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How Important is
The Lab of Ornithology is gearing up to begin the Birds and Calcium Project (BCP), a new study that will investigate birds’ use of calcium. We know that calcium is important to birds, especially during the breeding season when females are producing eggs and when birds are raising growing nestlings. A natural source of calcium for many birds is snail shells. In parts of Europe, however, snails are scarce and calcium has become an extremely limited resource. Researchers believe that acid rain—which leaches calcium and other nutrients from the soil—is probably to blame. The calcium deficiency found in Europe’s environment has caused a decrease in the nesting success of many species of birds. The evidence to support this conclusion is striking: many birds are producing eggs with thin or defective shells, and some cannot produce eggs at all. The new Birds and Calcium Project being set up by the Lab of Ornithology will begin to assess calcium use by birds across North America. Through the project we hope to determine which species use supplemental sources of calcium, what times of year or times of day that birds tend to consume calcium, and whether birds in different regions of North America consume differing amounts of calcium. To shape our ideas and questions about how birds use calcium most effectively, we needed answers to preliminary questions, so we turned to FeederWatchers for help. During the 1995-96 season, many FeederWatchers reported feeding eggshells—a rich source of calcium—to birds visiting their feeding stations. We composed a one-page survey and sent it to each of these participants, asking them several questions about feeding eggshells to birds. We mailed approximately 400 surveys in the first week of January, and within a few days, the responses were pouring in. Our response numbers have been amazing—thus far 68 percent of the surveys have been returned. The enthusiasm of FeederWatchers has been wonderful, and we appreciate the efforts of everyone who took time to fill out and return the form. The information we received has been very helpful, and although responses are still arriving, we have notable preliminary results. Though most of the FeederWatchers surveyed provide eggshells for birds throughout the year, some feed eggshells only during the spring and summer months, when birds are nesting, and a small percentage leave eggshells out only during winter. Most participants had birds visit their eggshell piles, though a few did not. Nevada’s Sherry Seese wrote, "I saw none feeding on the eggshells nor did any [eggshells] appear to be gone." Judy McGaughey, who had a similar experience in Indiana, wrote, "No birds seemed to feed on the eggshells I put out. I’ll try again this spring to see if the birds will take them this time. I’ll also keep better records." The survey strongly suggests that birds are opportunists, taking eggshells whenever and wherever they’re available. It appears that birds feed on eggshells during peak feeding times: early morning and late afternoon. Participants offer eggshells in a variety of ways—crushed on the ground, mixed with seed on the ground or in a feeder, in the tray of a feeder, or on a platform feeder. Many participants feed eggshells less deliberately. Some FeederWatchers reported seeing birds remove eggshells from their compost piles. North Carolina’s Margaret Spilker wrote, "While taking out the garbage, I accidentally dropped a few shells on the ground outside our kitchen window. Later that day, a Blue-gray Gnatcatcher came twice to eat the shells." To date, survey participants have observed 68 different species of birds dining on eggshells. Species frequently reported to take eggshells include American Crows, House Finches, House Sparrows, European Starlings, Tufted Titmice, Black-capped Chickadees, Carolina Chickadees, Northern Cardinals, and a variety of sparrows. Eggshells were also taken by Red-bellied Woodpeckers, Summer Tanagers, Wild Turkeys, Pine Warblers, American Robins, and various species of orioles, towhees, and wrens. Blue Jays appear to be the most common and certainly the most conspicuous species to take eggshells. Participants report that Blue Jays monopolize eggshell resources year-round. According to Robin DeLong of Minnesota, "Blue Jays were the biggest eggshell eaters. The chickadees got them only if the jays didn’t get there first." Many participants also described Blue Jays fighting over and carrying away large pieces of eggshells. "I have seen Blue Jays dive [into the compost pile], grab half an eggshell, and fly away with it," noted Connecticut’s Donna Potwin. Such gluttony, however, wasn’t limited to Blue Jays; American Crows, Pileated Woodpeckers, and other smaller species also took half shells. As Marion Bates of Maine observed, "Blue Jays aggressively take as many [eggshells] as they can at any time of the year. Hairy Woodpeckers tap off small pieces. Red-breasted Nuthatches take the biggest piece they can, even half a shell, which is comical to watch." The Lab’s calcium researchers extend their thanks to everyone who participated in this valuable survey. Their enthusiasm and desire to help our scientists was summed up well by Colorado FeederWatcher Jane Emery, who wrote, "Thanks for offering this opportunity. I now see how feeding shells to nesting birds is important to science as well as to the birds themselves." We hope that many of you will sign up for BCP this spring.
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