Calcium is an essential mineral for eggshell formation, bone strength and muscle and nerve function, but how can backyard birders offer calcium for visiting birds? Humans may take supplements to get all their necessary minerals, but offering supplements to backyard birds is trickier, but not impossible.

Without Enough Calcium

Proper calcium is vital for birds' health, and if they don't get enough in their diet, a range of different health problems can result.

Eggshells may be too thin and the eggs could never hatch, or young birds may have bone or bill deformities. Nesting females may lay fewer eggs, or some calcium could be drawn from the female's bones in order to make eggshells, putting her at greater risk for broken bones or other injuries. Birds will change their diets during the nesting season to seek out calcium-rich foods, and backyard birders can work to keep those foods in their yard and offer additional calcium at feeders to keep birds healthy.

Sources of Calcium

Wild birds can take advantage of a range of different calcium sources, particularly during the nesting season. Most insects and seeds have only a small amount of calcium, and while nesting, birds will consume larger quantities of calcium-rich insects such as millipedes. The high number of insects fed to hatchlings also helps them get enough calcium, but insects alone are not rich enough in calcium to meet all of a bird's nutritional needs.

Additional calcium sources wild birds will consume include:

• Snail shells
• Sand, dirt or ash (also valuable as grit for proper digestion)
• Bones from carrion carcasses
• Shed deer antlers
• Mortar and lime from between bricks

Many adult birds will also consume the discarded eggshells from their broods as a fine source of calcium, particularly if the species produces multiple broods during a single breeding season.

Providing Calcium to Backyard Birds

While it is not critical for birders to add extra calcium to the foods they offer backyard birds, doing so can ensure birds have adequate calcium available. Eggshells are the easiest option, and roasted chicken eggshells are rich in calcium carbonate that wild birds can easily absorb. Either white or brown eggshells are suitable, but shells that have been dyed are not. To provide the shells, rinse them thoroughly and dry them in an oven at 250-350 degrees for 10-20 minutes, until they are brittle but not browned or burned. When cool, the shells should be crushed to seed-sized pieces and offered in tray or platform feeders, or sprinkled directly on the grass, patio or garden for birds to take (leftover shells make fine compost). If the shells are crushed to dust or powder, they can be mixed into homemade suet as well.

In addition to eggshells, crushed oyster shells or snail shells can be offered for birds to consume. Some high quality seed and suet blends already incorporate shells or other calcium supplements, and while those mixes are more expensive, they can be a useful option for birds. Checking soil composition and lowering soil acidity with appropriate compost or fertilization can be useful to promote snails and insects rich in calcium as well, providing even more natural calcium for birds to use.

Ideally, calcium sources should be offered to birds in spring and early summer when nesting is in high gear. Because birds lay their eggs at night and the shell is formed in the last few hours before the egg is laid, calcium should be offered late in the day, but leaving it available all day gives birds more opportunities to take advantage of the supply as needed.

Depending on the foods and calcium sources offered, a wide range of bird species will enjoy them, including:

• Bluebirds
• Chickadees
• Cowbirds
• Gnatcatchers
• Kinglets
• Martins
• Orioles
• Sparrows
• Swallows
• Tanagers
• Towhees

With so many birds eager to take advantage of calcium sources, backyard birders can attract even more birds than normal to their feeders if they offer eggshells or other calcium sources in the spring. By providing birds this mineral supplement, they help ensure a safer, more effective nesting season for their backyard residents.

Photo – Crushed Eggshells © Danielle Davis