National Geographic : 1951 Feb
245 Young Osprey Guards the Family Castle with Raucous Screams and Menacing Gestures This tangled Isla Raza home points up an osprey habit. Long after the need of building materials has passed, parents gather sticks until fresh additions fall to the ground for lack of space (page 244). E. M. Blocher, director of the Fresno (California) zoo, inspected the nest, but did not take the young fish hawk. Not a soul lives on Angel de la Guarda, the 48-mile-long island in the background. vocal din which accompanies any disturb ance in a colony. Tracing the commotion, I found our boatmen squatting in the center of a milling mass of birds. A bucket of sea water was within reach. Sea Water Tells Good Eggs From Bad The men were lifting tern eggs from the nests and immersing them. If they floated, they were given back to the birds; but if they sank, they were packed in a box, to be eaten later. This use of specific gravity to determine development was an ingenious method of candling and was remarkably accurate. Later I met a Mexican who told me he sold 27,000 Raza bird eggs in 1947 to the markets of Santa Rosalia. If big boats start plying their trade again, the birds of Raza will be in serious danger. Since visiting my first sea bird colony, I had often wondered what controls the place ment of eggs. Do birds just roam around and call the spot where eggs happen to be laid home? Or do they select some particularly suitable area in advance and guard it against encroachment? The egg collecting by our crewmen presented an opportunity for study along this line. While the men were sorting in one section, I took pegs, poles, and string and marked off a 6-square-foot area. From the hillside I watched the birds return. After becoming familiar with the birds, I drew a rough chart of the placement of the eggs and then called the crewmen. Within a few moments the edible eggs were on their way to the boat. Those showing a tendency to float were placed outside the square.