How Plant Seeds Reshaped the Lives—and Evolution—of Birds

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Editor’s note: This article features an excerpt from Pete Dunne and David Allen Sibley’s new book “The Courage of Birds: And the Often Surprising Ways They Survive Winter.” For more details on the book, scroll to the bottom. 

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Approximately 370 million years ago, plants developed an ingenious advance in their reproductive strategy: They began encasing their embryos in protective external shells. These proto-seeds allowed plants to spread across the terrestrial world, and approximately 250 million years later, seeds were discovered by birds to be a nutritious, abundant, and durable food resource that retained their nutritional value even into the winter months, thus providing insect-eating birds with an alternative food resource when winter temperatures deprived them of their primary food. Supported by this new dependable food resource, birds were, now, able to take full advantage of their heat-generating metabolic capacity and remain in colder northern latitudes even during the winter. In short, plant seeds changed the game, giving many northern-breeding birds the latitude to avoid the risks and energetic demands of migration and remain closer to, even within, their breeding territories year-round, even into Arctic regions. Let’s explore the sexual revolution that changed the world of birds and led, ultimately, to the popular practice of backyard bird feeding—a multi-billion-dollar industry that has itself altered winter bird abundance and distribution.

Seed-bearing plants are divided into two groups. Gymnosperms (literally “naked seed”), mostly have needles and encase their seeds in cones (examples include pines, junipers, spruce, and cedar). The other seed-producing plant group, the angiosperms, encase their seeds in ovaries or fruits (including grasses, sunflowers, oaks, birches, hickories, grapes, bayberry, and fruit trees). Birds consume both seed types.

Plant seeds are highly nutritious (providing approximately 150 calories per ounce) with a very high protein content as well as polyunsaturated fats, in addition to other vital vitamins and minerals. Seeds are durable, drought resistant, and typically overwinter in a dormant state, maintaining their nutritional value and germinating only when conditions are favorable. Some seeds can remain viable for up to five years. Best of all, seeds are abundant, with single plants in some species producing thousands in a growing season.

It took birds some time to catch onto this new food resource. The first seed-eating birds (a determination based upon bill shape) did not appear until about 120 million years ago, the early Cretaceous Period. So approximately 250 million years after plant seeds evolved.

Seeds range widely in size, from the dust-sized orchid seed to the coconut. Seeds in North America that are favored by wild birds range from the 1/2-inch dandelion seed and 1- to 2-millimeter birch seed to the 1/2- to 2.5-inch-long acorn. Pine nuts, a popular seed type favored by many northern forest birds, are 1.5 to 2 inches. For comparison, the average sunflower seed is 1/2 inch. But since that game-changing dietary discovery by birds, many plants and birds have evolved a commensal relationship, with plants encouraging birds to forage on their fruits and bird species distributing seeds via caching and defecation. If you have ever wondered why poison ivy spreads so quickly, look to birds. Follow the trail of droppings, which contain viable seeds, back to the host poison ivy vine. Poison ivy berries are relished by multiple woodpeckers, mockingbirds, Yellow-rumped Warblers, bluebirds, and many other species during the colder months. The plants even signal their fruiting readiness by donning red leaves. Red is the universal plant-to-bird communication that reads: “Hey guys, soups on. Come and get it.” It’s why hummingbirds are particularly drawn to red blossoms.

Yes, plants want their seeds to be eaten.

It is the abundance of seeds as much as their durability and nutritional value that makes them such a vital food resource. The average mature oak will produce 2,200 acorns per season. The production of pine nuts is cyclic, varying greatly from year to year. But pinyon pine nuts, whose nutritional value has been compared to beefsteak, have high cone production every two to seven years. Pinyon pines cover 37 million acres in Arizona, Colorado, New Mexico, and Utah. Individual cones average 10 to 20 seeds per cone. In good years, a single acre of forest can produce 250 pounds of seed. And pinyon pine is just one pine species. All conifers produce seed-bearing cones. There are 600 conifer species on the planet, with juniper being the most common genus of conifer in North America. Juniper berries (modified cones produced by the female tree) are not only an essential component of gin but relished by a host of wintering birds, among them the American Robin, bluebirds, chickadees, Yellow-rumped Warbler, and Sharp-tailed Grouse. Robins and Townsend’s Solitaires are reported to consume up to 200 juniper berries per day.

While some species of birds like doves and jays swallow seeds whole, other species must dehusk the seed to access the kernel within. Some like chickadees, titmice, and nuthatches hammer them open; others, most notably the finches, have powerful seed-cracking bills that crush shells, allowing the bird to whisk out the exposed kernel with their tongue, swallowing the morsel whole. The bird then lets the husk fall from their mouth. Different finch species have bills calibrated for different sized seeds—a structural linkage first suggested by Charles Darwin in his study of Galapagos finches. In general, smaller-billed birds like American Tree Sparrow and American Goldfinch are more effective at husking small seeds, like goldenrod and millet. But larger-billed species, like Evening Grosbeak and Northern Cardinal, are able to forage on larger seed types. In times of shortage, larger-billed birds have a competitive advantage, able to access a wider range of seed sizes, large and small.

While the bills of all seed-eating birds have evolved to meet the task (or husk) at hand (or bill), the overlapping bills of the pine nut–consuming crossbills show, perhaps, the highest degree of evolutionary refinement. Crossbill bills differ from the bills of other finches by having curved and overlapping tips. Different Red Crossbill subspecies even have bills calibrated for the cones of different, specific pine species. By inserting the bill between the overlapping, shingle-like scales of the cone and exerting downward pressure, the unopened scale is pried apart, giving the bird access to the seed tucked within. The design of the bill is ingenious insofar as it allows crossbills the latitude to bring their stronger bill-closing muscles to bear upon the task, with the upturned tip of the lower bill now exerting scale-opening upward pressure. The large bills of the Cassia Crossbill (a newly designated species, endemic to the Cassia Mountains of southern Idaho) are perfectly sized to access the scales of lodgepole pinecones. The Pinyon Jay, a crestless, nomadic, highly social corvid of western pinyon-juniper environs, uses its pointed, probing bill to access seeds in green (unopened) cones. This messy process forces birds to habitually wipe pine sap from their bills, but it gives the jays access to a food source beyond the reach of many other bird species. Though irrevocably tied to its namesake pine whose range neatly coincides with the jays, like all jays, the Pinyon Jay is opportunistic and omnivorous, eating a variety of seed types and animal matter. Once extracted, the jays will consume a pinyon seed on the spot or cache it for later retrieval.

Caching seeds for later retrieval is a widely practiced safeguard against times of shortage. Some caching birds may store up to 60 per cent of their winter food stocks using this technique. The relocation of stored seeds requires extraordinary spatial memory capacity, a faculty facilitated among seed-caching birds, by the enlarged hippocampus region of the bird’s forebrain. Even so cerebrally endowed, not all hidden seeds are retrieved. Those seeds hidden early in the season are often overlooked and never recovered. Many of these forgotten seeds later germinate, facilitating the spread of the plant and insuring food for future generations of birds. It has been advanced that most eastern oaks sprout from acorns buried but never retrieved by Blue Jays. As for volume, one California homeowner estimated that his local troop of Acorn Woodpeckers had stashed 700 pounds of acorns in the walls of his home. Woodpecker granaries (storage bins) may contain 50,000 such stored nuts, festooning the walls of homes and outbuildings in a fresco of half-embedded acorns.

Rarely do birds deplete natural food stocks. Rather than exhaust a resource, birds rotate to more productive foraging sites. Among bird-eating hawks, these happy hunting grounds include your and your neighbors’ backyard bird feeding stations. Bird-eating hawks ensure themselves a dependable food reserve through the winter by rotating to different favored perches that overlook primary and secondary hunting areas. The hunters fly perch to perch as success or the lack of it warrants. And while many homeowners are dismayed by hawks killing birds at their feeders, the hawks are actually performing an important and natural service: removing diseased members from the flock before their debilitating affliction can spread. In this capacity, hawks serve as the guardians, not the enemies, of your flock.

If it is any consolation, by feeding birds, you are not causing them to be killed. The hawk is going to consume two songbirds per day, no matter what. By feeding birds in your yard, you are only locating this natural dynamic where you will see it—a National Geographic Special in your own backyard.

The Courage of Birds, by Pete Dunne, illustrated by David Allen Sibley, 192 pages, $28.00. Available here from Chelsea Green Publishing.