Seed Eaters
Many birds have figured out how to survive the frozen months when insects and nectar are unavailable – instead they feast on tasty and abundant seeds which are packed with fat and protein. Being granivores, animals that specialize in eating seeds and grains, these bird species evolved to take advantage of reliable and plentiful seeds, developing beaks specially designed to dig into this nutritious food.
Which birds eat seeds?
Few birds exclusively eat seeds, but some species are seed specialists. Leading the way are finches — goldfinches, siskins, redpolls, crossbills, and their kin. Sparrows, buntings, cardinals and their relatives also have a penchant for seeds, and outside of North America, weavers, the little waxbills and estrildid finches also love seeds. Beyond the songbirds, doves and pigeons are prodigious seed consumers, as are many parrots, and gamebirds like quail, pheasants, and grouse round out their diet with a great deal of grain and grass seed.
Seasonally, the number of species that eat seeds is even larger. Many birds that hunt insects in the breeding season shift to seeds in fall and winter when protein-rich bugs disappear and durable seeds ripen and become the most reliable source of food.
The beak story
A bird’s bill can be a solid clue that it’s a seed-eater. Look for a short, deep, conical beak – it’s a nutcracker built onto the bird’s face. The bill’s size and strength are finely tuned to the seeds a bird eats, with small-billed finches like goldfinches eating tiny, soft seeds, while birds with massive bills, like the Hawfinch and Evening Grosbeaks, cracking cherry and olive pits that would defeat almost any other bird (and even humans with tools).
This tight link between beak and seed is one of the most celebrated evolutionary stories in biology. On the Galápagos, the finches Darwin described have beaks which perfectly match their niche foods. More recent observation of these finches has shown that they can still evolve quickly to adapt to changing environmental conditions. For example, recent drought conditions produced large and hard seeds, so birds with bigger, stronger beaks survived, leading to a dramatic and quick increase in the population’s average beak size.
A superb example of bird evolution can be found among crossbills – finches whose upper and lower mandibles cross at the tips. It’s a bizarre-looking arrangement that’s actually a precision instrument for prying apart the scales of conifer cones to reach the seeds inside. Different crossbill types, distinguishable by their vocalizations, are adapted to different species of pine and spruce, essentially separate keys cut for separate locks.
Cracking, husking, and grinding
Seed-eaters have two broad strategies to open a hard seed so they can eat the digestible inner parts.
Huskers — most finches and sparrows — do the work in the bill. A finch wedges a seed into a special groove in its palate, uses its tongue to position it, and applies pressure to split the shell, deftly discarding the husk and swallowing the kernel. Watch a goldfinch at a feeder and you'll see a steady rain of empty hulls.
Swallowers — doves, pigeons, and gamebirds — skip the cracking and gulp seeds whole, often storing them first in a pouch called the crop, then grinding them in the gizzard, a muscular chamber that acts like a gravel mill. Many seed-eaters deliberately swallow grit and small stones to help pulverize the seeds they’ve foraged.
Most birds, even devout seed-eaters, feed their chicks insects, because babies need protein to grow fast, and bugs require no husking or grinding. But a few finches, notably goldfinches, raise their young exclusively on regurgitated seeds. For this reason, they raise their young later in the summer, when seeds are more abundant.
Seed predators & planters
Most seed eating birds are seed predators: when they find and digest a seed, that seed is destroyed and its future as a plant is finished. Granivores are a powerful check on plant reproduction and a major force controlling weeds and shaping which plants dominate a landscape.
But some seed-eaters are among nature's greatest gardeners, storing seeds that are never eaten. A Clark's Nutcracker can bury tens of thousands of pine seeds across a mountainside in a single autumn. When winter arrives, they remember seed locations and feed off their stored food cache. But an individual won’t remember every hidden spot, leaving behind forgotten seeds which grow into the next generation of pines. In many high-mountain forests, these trees and birds depend on each other so completely that neither thrives without the other.
Some seeds have evolved to require a trip through the digestive system of a hungry bird (or other animal). The animal’s stomach acid and digestive enzymes break down the seed’s outer coat and the resulting chemical scarification triggers germination when the seed exits the body. Because a bird may travel wide and far, plants benefit by having their seeds deposited by the birds in new locations, along with nutrient-rich bird excrement.
Boom, bust, and irruption years
Because seeds are produced in unpredictable pulses — a spruce forest may mast heavily one year and barely at all the next — seed specialists live a feast-or-famine life. When the northern conifer crop fails, boreal finches like crossbills, redpolls, pine siskins, and evening grosbeaks pour southward in dramatic "irruptions," suddenly appearing at feeders hundreds of miles from their usual range. Birders track these "winter finch" movements each year, because a good irruption can bring unexpected visitors. (Having a Haikubox to listen for these unusual but welcome visitors might help you spot one!)
Seeds also allow many granivores to stay put. Unlike the long-distance insect-eaters that must travel to south to find locations still teeming with bugs, many seed-eaters are year-round residents or short-distance travelers. The stored energy of seeds is exactly what makes toughing out a northern winter possible.
A quieter crisis
Grassland birds that rely on seeds are in trouble – sparrows, longspurs, and buntings have declined more steeply than birds of almost any other habitat in North America, as native prairie has been plowed under, and remaining grasslands are farmed intensively. Their fate tracks the health of the world's grasslands, one of the planet's most altered and least protected ecosystems.
For most backyards, attracting and spotting seed-loving birds is easy –fill a feeder with black-oil sunflower and nyjer, plant native grasses and seed-bearing flowers, and leave the seed heads standing through winter instead of tidying them away. Before long, you'll host cardinals, finches, juncos, sparrows, and doves through the year, including during the coldest months.