Reprinted from: Purple Martin Update 6(4): 13
Save your own chicken eggshells or ask your local McDonald's restaurant for theirs. Rinse the eggs in plain water (no soap) to remove the gooey albumin. To kill potentially-lethal Salmonella bacteria, thoroughly dry the eggs in an oven. Be sure every eggshell hemisphere is unnestled from other hemispheres. After the shells are dry, crush them into tiny (1/4" x 1/4") pieces in your hands. Offer them to the martins all season long in an elevated feeder ( similar to those sold by the PMCA) placed just below the martin housing. Or place the shells on the bare ground (or on a driveway) near the martin housing. Replenish often, especially after a rain. Your martins will appreciate these shells for their calcium and grit value, plus other species will join the feast, too.
*Additional information added 01-23-04
Many landlords offer their martins eggshells or oystershell as a source of grit and calcium. Crushed oystershell can be purchased pretty cheaply - usually from feed stores, in a 50# bag - but it can be stored for several years of use. Our martins eat oystershell readily, and also feed it to their nestlings.
The martins' diet of insects offers them plenty of protein, but no calcium - so the supplemental eggshell or oystershell is a good extra to supply. The martins will collect pieces of shell on their own from nearby lakes, etc - we have found pieces of clam shell in the nests, also fish bones, mussels, and snail shells.
The eggshells should be rinsed in cold water to remove the gooey albumin that lines the shells, but it is not necessary to remove the papery membrane that lines the shells. To kill bacteria such as salmonella, toast the eggshells in a 200 degree oven for about 10 minutes - spread them out on cookie sheets to make sure all surfaces are completely dry. Then crush the shells to roughly 1/4 inch size pieces and store in a ventilated container. Since microwave wattage varies, we cannot give you exact times or heat settings - if you use a microwave, just be sure all surfaces of the shells are completely dry, so no bacteria can survive. Eggshells can also be processed by boiling them for about two minutes. Using the shells from hardboiled eggs (including dyed eggs) is not recommended, as it's difficult to remove the all bits of cooked egg from the shells, and there is a chance it will spoil and could sicken the martins.
You can spread the shells on the ground below the martin housing, or place them in an elevated platform feeder. Replenish as needed - you may need to stir them up after it rains, too. Other birds may also come for the eggshells, including barn swallows. You can put eggshells out in the winter, too - according to a study done by Cornell Lab of Ornithology, about 60 species of birds will consume them for the grit and calcium they contain.
Copyright 1996 by Purple Martin Conservation Association. All Rights Reserved.
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