As summer gives way to fall, the landscape changes its menu. Insects become less abundant while berries, fruits, seeds, and nuts become increasingly important. For birds preparing for migration, those changes can be critical: they need to build up energy reserves before undertaking journeys that can span thousands of miles.
Some species dramatically change what they eat as the seasons turn. Baltimore Orioles, for example, feed heavily on insects during the breeding season, when they need protein to raise their young, but fruit and nectar become more important in spring and fall. Those sugary foods are readily converted into fat, providing a concentrated source of energy for migration. Orchard Orioles make an even more pronounced seasonal switch, moving towards a diet dominated by ripe fruit just before their fall migration.
Other birds arrive at the autumn buffet with a different strategy. American Robins are familiar consumers of berries and other fruit, while Cedar Waxwings can descend on berry-laden trees in large flocks, sometimes swallowing fruit almost as quickly as they can find it.
For birds such as these, fruit isn’t simply a convenient seasonal snack. It can provide the high-energy food needed to fuel migration, while the plants benefit from having their seeds carried away and deposited elsewhere. The relationship can therefore work both ways: birds get food, and plants get a highly mobile seed-distribution service.
But autumn’s menu is considerably more varied than berries. Seeds and nuts become increasingly important as they mature, while insects remain part of the food supply even as their numbers decline. Red-bellied Woodpeckers, for example, have a broad diet that includes insects, fruits, nuts, and seeds, allowing them to take advantage of whatever the season provides. Even the familiar European Starling is an adaptable forager, able to switch among insects, fruit, and seeds as different foods become available.
These changes in the food supply also help explain why birds turn up in particular places at particular times. A fruit-laden tree, a stand of seed-bearing grasses, or a patch of ground rich in insects can become an important feeding site for birds that are simply passing through. The quality of a stopover site for a migrant may determine how quickly it can replenish its reserves and continue its journey.
And that creates a much bigger ecological question. Migration is carefully timed around the seasonal availability of food, but the plants and animals that provide that food do not necessarily respond to the same environmental cues. Temperature and weather can cause plants to flower or fruit earlier, while insects may emerge earlier too. Birds may also adjust the timing of migration, but not necessarily at the same rate. Scientists refer to the resulting loss of synchrony between a migrant and its food supply as a “phenological mismatch.”
The consequences could be significant: a bird arriving at a stopover site after its main food source has peaked may find that the landscape no longer offers the abundance it needs, potentially making it harder to replenish its reserves and continue its journey. Research on migratory birds has shown how closely their movements can track seasonal peaks in food resources, making changes in the timing of those resources potentially important for survival.
For birders, this provides another way to look at migration. Birds aren’t simply responding to shorter days or falling temperatures; they are following an ever-changing landscape of food and opportunity. If the tree in your garden suddenly becomes full of berries, the grasslands turn to seed, or the woodlands come alive with insects, watch what happens next. A tree heavy with ripe berries can become a magnet for hungry migrants, while a seed-rich grassland or insect-filled woodland can provide an essential refuelling stop. Suddenly, the birds around you make a little more sense.
