Humans are not the only species to build with stone. Several species of birds are well known for this trait. The desert lark (Ammomanes deserti), a ground nester found in the Middle East, builds low stone walls (about 4cm high) in front of its nest. One ornithologist described the stone structure as a “pebble glacis,” a type of defensive barrier found in medieval fortresses. In contrast, the rock wrens (Salpinctes obsoletus) of the American southwest appears to be more friendly. The birds live in labyrinthine world of crevices, nooks, passageways, and recesses and construct nests with stone driveways, one of which consisted of 728 flat pebbles ranging from ½ to 2 inches long.
Most impressive, though, is the black wheatear (Oenanthe leucura), known in Spain as pedrero, the stone quarrier or stonemason. During breeding season, a male will pick up an average of 277 stones and fly them back to the nest area. He deposits them in a pile, some of which contain up to four pounds of rock. The piles provide no benefit; the stones neither discourage predators, protect against weather, nor moderate temperatures. Instead, the females use the males’ stone toting facility to determine how much effort he will provide in raising their brood.
I can just imagine her thinking “Now, there is some good mating material. If he spends this much energy building a pile of rocks, imagine how devoted he will be to the kids.” Then again, I worry that she may be disappointed. He may just be a typical rock hound who likes to collect rocks.