The first year I started using egg shells, I visited one of our local restaurants to ask them to save a few buckets of egg shells from their morning breakfasts. I live in the country, so the local breakfast joints are very small and full of old-timers. I got some very strange looks and a lot of questions when I was asking them to save the egg shells for me and I noticed some of them looking at each other with a knowing glance as if to say, "she's feeding egg shells to birds?". If you're not afraid of being categorized as *special*, your local restaurants are certainly a good source of a large supply of egg shells to get you started.
Eggshells are an excellent source of grit and calcium, but chicken eggs may harbor the salmonella bacteria so, I always wash and prepare the egg shells according to the PMCA guidelines here. One of the most frequently-asked question this time of year is "When should I put out the eggshells for the birds"? The answer is, "As soon as they arrive".
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| Left - my Tupperware freezer container; Right - 1 gallon Ziploc baggie full of egg shells. After preparing the egg shells, I store them in the freezer all year just to make sure no bacteria can grow on them. |
Every female mother in every species on earth experiences calcium depletion of her body when giving birth. It's no different for purple martins. The female martins already have a tougher time than the males, often experiencing multiple copulations, egg-laying and finally, brooding the eggs for hours and days on end. All of this will sap her energy and her own calcium storage in her bones. The crystalline layer of her eggs (the shell), which is responsible for its mechanical strength, consists of more than 90% calcium in the form of calcium carbonate. Calcium is absorbed from the food in the intestine. Thus, you can see why she needs replacement calcium to help properly both form and strengthen her eggs and keep her own bones strong. I've also observed my purple martins, males and females alike, taking crushed egg shells in to feed their young.
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| Providing birds with supplemental calcium helps them produce stronger egg shells and nestlings. |
According to this reference: "The diet of a particular species of bird may play a role in determining whether the species will seek supplemental calcium. For example, insects are low in calcium; therefore, birds such as swallows that eat primarily insects probably need to find additional sources of calcium. BCaP results suggest this is true. Although they’re not typical feeder visitors, 10 percent of the swallows that came to yards took shells."
How to Offer Supplemental Calcium:
You can offer calcium in either a platform feeding tray or spread out on the ground. This site recommends the following: "The Lab's Birds and Calcium Project determines we should offer calcium on the ground and on platform feeders".
I tend to agree. I prefer to offer them in a platform feeder for my martins as I once had a hawk attack a female that was on the ground eating egg shells. He grabbed her before she could take off. Fortunately, he dropped her when I ran across the yard and scared the heck out of him, but my lesson that day was to move their calcium up to a height where they could more quickly take off if attacked by a predator.
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| My tray is simply a drip tray from a plastic planter that I purchased at Lowe's. I drilled multiple small holes around the bottom edges so that water can drain out. Then I mounted a shelter over the tray so the rain wouldn't wash all the shells out. |
Egg shells are not the only source of calcium that you can offer. You can also purchase 50 lb. bags of crushed oyster shell here in Missouri at your local MFA stores. My last bag lasted me almost 3 years, but as I mentioned, my martins show a distinct preference for the egg shells.
Offering calcium to purple martins is listed as one of the potential attraction techniques. Having a ready-supply of calcium nearby certainly can't hurt your chances of attracting them and it's a cheap and easy thing to do, so why not try it?
