It's a fairly common practice to help certain endangered species in the wild by providing them with extra food or prey. But could these activities actually end up harming the very species conservationists are trying to help?

Researchers from Spain's National Museum of Natural Sciences, the Doñana Biological Station and GIR Diagnostics asked that question in a recent study of the Spanish imperial eagle, also known as the Iberian imperial eagle or Adalbert's eagle (Aquila adalberti). One of the world's rarest raptors, conservationists frequently supplement the bird's natural prey with farm-raised rabbits. According to a study published in this month's issue of Ecological Applications, such rabbits are often treated with antibiotics and antiparasitics, and young eagles who eat this meat face depressed immune systems and higher levels of pathogens in their systems.

The researchers took blood samples from 191 young eaglets in three different groups. The first group did not receive any supplemental food; the second received wild and/or domestic rabbits that were first tested and found to be chemical-free; and the third was fed rabbits that came from farms and could have contained the dangerous chemicals.

The results were striking. The presence of antibiotics in their food actually made the eaglets more susceptible to illness: Seven different avian pathogens were found in the third group of eaglets, and the vast majority of these birds required extra medical care. According to the study's abstract, "a higher presence of antibiotics (fluoroquinolones) was found in sick as opposed to healthy individuals among eaglets with supplementary feeding, which points directly toward a causal effect of these drugs in disease and other health impairments."

The authors call this "a telling example of well-meaning management strategies not based on sound scientific evidence becoming a 'contraindicated' action with detrimental repercussions undermining possible beneficial effects by increasing the impact of stochastic [random] factors on extinction risk of endangered wildlife."

Luckily, these sick birds all received extra care and treatment to protect them from these pathogens. That's because the Spanish imperial eagle is one of the most heavily managed endangered birds in the world—so intensely managed, in fact, that the International Union for Conservation of Nature and Natural Resources upgraded the species from "endangered" to "vulnerable" in 2005, in spite of their still-limited population. But other endangered species in the wild might not be so lucky if they encountered similarly tainted food.

The Spanish imperial eagle almost disappeared a few decades ago due to habitat loss, accidental poisonings and electrocution on power lines. Only 30 pairs of birds remained on Earth in the 1960s. Conservation efforts helped raise that number to 253 pairs in 2008 and then to 283 pairs in May of this year, when the first eagle chick born through artificial insemination was hatched at the Center for the Study of Iberian Raptors in Sevilleja de la Jara, Spain.

Even with conservation efforts, illegal wildlife trafficking continues to be a threat to this species. In June 16 people—including the staff director of the San Jerónimo breeding center in Seville, Spain—were arrested for illegally gathering eagle eggs from the wild and selling the chicks for as much as $24,800. According to the English-language Spanish news site The Olive Press, police rescued 101 eagles from the center, where they also found 11 dead birds. The center staff has also been accused of inflating their reported breeding counts in order to boost their grant revenue.

(Correction July 11, 2012: An earlier version of this article incorrectly cited the birthplace of the artificially inseminated eagle chick as the Smithsonian National Zoo. We regret the error.)

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Photo by Antonio Lucio Carrasco Gómez via Wikipedia. Used under GNU Free Documentation License