Cornell Lab of Ornithology

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AUTUMN 1999/VOLUME 13, NUMBER 4

Citizen Science
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Feeding Calcium to Birds
BY André A. Dhondt


Please cite this Page as:
Dhondt, A. A. 1999.  Feeding Calcium to Birds. Birdscope, Volume 13, Number 4:  15-16.


The Lab's Birds and Calcium Project determines we should offer calcium on the ground and on platform feeders

Birds need calcium to form eggshells and to feed their nestlings. Yet recent scientific work has shown that under certain conditions, birds cannot find sufficient sources of natural calcium, and researchers believe that acid rain may play a role in this. With that in mind, we began the Birds and Calcium Project at the Lab in 1997. The idea was that by observing differences between species and geographic regions in the frequency that birds take artificial calcium offered by participants, we could determine to what extent acid rain might have adverse effects on birds.

Six-hundred-seventy-nine participants from 49 states and provinces submitted data. In the Winter ’98 issue of Birdscope (page 7), we showed that very large differences exist between species when comparing the likelihood that they would be seen taking calcium. Jays, for instance, took eggshells often, whereas thrushes, wrens, and woodpeckers were observed taking calcium on rare occasions.

A number of our participants put eggshells on the ground, others used platform feeders, and some put eggshells in both places. This permitted us to address the question, “Does the place where we provide calcium influence our chances of seeing a bird take it?”

Before we look at data, we should, perhaps, think about what to expect. I would think a relationship exists between the place where a bird generally looks for food and its preference (if any) about where to take calcium. Thus, the quails that forage on the ground would be expected to prefer to take calcium from the ground, whereas the tree-living chickadees would be expected to take calcium mainly from a platform feeder. It is unclear, however, what to expect the aerial swallows to do. Would they prefer platforms, because of their height, or would they prefer to land on the ground?

The data were analyzed as follows. We recorded where all participants provided eggshells: on a platform, on the ground, or in both locations. We summarized the observations of each participant by month. The number of observations (or sample size) is then the total number of monthly observations made by all participants. For each month, we determined whether the participant had observed a particular bird species and whether that species had been seen taking calcium. We grouped the birds into “families” to increase sample size, assuming that related birds had similar feeding habits. We then calculated the percentage of monthly reports within each family in which a bird had been seen taking calcium.

The percentages in the table below represent the chance that an observer would see a bird taking calcium in a given month.

Percentage taken N=observer
Ground Platform   Both   -months
Thrushes and Mimics 1.9 1.1 1.4 2618
Woodpeckers 2.5 2.6 3.7 3932
Wrens 2.7 2.4 3.1 1242
Nuthatches 2.5 6.1 7.3 1598
Titmice and Chickadees 2.7 6.9 7.4 1728
Towhees and Sparrows 3.5 5.2 5.8 3293
Finches 5.2 7.6 8.3 4551
Blackbirds 9.5 8.0 9.1 3499
Grosbeaks, Cardinals, and Buntings 9.8 10.2 9.6 2885
Quails and Doves 14.6 7.7 10.6 2623
Swallows 21.1 1.8 11.5 648
Crows and Jays 27.3 20.4 23.1 3273
This table indicates the percentage of participants who reported seeing birds take calcium from their yard. The numbers in bold, italic face indicate that the percentage of calcium taken from ground or platform feeders was significantly different.

Our data show that thrushes, mimics, woodpeckers, and wrens took calcium so rarely, we do not know if they have a location preference. Although nuthatches and titmice also do not take calcium very often, they clearly prefer calcium offered on a platform feeder. Participants offering calcium on a platform feeder either by itself or in combination with calcium on the ground were two to three times as likely to observe one of these birds taking calcium than if they provided the calcium only on the ground. Surprisingly, towhees and sparrows also showed a preference for calcium on a platform, whereas the effect on finches was not statistically significant.

Blackbirds, grosbeaks, cardinals, and buntings took calcium in equal proportions from either location, and the difference between locations for crows and jays was small. Quails and doves preferred to take calcium on the ground.

Swallows and martins provided the biggest surprise. Participants who provided calcium on the ground were 5 to 10 times more likely to observe one of these birds landing on the ground and eating calcium than if they provided the eggshells on a platform. It can be very dramatic to observe a swallow circling around several times before landing on the ground. If the swallow is a female in the process of laying eggs, she may sit on the ground for several minutes eating piece after piece of eggshell.

Our conclusion, therefore, is that we should provide calcium (as crushed eggshells or in any other form) both on the ground and on platform feeders.

Because we knew so little about birds taking calcium in the wild, the observation protocol for the Birds and Calcium Project was relatively complicated. That made it possible to determine if this project could be done at all and how best to collect data. I believe we have learned quite a bit. Birds take artificial calcium-some species rarely take calcium, whereas others take it frequently-and location matters. We are developing a new protocol for the 2000 breeding season that will enable us to collect data in a more focused way to address the initial question about the possible effects of acid rain. As always, we encourage more citizen scientists to participate and help us find out to what extent the rate of birds’ consumption of supplemental calcium can be used to indicate the impact of acid rain on the natural environment.