a stressful time for birds, but they have evolved different physiological changes to prepare for the journey, and with backyard birders' help, every bird can be well equipped to survive even the most grueling migration.