By Michele S. Byers
With the onset of the first winter storm, you're probably envious of robins' and other birds' migration south to warmer climates. And you may wonder how American robins arrive back in our gardens so quickly in March.
But you may be surprised to learn that the first robins of spring, especially the males, don't leave this state we're in during winter in the first place. They just go into hiding - congregating in New Jersey's maritime, or coastal, swamp forests, where abundant native fruits make up their winter diet.
Visit the magnificent holly forests of Sandy Hook in the Gateway National Recreation Area on a calm winter day, and you'll see huge flocks of robins devouring holly berries. American holly berries start to ripen now, during cold weather after Thanksgiving.
The American holly and many other shrubs and vines have coevolved with fruit-eating birds that winter in New Jersey. Each helps the other.
Many plants, especially those with large seeds, have evolved chemicals to discourage seed-destroying mammals from gnawing or chomping their fruit. American hollies, for example, have big seeds that taste bitter to humans and other mammals. But birds love them.
Fruit-eating birds have no teeth or gizzards, so seeds pass quickly through their guts unharmed. In fact, their digestive juices vastly improve seed germination by providing an initial breakdown of the seed coating. Experiments reveal that frequency of germination may increase by as much as 50 times if seeds first are digested by a fruit-eating bird.
Bayberry, juniper, and poison ivy plants have wax in their fruits as a reward instead of sugar. Many species of birds possess enzymes that digest this wax. For instance, cedar waxwings devour the waxy, berry-like cones of juniper, or red cedar. Yellow-rumped warblers consume waxy bayberries all winter. Robins, northern flickers, tree swallows, and eastern bluebirds eat all these fruits during winter months.
The few remnants of the forests that once covered the entire length of the Garden State's Atlantic coastline are dominated by trees, shrubs, and vines whose seeds are dispersed by fruit-eating birds. From holly and hackberry to black cherry and red cedar to poison ivy and Virginia creeper, almost every woody plant that you see on a barrier island forest germinated from a fresh bird dropping. No better way exists for seeds to colonize shifting, ever-moving sand dunes after a nor'easter than to be dropped by a winged seed-pooper.
This winter, visit New Jersey's barrier island forests at Sandy Hook or Island Beach on a nice day and witness the result of hundreds of thousands of years of coevolution: the interdependence between birds and woody plants.
And when you wake up to see the first robin of spring - its head cocked to one side, listening, looking, and feeling with its toes for the stir of an earthworm - know that it spent the winter in New Jersey, feasting on native berries.