There are two likely reasons to eat dirt: to bind and neutralize plant toxins or to obtain rare nutrients

If you visit Peru, you may be lucky enough to witness one of nature’s most colorful spectacles: hundreds of parrots of up to 18 species congregating on the clay cliffs — known as clay licks — alongside the Tambopata river. These parrots visit these cliffs so they can eat dirt. But why? What makes this particular dirt so special?

Geophagy — eating soil — is widespread amongst animals and birds, yet no one knows why it occurs. Over the years, two alternative hypotheses have been proposed to explain what may drive this behavior in wild parrots of the central Amazon Basin. One hypothesis proposes that consumption of toxic plant foods — foliage, fruits or seeds — is necessarily accompanied by geophagy since some soils may absorb and neutralize plant toxins. The other hypothesis proposes that additional nutritional demands that accompany reproduction may drive an increase in geophagy.

Although there is scant evidence to support the toxic foods hypothesis, most evidence indicates that seasonal nutrient depletion is the main driver of geophagy in parrots of the Amazon Basin.

“There’s lots of evidence that’s pointing in that direction,” said study co-author Elizabeth Hobson, a postdoctoral fellow at the Santa Fe Institute.

Further, it has not escaped the notice of scientists, naturalists and birders that regions where parrot geophagy occurs are located far from the ocean and the local plant foods favored by parrots contain extremely low amounts of sodium.

“Sodium in the rainforest is really rare, and the place on these clay licks most preferred by the birds also has the highest sodium content,” said Dr. Hobson.

Sodium is an essential nutrient that helps the body maintain the proper balance between water and electrolytes, and it supports proper nerve function and muscle contraction.

Previous analyses determined that clay lick soils in the Amazon Basin are particularly rich in sodium (i.e.; ref): sodium concentrations are 40 times greater than those found in typical plant-based foods consumed by wild parrots, and the sodium-to-potassium ratios of these soils are 4,500 times greater — making clay licks an important source of sodium for wild parrots, other birds and wildlife.

Why are these soils so rich in sodium?

“There is some debate why the soils of the clay licks have so much sodium,” said lead author Donald Brightsmith, an assistant professor at the Schubot Exotic Bird Health Center at Texas A&M University. “I think it is because of ancient inland seas that laid down sodium rich clay deposits.”

In contrast to animal-based foods, which are sodium-rich, plant foods are rich in potassium. Not only does potassium interfere with the uptake of sodium, but it also increases sodium excretion, thereby intensifying sodium deprivation. Consequently, for animals in the Amazon Basin that dine exclusively upon plant-based foods, sodium is an important, albeit critically scarce, nutrient.

Field researchers and birders have noticed that geophagy amongst wild parrots varies seasonally. This provides a convenient opportunity to distinguish between the two proposed hypotheses for what drives this behavior in free-ranging parrots. If the plant toxins hypothesis is the primary driver for geophagy, then use of clay licks will peak when plant-based foods are low, because the birds are forced to consume those plant foods that are more toxic. In contrast, if the supplemental nutritional demands hypothesis is the main driver, then clay lick use will peak when parrot nutritional demands are highest — when they are breeding.

The Tambopata Macaw Project is a long-term parrot ecology and conservation program in southeastern Peru. Under the direction of Professor Brightsmith, this project monitors wild free-ranging parrots with the goals of improving the health and welfare of captive birds and aiding the conservation of wild birds in the tropics. Throughout the decades, 18 parrot species have been recorded as they visited the clay licks near Tambopata, and nearly two dozen of these species live nearby in the dense lowland rainforest. When the parrots emerge from the dense jungle to consume beakfuls of soil, they create “a crazy, screaming kaleidoscope of color,” according to Dr. Hobson.

But parrots aren’t the only animals that frequent the clay licks. Pigeons and guans are common visitors, too (ref).

“The birds that eat clay are usually ones with basically no insect or other animal protein in the diet,” Professor Brightsmith said. “[Eating] insects and animals provide an alternative sodium source.”

Professor Brightsmith, Dr. Hobson and Gustavo Martinez, who studies interactions between climate, food availability and parrot abundance at the Tambopata Macaw Project, collected data on seasonal fluctuations in rainfall (19 years), parrot clay lick use (13 years) and tree fruiting (four years). These seasonal patterns were compared to food availability patterns, and to parrot nesting cycles (ref), to distinguish between the food toxicity hypothesis and the supplemental nutrition hypothesis.

“When I started this work I was sure [geophagy] was due to toxin protection during the dry season when there was little food,” said Professor Brightsmith in email.

“But no. It is related to breeding season and yes, it is likely sodium that drives it,” Professor Brightsmith continued. “I still remember when I made the first graph and realized that was the pattern.”

The data revealed, for all parrot species in this community, that clay lick use peaked during the breeding season — not during annual lows in overall plant food availability. These findings support a large body of evidence indicating that the need for sodium is the primary driver of geophagy in parrots.

Of course, this finding does not mean that clay lick soils do not also bind some dietary toxins, but it does call into question the idea that toxin binding is the main reason driving this behavior.

Professor Brightsmith and his colleagues were also surprised to learn that wild macaw chicks fledge at what appears to be the worst time of the year.

“Also I am amazed by the fact that the macaw chicks fledge just as food is going downhill so fast,” Professor Brightsmith said. He noted that some of the larger macaws breed immediately before a seasonal crash in their food supply, thus requiring that parents take their fledgling young on long flights in search of food.

“It was really surprising, but explains a great deal of the movement patterns we see,” Professor Brightsmith added. “This paper really describes the base of the annual natural history pattern for these birds.”

This study also underscores the importance of long-term in situ research projects for understanding natural history patterns as well as sweeping climactic effects on natural communities.

“Long term multifaceted data really allow you to see the big picture,” Professor Brightsmith explained, adding that he is concerned about the effects of climate change on this inland rainforest ecosystem.

“I am curious about the impact of climate change — I hope it does not mess with the birds too much. It may be that the concept of shifting food sources may impact some of the many species at our site. But we are not sure which ones,” Professor Brightsmith cautioned.

“If climate change starts messing with the macaw’s food supply, it could disrupt their ability to breed.”

Source:

Donald J. Brightsmith, Elizabeth A. Hobson, and Gustavo Martinez (2017). Food availability and breeding season as predictors of geophagy in Amazonian parrots, Ibis, published online on 3 August 2017 ahead of print | doi:10.1111/ibi.12515

Also mentioned:

Donald J. Brightsmith (2008). The Roles of Soil Characteristics and Toxin Adsorption in Avian Geophagy, Biotropica 40(6):766–774 | doi:10.1111/j.1744-7429.2008.00429.x

Donald J. Brightsmith (2004). Avian Geophagy and Soil Characteristics in Southeastern Peru, Biotropica 36(4):534-543 | doi:10.1111/j.1744-7429.2004.tb00348.x

Donald J. Brightsmith (2005). Parrot nesting in southeastern Peru: seasonal patterns and keystone trees, Wilson Bulletin 117(3):296–305 | doi:10.1676/03-087A.1