Article by Braden DeForge

People who watch wildlife in Burlington wonder about how animals survive winter. We marvel at the ways in which adequate shelter, food resources, the harsh temperatures and light availability become challenges but not limiting factors for those well adapted for this climate.

This time of year, I put out a bird feeder filled with black oil sunflower seeds. I get to see the world of our winter residents up close while giving them an energy boost. As I watch Northern Cardinals, nuthatches, Tufted titmice, and Blue Jays flit between my feeder and a nearby tree, I wonder what else keeps them going through the winter and what it takes to be successful.

Fortunately, Bernd Heinrich has a great book, Winter World, in which he dives deep into scientific thinking about the lives of animals in winter. Heinrich, a professor of biology emeritus at the University of Vermont has a great skill for observation. His chapter Berries Preserved, describes patterns Heinrich saw in the birds who visit the fruiting trees near his home, and a nearby beaver bog.  

Growing by the beaver pond, Heinrich notes various kinds of berries and how they are eaten. Arrowwood berries are a choice food for fall-migrating robins, while nannyberry is left alone and eaten throughout the winter by resident bird species. Migrating birds arrive in flocks rather than solitary and this is an intentional pattern. Much like how the v-shape formation of Canada Geese in flight yields lower air resistance, the many eyes scanning the landscape for berries makes finding them easier.

Migrating bird species choose to avoid winter weather while also achieving great physical feats. In order to fly, in some cases, thousands of miles, birds can double their body weight and even rearrange or absorb certain internal organs. Though not all individuals in a given species may migrate and distances traveled will vary, it remains true that migration comes at a high energetic cost. As their digestive systems generally shut down for extended flights, finding enough food beforehand is essential. One key to fueling up for this journey is winter berries.

Heinrich describes the key difference in berry types: summer versus winter berries. You’re probably quite acquainted with summer berries; juicy, soft, and sweet such as raspberries, serviceberries, or blueberries, these are the quick to go bad, summer diet of birds. Winter berries which ripen in the fall are typically less palatable to humans but that is the key to their winter persistence. 

Lower sugar content and hardier berries means spoilage takes longer, and therefore trees such as winterberry, mountain ash, and nannyberry can provide food for winter residents throughout the season. Berry preservation is taken to an extreme with staghorn sumac which has velvety stems and furry, dry red berries which are preserved with acid giving them a tart, citrusy taste. Come late winter, staghorn sumac berries become an important food source in a time of scarcity. Staghorn sumac grows along forest edges throughout Burlington and its berries are used today and historically by indigenous communities in Vitamin C rich tea preparations. 

Species of waxwings, and in particular Cedar waxwings, are one of the most frugivorous birds in North America. Their winter diet, composed almost entirely of fruit and berries, means that the persistence of winter berries is essential to their survival as they migrate and forage in the colder months. The only time of year that waxwings stray from their fruit heavy diet is in the spring when flowers buds become choice morsels due to their abundance and the fact that their blooms have not yet supplied a new crop of fruit. 

Though species of plants provide throughout the season, berry crops are variable and in some cases fail or mast. Mast year means a synchronicity within a species of tree when all the individuals produce a far greater amount of seed compared with an average year’s crop. This means you may spot a bird where uncommon for the season or in a frequency not usually observed in such a part of its range. The Finch Research Network’s Winter Finch Forecast is a resource for understanding how you might expect various species to respond and understand what birds turn to in times of scarcity and abundance.

This year, between here and western Lake Superior, the mountain-ash berry crop has been small. The Finch Research Network forecasts that flocks of Pine Grosbeak will be seen looking for ornamental fruit and bird feeders as they make their way toward the Atlantic coast where the berries are far more abundant. You may see Pine Grosbeaks eating your crab apples while neighboring squirrels try and stash them in crooks of trees for later. 

Though researchers have determined that birds prefer their native berries, in times of necessity and especially for migratory birds, non-native berries are an option. The coevolution of plant and bird species means that native berries provide high-value energy and nutrition. As it turns out, berry-producing invasive plants studied in Vermont and other areas of the Northeast bore maximum fruit 20 days later into the fall than various native species. As Climate change impacts bird migration patterns, some birds may find themselves missing their native berry window and instead finding the non-natives. Further research has shown that when the preferred native berry crops fail, birds will turn to non-native berries such as European buckthorn.

As our climate changes, trends are becoming clear in the patterns of bird populations. The eBird trends map is a great tool for understanding how populations are fluctuating. The Wood Thrush for example is seeing population losses in the Northeast while there are significant gains west of the Appalachians. Any bird species with trend data in eBird can be selected to view their changes on the map. The 2022 state of the birds notes an overall decline of bird populations in all habitats except wetlands where investment in improving and restoring these areas has resulted in “dramatic gains”.

Now is our chance to improve the picture in habitats seeing losses, and that can start right in your backyard! Audubon Vermont has a great superstar native plant list which includes species best adapted for our area that provide the most food for our birds and the caterpillars they eat during the spring and summer. It’s a great place to get started. But when in doubt, do as Bernd Heinrich does. Take a walk and notice what food you can see birds eating.