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Many people have probably seen birds picking up small pieces of man-made materials, like strips of a plastic bag or paper litter, and taking them into their nest. This behavior appears to be fairly widespread among birds. What's unique about some larger bird species, like certain vultures, eagles, and falcons, is that the same nest is used for centuries if it continues to be in a safe space. Generations of birds will continue to occupy and add materials to these nests for hundreds of years.

These behaviors are well documented in the Bearded Vulture (Gypaetus barbatus), a threatened species that builds nests in cliff caves, rock shelters, or on cornices. The Bearded Vulture can most often be found in European mountain ranges, particularly the Pyrenees. The environment in many of these regions is dry, particularly in the cave-like structures where nests are found, creating an ideal environment for long-term preservation.

Over a decade ago, a group of researchers in Spain had the opportunity to examine 12 of these nests in detail. Their study was recently in the journal Ecology and discusses a number of surprising findings.

They say, "Between 2008 and 2014, we carried out intense research focusing on more than 50 well-preserved historical Bearded Vulture nests in parts of southern Spain, where the species became extinct around 70–130 years ago. A total of 12 nests were examined, and the remains were identified and analyzed layer by layer, following established archaeological stratigraphic methods."

The team rifled through centuries worth of vulture eggshells, remains of prey, and nesting material and among these they also found 226 items that had been made or altered by humans—providing a window into both past ecosystems and human cultures from the region. The hoard included items like a slingshot made from esparto grass, shoes, a crossbow bolt, a decorated piece of sheep leather and a wooden lance.

Even more surprising was that several items were well over 600 years old, according to carbon-14 dating. Results from one shoe dated back to around 675 years ago, while the decorated leather dated to around 650 years ago. However, the dating revealed a range of time periods, with a piece of basket dating to about 150 years ago.

"Thanks to the solidity of Bearded Vulture nest structures and their locations in the western Mediterranean, generally in protected places such as caves and rock shelters with relatively stable temperature and low humidity conditions, they have acted as natural museums, conserving historical material in good condition," the authors write.

In addition to the human-made items, the researchers found 2,117 bones, 86 hooves, 72 leather remains, 11 hair remains and 43 eggshells. The team notes that this study and its findings "can provide information about temporal

changes in the trophic spectrum, past environment, and the wild and domestic species present."

They also call these nests a powerful tool for investigating and understanding more about the ecology, biodiversity trends, and environmental changes that the vultures are subject to. The findings could potentially inform habitat restoration and species reintroduction efforts.

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