Bluebird and Small Cavity Nester Conservation
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Supplementing Calcium
Feeding Crushed Chicken Eggshells, etc. to Birds

QUICK TIP: Supplement calcium by offering dry, sterilized, crumbled chicken eggshells (or oyster shells) mixed in with mealworms, birdseed, suet, or spread on the ground.

Both young and adult birds need calcium in their diet. Calcium deficiency can cause developmental problems in growing birds. Lack of calcium can cause weak bones in adults. In breeding females, it can cause egg binding or weak eggshells.

chicken egg. Wikimedia Commons photo.
A dry chicken eggshell is about 78-94% calcium carbonate (CaCO3). Calcium carbonate gives shells hardness and strength. Laying chickens need about 4 grams of calcium a day to produce quality eggs (source: Butcher, Concepts of Eggshell Quality).

Note that Mealworms are calcium depleting. Consider supplementing calcium when feeding them to nestlings.

Jump to: Sources | Preparation| Offering | Supplementing Mealworms | How much to Offer | Grit | More Info

Sources: One source you can provide is crushed eggshells. Bluebird parents often eat the eggshells of their hatchlings - perhaps to benefit from the calcium content and also to clean up the nest. You can also buy crushed oyster shells at a feed store - they contain about 38% calcium in a form that is biologically available for eggshell formation.

Preparation: It is probably a good idea to sterilize chicken eggshells first, in case they are contaminated with bacteria. They will also break up more easily if dried. You can do it in the oven or microwave. Don't overcook them, as they will turn brown and smell like burnt popcorn.

  • Rinse the shells, break in half. Then dry in an oven or microwave.
    • OVEN: Put them on a cookie sheet in a 250 F degree oven for 10 minutes until dry, but not brown. (Note: some people use a 350 F oven, but since commercial oyster shell producers use 250, I figure that is sufficient.)
    • MICROWAVE: put them on a plate or paper towel in the microwave on high for about 1.5 to 3 minutes (for a dozen, go 3-4 minutes.) (If you don't rinse them first, be prepared for some loud popping noises from residual egg whites.
  • Then break into small pieces - don't worry too much about the size but you want it to be big enough for them to pick up with their beak and small enough to swallow - maybe about the size of sunflower seed is good. If you cooked them in the microwave on a paper towel, just smunch it up. You can use a blender/processor to get a finer powder to mix with suet or to toss with mealworms.
  • Store extras in a paper bag.

How to offer calcium:

  • Scatter crushed eggshells on sidewalks, patios, deck railing, a rock, or driveways - anywhere birds can see and find them.
  • Mix finely crushed eggshells with seeds and grains normally put in a bird feeder (e.g., platform feeder)
  • Purple Martin landlords sometimes scatter eggshells on the ground where the birds are picking up mud for nests. Some landlords attach shelves to Martin house poles and put eggshells in there.
  • Crushing: Note that most commercial egg producers do NOT offer whole eggs/eggshells to birds, as the birds can learn to eat their own eggs.

Supplement calcium when feeding mealworms to nestlings: Mealworms are calcium depleting, and birds a week away from fledging need to have strong bones for that first flight/crash landing. If you are feeding a LOT of mealworms (perhaps to help a widow/widower, or during cold rainy weather), the following is recommended (based on nutritional research on poultry and parrots) by veterinarian and bluebirder Linda Ruth. You can also mix pulverized chicken eggshells (prepared per above) with the mealworms.

  1. Put mealworms/other insects in to a plastic bag with calcium carbonate or calcium citrate powder, and shake it gently to coat them. Both are readily available at health food stores or on the internet. Calcium carbonate is less expensive. You can also sprinkle on a small amount of avian vitamins at the same time.
  2. Add calcium to the diet of feeder insects (called "gut loading) you offer, using vitamin and mineral supplements sold in pet stores, mostly for feeding pet reptiles. They are expensive though, and have not been proven to work.
  3. Another option is to make up the following mixture which wildlife rehabbers use, and coat mealworms with it before offering them to birds (thanks Casey!)
    • 1 cup high quality kitten kibble. Wal-Mart's Maxximum or Iams is good.
      Soak in 1 cup warm water until the water is absorbed and the kibble is no longer crunchy.
    • Two hard cooked egg whites - no yolk - chopped
    • Calcium carbonate tablets plus vitamin D that will equal 1800-2000mg. calcium (probably 3 or 4). Crush tablets.
    • Combine in a blender. Final consistency similar to thin Dijon mustard.

How much to provide: Studies have been done of how much calcium chickens need, and there is some information on caged birds, but we know very little about the nutrition requirements of wild birds. Their diet is more varied, and they lay far fewer eggs (maybe 20 per year vs. 200 for a laying chicken or duck.) Also, the needs of a growing nestling vs. a male vs. a breeding female will differ. Other factors, such as the form the calcium is in (which affects availability) and the ratio to phosphorus can play into the equation. If you offer bits of eggshells that adults can eat as desired, you probably have the best bet of getting them the "right" amount.

Grit: Note that seed eating birds also need a source of grit like sand. Eggshells are probably not sufficient. Oyster shells are a source of grit. House sparrows have been observed eating mortar between bricks, probably to obtain either/both calcium (from the lime) and grit.

References and More Information:


I'm thankful for the birds who tolerate us, survive despite us, and brighten our lives.
- Bluebars, Gardenweb Bluebirding Forum, 2008

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