784 EMERY [chap. 28 



p. 357), but according to W. E. Schevill (Woods Hole Oceanographic Institu- 

 tion, Massachusetts, m litt.) in at least several instances these are only poorly 

 polished. Some modern reptiles do carry stones. An 8-m cayman in the 

 Philippine Islands was found to contain 70 kg of stones to about 12 cm diameter 

 (de la Gironiere, 1853), and 545 out of 681 specimens of the Nile crocodile 

 examined by Cott (1961) contained stones averaging in total about 1% of 

 body weight. 



6. Birds 



It is well known that chickens must have available a supply of grit or small 

 pebbles to serve as grinding tools against seeds in their gizzards. Many other 

 seed-eating land birds pick up sand and grit for the same purpose. The largest 

 land bird, the extinct moa of New Zealand, carried gizzard stones as much as 

 2 cm in diameter, as noted by the writer during the excavation of a moa 

 skeleton, and according to Hamilton (1892) stones associated with a single 

 skeleton may weigh in aggregate nearly 3 kg. The instinct or need for grinders 

 is so well developed that in the absence of sand or pebbles swans at Tule Lake 

 and nearby game refuges in California pick up so much of the estimated 52-ton 

 annual fall-out of lead shot that many sicken and die of lead poisoning (Rosen 

 and Bankowski, 1960). 



Transportation of pebbles by shore-based sea birds is less well known but 

 definitely occurs. Penguins, living far from known sources, commonly carry 

 stones of a wide variety of rock types (Murphy, 1936, pp. 353, 364, 377, 397, 

 451). As much as 300 g of pebbles having diameters up to about 1 cm have been 

 found in young penguin chicks (J. M. Sieburth, Virginia Polytechnic Institute, 

 in litt.). Adults of the large emperor penguin have been reported (Sclater, 1888) 

 to contain pebbles of 3.3 cm diameter with an aggregate weight of 5 kg. Since the 

 stomachs are not muscular, the stones cannot serve as trituration aids, but possibly 

 are used for ballast. They are commonly regurgitated near the nests and picked 

 up again when the penguin returns to the water. Similar pebbles have been 

 dredged by M. Ewing of Lamont Geological Observatory (Fellows, 1958) from 

 deep water near Antarctica where they may have fallen from floating ice on 

 which nests had been built. Many other sea birds from the Antarctic, such as 

 shearwaters, storm petrels, diving petrels, steamer ducks, oyster catchers, 

 sheath bills, cape pigeons and ducks are reported by Murphy (1936, pp. 666, 

 672, 770, 786, 962, 968, 980, 981, 1006, 1056, 1081) to contain pebbles, shells 

 and sand in their stomachs. At Cedros Island, Mexico, Kelly found three sub- 

 round 1.5-cm pebbles of a local kind of schist in a cormorant, but two other 

 cormorants taken at La Jolla, California, contained none. At least one 

 instance is recorded of fossil gizzard stones — in phosphate deposits of Kita- 

 Daito-Zima (Yabe and Sugiyama, 1934). 



Some sea birds, especially gulls, are well known for their shell-opening tactics, 

 by dropping clams, mussels and other mollusks from a height of 10 to 20 m onto 

 sandy or rocky shores. An entertaining description of the technique is given by 



