Blog #264 Building with Stone

 


As discussed in the earlier blogs, Ospreys are compulsive collectors, building their nests out of whatever materials are available throughout the breeding season, including eclectic human detritus. Natural materials, such as branches, twigs, grasses, algae,  seaweed, and reeds form the basis of most nests. In a rural area like the Finger Lakes,  cornstalks are common. Just before egg laying, lining materials are supplemented by an endless collection of human garbage. In dairy country, you may find milking machine and tractor parts, rubber boots, fertilizer bags, tools, hats, dried cow manure, plastic garbage bags, plastic toy parts, lost clothing, bicycle tires, hula hoops, telephone cords, and ropes of all kinds. Particularly hazardous garbage includes twine, plastic netting,  string, six-pack holders, snarls of fishing line, and fishing nets. 

Young osprey in nest with plastic bags, courtesy of Conserve Wildlife Foundation. 

Perhaps one of the strangest nesting materials used are rhizoliths, first documented on the island of Boavista, Cape Verde, off the coast of Senegal. Boavista is an arid island that consists primarily of stony tablelands and dunes, with a few rugged regions of 

eroded mountains and low coastal cliffs. Rhizoliths are bizarre concretions that project outward from the dunes, closely resembling a dry branch or stick. Such concretions have now been found in other coastal Osprey nests near dunes, including half of the active and old nests found at Boavista.  

Stick-like rhizoliths form around roots, courtesy of Twitter. 

Rhizoliths are root systems that have been encased by calcareous (calcium carbonate)  sediment. They look like cylindrical animal burrows measuring a few inches to several yards long. Root calcification occurs during dry climatic conditions in highly seasonal moisture regimes. Rhizoliths are also abundant in coastal regions of the western  Mediterranean. 

A close up of rhizoliths forming around plant roots, courtesy of wordnik.com. 

The thought that Ospreys carry rhizoliths and incorporate them into their nests is  amusing: these determined creatures are consummate pack rats. Is the similarity of  rhizoliths and sticks the only reason Ospreys collect them? It turns out that rhizoliths

make excellent building supports, reinforcing, and giving structure to the nest. Even if  the rest of nest material disappears through time and weather, the rhizoliths remain, like  Roman pillars, ensuring the nest’s durability over time. It’s suspected that such mounds  may stimulate future Osprey pairs to nest there. 

Osprey nest incorporating rhizoliths, courtesy of Bioone.com. 

Eyes to the sky! 

Candace 

Candace E. Cornell 

Friends of Salt Point 

Lansing, NY 

cec22@cornell.edu 

Reference: Felipe Siverio, Pedro López, Domingo Trujillo (2010). Osprey (Pandion  haliaetus) Use of Rhizoliths in Nest Building. Journal of Raptor Research, Vol. 44, Issue  2 (Jun 2010), pg. (s) 159-161.

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