Bird Food

Grit, Calcium and Eggshells

Grit is not food. It is equipment: hard fragments a bird stores in its gizzard to grind seed it has no teeth to chew. Calcium is a different problem with a different answer.

Birds have no teeth. A seed eater swallows seed into a crop, passes it to a muscular gizzard, and grinds it there against hard particles it has deliberately swallowed. Those particles are grit, and without them a diet of whole seed is only partly digestible.

Calcium is a separate matter that gets folded into the same conversation because both are often offered as crushed shell. Grit is machinery; calcium is a nutrient, and it is in genuine short supply for female birds in the days before they lay.

This page covers which birds need grit, the difference between insoluble and soluble grit, how to sterilize and crush eggshells so they are safe to put out, and what the evidence does and does not support about supplemental calcium in a backyard.

What grit does inside a bird#

The gizzard is a thick-walled muscular chamber that contracts hard enough to crush seed coats. Grit gives it something to crush against. In heavy seed eaters the gizzard can generate pressures that would surprise anyone who has only seen the outside of a mourning dove.

Particles stay in the gizzard for days to weeks, wearing smooth and eventually passing through, which is why birds top up regularly. Grit size scales with the bird: a dove takes particles a couple of millimeters across, a junco takes something closer to coarse sand.

Grit also has a secondary role that is easy to miss. Coarse mineral particles carry trace elements, and in some regions birds visibly seek out particular soils, road edges and mineral seeps. The habit is real even where the nutritional reason is not fully worked out.

Which birds need grit and which do not#

The dividing line is whether the bird swallows the seed whole or shells it first.

Whole swallowers need grit most: doves and pigeons, quail, native sparrows, juncos and other ground foragers working through millet and cracked corn. A dove's gizzard is a working mill and it needs stone in it.

Shellers need much less. Cardinals, finches, chickadees, titmice and nuthatches remove the hull before swallowing, so the kernel arriving in the gizzard is already soft. They still take some grit, but the requirement is modest.

Insect, fruit and nectar feeders need effectively none. Bluebirds, wrens, orioles, hummingbirds and robins are not grinding seed coats. Their calcium needs are real, but grit is not part of it.

In most yards, birds find their own grit in driveway dust, bare soil, gravel paths and the sand along road edges. Offering grit is a convenience, not a rescue, and it matters most where deep snow covers every source for weeks.

Insoluble versus soluble grit#

Two different materials do two different jobs, and the packaging rarely explains it.

Poultry suppliers and farm stores sell both granite grit and oyster shell in bags sized for chickens, at a price that makes a backyard supply almost free. Buy the smallest particle grade they stock, usually sold as chick or starter grit, because layer grit is too coarse for songbirds.

Calcium and the nesting season#

A female bird building a clutch has to produce an eggshell for every egg, and the shell is almost pure calcium carbonate. She cannot store enough in advance, so she draws it from bone and from whatever she can find in the days before and during laying.

That demand is concentrated. It peaks in the week or two before the first egg and stays high through laying, then drops off. For most of the United States that means late March through June, earlier in the South, later in the mountains and in southern Canada. The broader seasonal picture is in feeding during nesting season.

Where does a wild bird normally get it? Snail shells, millipedes, isopods, bone fragments and calcium-rich soils. Ornithologists working in areas affected by acid rain have long linked declining snail populations to thin eggshells and failed clutches in woodland songbirds, which is the clearest evidence that environmental calcium supply genuinely limits breeding success.

Whether backyard eggshell moves the needle in an ordinary suburb is honestly not established. Birds take it readily, it costs nothing, and the downside is zero, which is a reasonable basis for doing it without overselling the result.

Preparing eggshells safely#

Do not put raw shells out. Residual egg white can carry salmonella, and a feeder is exactly where that spreads between birds. The background is in salmonella outbreaks at feeders.

The whole procedure takes five minutes of attention:

1. Rinse each shell under cold running water as you use the egg, and let it dry on the counter. Do not worry about removing the membrane.
2. Collect shells in an open bowl or paper bag until you have a dozen or more. They must stay dry; a sealed container grows mold.
3. Bake on a sheet pan at about 250°F (120°C) for 20 minutes. They should come out dry, brittle and faintly toasted, not browned.
4. Cool, then crush. Roll a bottle over them or crush them in a bag by hand. Aim for pieces smaller than about 1/4 inch (6 mm), the size of coarse gravel. Powder blows away and large curved shards are awkward for a small bird to pick up.
5. Store the crushed shell in a jar with the lid on, somewhere dry.

How to offer grit and eggshell#

Keep it separate from seed. Mixed into a feeder, crushed shell sinks to the bottom and gets wet.

The simplest option is a shallow dish, a saucer or a flat rock in an open, sunny spot with clear sightlines, near where ground feeders already work. A dedicated corner of a platform feeder works too, as does a small tray on the ground under a ground feeder setup. Birds find it faster if it is visible from above and not tucked against cover.

Put out a small handful at a time, a few tablespoons rather than a cup, and refresh it after heavy rain. Crushed eggshell washes into a paste if it sits in a puddle.

Watch what turns up. Doves, sparrows and juncos work grit steadily. Swallows, robins, bluebirds and chickadees will drop in for calcium specifically during nesting, sometimes in a brief flurry of visits in April and then not again.

What not to use#

• Salted or beach sand. Sodium is a real hazard for small birds, and beach sand carries it.
• Sand from a play area that has been treated, or sand mixed with any product.
• Limestone or dolomite dust in quantity. Fine dust cakes when wet and does nothing useful.
• Wood ash and fireplace ash. Strongly alkaline and no benefit whatsoever.
• Antacid tablets or dairy as a calcium source. Birds have essentially no lactase, a point covered under what not to feed birds.
• Shells from a driveway seashell mix, which are usually salted from marine origin unless sold as poultry-grade and rinsed.

A nesting-season checklist#

• Late winter: start saving eggshells. Rinse, dry, collect in an open bag.
• Early spring: bake, crush and put out the first batch on a flat dish in the open.
• Through spring: refresh after rain, keep the quantity small, and add poultry-grade granite grit if your yard is all lawn and paving with no bare soil at all.
• Alongside it: keep the seed offering simple. Millet and cracked corn on the ground draw exactly the species that use grit most, and the pairing works.
• Midsummer onward: stop. Calcium demand drops once clutches are done, and uneaten shell just sits there getting rained on.