Birds Recover from Breeding and Prepare for Winter
As fall settles in, birds replenish nutrients drained by reproduction and replace their tattered feathers with a new set for migration or protection against winter's cold. They also start storing fat reserves that will help residents overwinter and migrants fuel their southward flights.
The Types and Availability of Food Change Rapidly in Autumn
Insect populations, especially nutrient-rich young prey, rapidly decline as fall progresses. This food source is replaced by high-energy foods such as nuts, seeds, and fruits, which are ideal for fueling molt and depositing fat reserves.
Small sparrows and finches favor the abundant but very small seeds produced by most plants. Enormous masts of large seeds from oaks, beech and other trees provide a bounty for those species that can crack them. Year-round residents like jays, nuthatches, and some species of woodpeckers cache away an ample store that will be tapped during winter. As fall progresses, the rapid decline in smaller seeds and insects combined compels many populations to migrate.
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Molting is a Nutritionally Demanding Process
Feathers are made of protein. That's why birds must consume a wide variety of foods in order to obtain the appropriate amounts and balance of critical amino acids needed for replacing their plumage. Additionally, thermal losses due to shedding feathers together with the high costs of building a new coat of these highly intricate structures markedly increases energy needs. Fortunately, birds are adept at blending food items in order to meet their specific amino-acid and energy needs. Molting is a very high priority process and will proceed even when food availability is sub-optimal; however, the quality of the new feathers suffers and this affects overwintering success.
Special Challenges of Migration
Many birds deal with the dual problem of declining food availability and colder temperatures by migrating to warmer climates. Some will travel only a few hundred miles to find areas of greater food supplies, while others will make the long flight to tropical areas. In preparation, short-distance migrants often bulk up by depositing sufficient fat to increase their body weight by 10-20%. Long-distance migrants that must cross oceans, deserts, or mountains may increase their weight by 50% or more.
Of course, those birds that do not migrate face the challenge of winter's cold coupled with variable food availability due to weather or sickness. Resident birds often store considerable fat during the fall in preparation for winter.
Role of Bird Feeding
Autumn is a time when birds need readily available high-energy foods such as nuts and seeds. However, during early autumn many seeds drop to the ground and may be difficult for birds to find due to thick vegetative cover. Furthermore, birds are often at the peak of their molt and have a difficult time flying effectively in order to procure insects or find new food sources. Consequently, fall is a time of heightened activity at the bird feeder. As fall progresses, birds increase their appetite in order to fatten up for migration or in preparation for winter, and they make frequent trips to the feeder. Migrants will often stop over for an easy meal at a bird feeder in order to replenish their reserves.
Article by Kirk Klasing. Dr. Klasing is a professor of Comparative Avian Nutrition in the Department of Animal Science, University of California, Davis.