The four-story Casa Grande Ruins, the hand-built centerpiece of a prehistoric Hohokam Indian village, have baked under Arizona's desert sun and withstood its rains for perhaps seven centuries.
But now, even as the ruin made of concrete-like clay becomes an island surrounded by urban development, those walls face an unlikely threat from the air and ground: birds and squirrels.
Cooing pigeons roost in round holes that once held the stout wooden ceiling and floor beams, which were hauled at least 30 miles from the nearest mountaintops.
The birds peck at the hardened clay and foul the ruins with droppings, while underground burrows dug by the native ground squirrels hold water that undermines some of the low-lying perimeter walls in the compound surrounding the Great House.
Officials at the ruins 60 miles south of Phoenix are pondering ways to cope with the threats and minimize further degradation of the nation's first archeological preserve and one of its first national monuments.
"It's serious if we don't deal with it right now," said Paige Baker, the monument's superintendent.
If not addressed, Baker said, "Then we'll see this building start to deteriorate. If water gets underneath and starts to settle, and there's some holes right there ... that's a real issue."
"The birds are at least equal to the problem and the threat that we're having with the ground squirrels," said Carol West, chief ranger at the monument operated by the National Park Service.
Primarily pigeons, but also finches, starlings and other migratory birds have been attracted to the area by ample open feed available at nearby cattle feedlots, dairy and other farm operations.
"So the community's feeding them and then we have this absolutely fabulous place for them to sleep and nest," West said.
At least a few dozen pigeons have adopted the Casa Grande, or Great House, for their home, and in migratory periods, starlings can number in the hundreds -- despite the presence of a great horned owl in the canopy's rafters.
"Their droppings are pretty bad, but worse than that, they enlarge the cracks and the already existing holes in the walls in order to roost and nest," West said of the pigeons.
A giant metal canopy has kept rainwater from pooling under the walls of the now-roofless Casa Grande since 1932.
The canopy helped with long-term preservation of the house "but it provided just the perfect harbor for birds, and now they just love it and they're causing all kinds of trouble," West said.
Volunteers collect two or three gallons of debris weekly from inside the structure, looking for droppings and loosened clay to quantify how much material is being lost from the walls, West said. "It's not difficult to imagine how, at that rate, it's accelerating deterioration of the building."
The squirrels have dug scores of tunnels in the open compound surrounding the main structure and countless passages beneath much of the 472-acre complex, which is walled or fenced off from Coolidge on three sides.
The problem, West said, is that in many places, rainwater has collected in the critters' burrows beneath berm-like perimeter walls. "That standing water's the worst thing that could happen to caliche, because it'll soften it up and it will be more pliable," she said.
The squirrel problem has been noted before, including in the early 1970s.
A University of Arizona researcher reported in August that she had caught and tagged 135 adult squirrels and 55 juveniles in a small portion of the monument.
The squirrel population has swelled to the thousands in large part because, with development on the monument's perimeters, "we're cut off from the natural ecosystem that did exist," Baker said.
Large predators that once dined on squirrels are gone, the superintendent said. Only an occasional coyote still wanders through the park. Badgers, foxes and mountain lions are gone.
The vegetation also has changed: Mesquite has given way to creosote, a desert plant that the squirrels love.
Even hawks and other predatory birds come looking for squirrels less frequently; development has "affected those migration patterns or impacted their interest in looking for prey around here," Baker said.
A Wal-Mart and a Safeway strip mall sit to the east, across Arizona 87. Irrigation canals have been in place for years on the south and northwest.
Now, about 1,700 homes are planned to the west on the 489-acre Cross Creek Ranch planned development, said Sue Laybourn, senior planner for the city of Coolidge.
Baker is checking research for possible solutions to the squirrel and bird troubles.
Whatever action is taken must address the squirrels, as well as the pigeons, he said, to assure that the rest of the community is acting together to address the concerns.
Finding a solution won't be easy, but Baker believes it can be addressed.
Trapping would be overwhelming, given the number of squirrels, while poisoning has problems on several levels.
"The reason why we're here also is to protect animals, not destroy them, so that's part of the challenge. How do we do this in some manner that protects the Great House and at the same time protects the ecosystem?" Baker said.
Then too, he said, "If you destroy one species, what are the impacts of that with other species around?"