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The Birdhouse Network |
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Birds
and Calcium
Study shows calcium use varies among species Calcium is an essential element in the diet of birds, especially for producing eggs. Although we know that wild birds find calcium in a variety of places, relatively little is known about the use of calcium by birds. Published accounts report that Black-capped Chickadees eat ash that is high in calcium and that Anna’s Hummingbirds feed on calcium-rich sand. Arctic sandpipers use the teeth and bones of brown lemmings as a source of calcium. Other sources include snail shells, grit, and eggshells. Several years ago in Europe, researchers began observing birds incubating empty nests or eggs with defective shells, which led them to question whether these birds lacked sufficient calcium in their bodies to produce viable eggs. In subsequent experiments, European researchers found that if they provided calcium to birds, they would produce defective eggshells less frequently and their rate of hatching failure would decrease. Apparently, these birds were unable to find the calcium they needed in the environment, perhaps because of acid rain, which leaches calcium and other elements from the soil. The Lab’s Birds and Calcium Project (BCaP), which began this past spring, is the first large-scale study to investigate the use of calcium by North American birds. This year, the project focused on acquiring general information about birds’ calcium use. BCaP participants offered calcium to birds in the form of chicken eggshells and made monthly records of the bird species that visited their yards and which ones took eggshells. Although a participant may have seen a given species repeatedly during the same month, for the initial results reported here, multiple observations in each month are treated as one sighting. The results are expressed as the percentage of participants who observed a particular species of bird taking eggshells during a given month, based on the total number of participants who observed the species in that month. Participants reported seeing 67 species taking eggshells, including Ovenbirds, Eastern Phoebes, Scarlet Tanagers, Baltimore Orioles, and more. Jays and American Crows were by far the most frequent consumers, taking shells during 30 percent of the observations. Scrub Jays and Blue Jays took eggshells more than three times as frequently as any other species, 63 percent and 43 percent respectively. Steller’s Jays were also frequent takers at 16.7 percent. Some participants commented on the manner in which jays took shells. Brandon Keinath of Circle Pines, Minnesota, saw Blue Jays sampling eggshells with great gluttony. "Toward the end of the month, Blue Jays were practically gulping the eggshells," he wrote. The data and observations on jays lead us to wonder why these species take so many eggshells. Blackbirds were also big consumers of eggshells, taking shells in 11 percent of the yards they visited. Because Brown-headed Cowbirds are brood parasites, laying their eggs in the nests of other bird species, they do not incubate their eggs and can thus produce eggs virtually continuously through the breeding season. To support such high rates of egg production, cowbirds presumably have a great need for calcium, and we would, therefore, expect them to take eggshells frequently. Indeed, BCaP data show that cowbirds do take calcium frequently, with a value of 13 percent. Other blackbirds, however, also use calcium frequently, with values of 14 percent for Common Grackles and 9 percent for Brewer’s Blackbirds. 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. If, as these results show, swallows need supplemental calcium and will visit feeders to get it, why aren’t the numbers of swallows taking shells higher? In addition, why do woodpeckers (2.9 percent), wrens (2.4 percent), and thrushes (1.9 percent) use supplemental calcium so rarely? The data from BCaP provide us with the first information on how various species of birds differ in their use of supplemental calcium. We’re also obtaining vital information from participants’ comments. For example, some participants reported the sex and age of individuals feasting on eggshells. Katherine Aycrigg of Pittsboro, North Carolina, saw a "female adult cardinal feeding eggshells to two juvenile cardinals." Other participants reported Common Grackles, Purple Martins, and several species of sparrows feeding eggshells to their young. These observations, combined with questions prompted by the data, suggest possible avenues for BCaP to explore and illustrate how much more we need to learn about birds and calcium. Are birds in regions affected by acid rain (for example, the Adirondacks of New York) more likely to take eggshells? And, of the birds that do take shells, is there a pattern to when they take them? Look for more results from BCaP in future issues of Birdscope. If you participated in the project but have not yet returned your forms, it’s not too late. Please send us your data as soon as possible. If you did not take part in this project and would like to sign up or learn more about it, please call (607) 254-2446 or send e-mail to birdeducation@cornell.edu Return . |
