For long-distance migratory songbirds, consuming more calories during stop-over periods than they expend during nocturnal migratory flights is a critical balancing act. Warblers, orioles, Gray Catbirds, and other neotropical migrants need to fatten up before they depart on each leg of their journey. These fat deposits are the fuel that powers their flights on calm, fall evenings. If they haven’t found enough food during the day to replenish their fat stores, or if they expended too much energy locating food, they’re likely to be at a significant disadvantage. The migration of these individuals may get delayed, exposing them to harsher weather and ever diminishing resources, or they may not have the energy to locate prime habitats for foraging.
Insects, seeds, and berries make up the bulk of a birds’ fall diet. When given a choice, birds will invariably choose the food that offers them the most energy value for their effort. For instance, at a bird feeding station, most species will choose sunflower seeds over cracked corn or millet. These preferred seeds are high in fat and provide more calories than the others. Similarly, a bird that we normally think of as insectivorous during the summer may choose to stuff itself with easy-to-find berries in September, rather than expend energy catching insects.
Some berries are high in carbohydrates and low in fats and sugar, while others are just the opposite. Most berries that ripen in summer such as blackberries, strawberries and Juneberries, are high in carbohydrates. These fruits are relished by many birds during a time when other nourishing food items are plentiful and energy demands are relatively low. But during the high energy demands of fall migration, both low-fat and high-fat berries are available. As would be expected, migrating birds often pig-out on poison-ivy berries, chokecherries, hackberries, elderberries, autumn olive, and many other high-fat fruits to help fuel their southbound journeys, while ignoring low-fat berries.
Many of these high-fat fruits undergo a dual color change as they ripen. For example, chokecherries start out green, and turn pink a few days prior to turning a deep purple. This intermediate color change has been shown to “flag down” passing migrants as an indicator of an imminent food source, much like the Golden Arches might flag down a hungry tourist. In nature’s grand evolutionary scheme of things, this is truly a wonderful system. Those high-fat berries, if not eaten, would quickly be attacked by bacteria and rot on the parent plant, its seeds, contained within the highly nutritious and edible package, not being dispersed.
But what happens to all those low-fat (junkfood) berries that the migrants passed up in favor of the high-energy snacks? Because these berries are low in fats they persist on the plant without rotting. These become important winter foods for Cedar Waxwings, Evening Grosbeaks, woodpeckers, American Robins, and other species that brave our northern winters. But depending on what other foods are available these low-fat fruits, including mountain ash, crab apple, viburnums, sumac, and winterberry, may last well into March before being consumed as a last resort.
VINS Conservation Biologist