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National Geographic : 1951 Feb
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The National Geographic Magazine Drawn by Harry S. Oliver and Irvin E. Alleman Raza: Bird City on an Uninhabited Isle Less than a mile square, the island in Gulf of (Golfo de) California is so unimportant that most continental maps ignore it. Boulders, saltweed, and cacti compose its dreary scenery. This very monotony favors a bird colony, which finds security against ordinary risks. Egg robbers, however, pillage the rookeries at times. ton, and Vancouver Island. Here they leave the beaches at the first sight of man and spend much of their time riding the breakers a quar ter-mile from shore. In setting up our collapsible blinds for photographic work, we tried without success to find a spot where they could be placed without disturbing nesting birds. We were finally forced to move several sets of eggs. Ten minutes later the birds, having waited a few yards away, walked back to their homes and settled upon their nests, seemingly un concerned about the strange canvas structure. Owners whose eggs had been moved, however, were baffled. They discovered them in new locations, sat on them for a few minutes, then walked to the blind that hid the original sites. One even stuck its head inside before accepting its new nest position. Ceremony of Changing Shifts Within an hour the life of the colony was going on just as it had when the nearest human was 50 miles away. Birds returning from the Gulf sidled up to their mates with stick, pebble, or feather in beak. Those to be relieved picked up these offerings cere moniously, tucked them under breast feathers, then moved aside to let the newcomers take over home duties. Throughout the entire rookery these changes were constantly being made as returning mates drifted in, singly or in small groups. The birds instinctively knew the killing power of noonday sun on their eggs and young and kept them covered during the heat of the day. Generally, the gull family is notorious for nest robbing and will usually eat the eggs or young in any unguarded nest. Gulls even raid nests of their own species. The Heermann's gulls of Raza, however, seem to have a code of ethics which prohibits them from eating eggs or young of their own kind. Nest Sites, Not Eggs, Recognized A long, narrow valley bisects the north eastern side of Raza. As we came over its side ridges, we were forewarned by a deafening clamor that something really different was in store, but no one could begin to picture the colony of nesting terns that burst into view. At the bottom of this secluded depression the birds were spread out like a silvery blan ket, packed so tight together that from this distance earth could not be seen between them. Others were milling overhead or landing on the backs of those below. The large royals and the smaller elegants, the latter real natives of the Gulf area, min gled indiscriminately (pages 246, 248). The royals laid eggs similar in marking and usually two to the nest; the elegants laid only one, of extremely variable color and pattern. These variations have led some ornithol ogists to advance the theory that the egg number and color pattern are used by adults for home recognition. I wanted to prove or disprove this theory. Several eggs of elegant terns among the thousands near by were almost green. These we traded for ones not so distinctive in color and placed them in nesting depressions within arm's reach of the blind where the rightful owners of the nests were known to us. The owners of these depressions settled to incuba tion immediately without so much as a glance at the strange eggs under them. Next, pairs were made of these green eggs and placed where only one egg had been be fore. Again the results were the same, the bird incubating the two without any sign of indecision. Even when the larger eggs of the royal were placed in elegant nests, and vice versa, the rightful owners of the site, not the eggs, took over. For two full days I juggled these eggs from place to place until I became so confused I couldn't tell where they rightfully belonged; but the individual terns still resided on their original nests. From far up this winding valley of terns came a steady roar of wings mingled with the 240
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