Pleistocene, Holocene and Recent Bird Gastroliths from Interior Alaska

Abstract

ABSTRACT. Polished and rounded grains of quartz, secondary quartz and chert have been found in loess and peat in interior Alaska. The intermediate diameter of these polished and rounded grains is between 35 and 0.07 mm., with grains of 0.7 mm., 2 to 4 mm. and perhaps 10 to 20 mm. being most abundant. Polish on these grains has been produced by abrasion in bird gizzards. Gizzard-bearing birds are living in interior Alaska today and produce glossy polish on their gastroliths comparable to the polish on gastroliths from the Holocene and Pleistocene deposits. The polish and roundness of Recent bird gastroliths vary, being more pronounced in those of living birds in late winter. Some of the Holocene and Pleistocene polished gastroliths are believed to have been produced by species of birds not now living in interior Alaska, because modal size classes of these ancient gastroliths do not coincide with modal size classes of polished gastroliths from birds living there today.

Double-brooding observed in a Columbian Sharp-tailed Grouse (Tympanachus phasianellus columbianus) in Idaho

“…Sharp-tailed grouse, as well as other gallinaceous species, ingest small stones (grit) that accumulate in the gizzard (McCann 1939, Hoskin et al 1970, May and Braun 1973, Norris et al 1975, Schneider 1994. Grit facilitates digestion by assisting in mechanical abrasion of coarse or hard foods (Nestler 1946, May andBraun 1973).…”

Evaluation of factors influencing grit use by birds

“…Grit size varies gready, both within and among avian species, ranging from particles less than 0.1 mm in diameter (Hoskin et al 1970, Siegfried 1973 to stones that measure more than 2.5 cm aaoss, used by large birds such as Ostriches . In general, grit size is positively correlated with body size Rastall 1911, Li^eld 1983).…”

“…Grit size also is related to diet. For a given bird body size, the consumption of harder and coarser foods generally is associated with the use of larger grit , Hoskin et al 1970. Larger grit particles may increase the gizzard's efficiency in mechanically breaking down hard, coarse foods.…”

“…It is usually not possible to determine how much rounding of grit particles occurs in the gizzard (but see Hoskin et al 1970, Selden andSmith 1978). Hoskin et al (1970) reported that Alaskan Rock Ptarmigan grit was more angular and dull-surfaced in summer, when grit sources were always available, and more rounded and polished in winter, when fresh grit was unavailable due to snow cover. They concluded that grit availability determined retention rates and thereby influenced the shape of gizzard grit.…”

“…Stomach stones were used by at least two groups of Cretaceous reptiles, including sauropod dinosaurs and aquatic plesiosaurs (Darby andOjakangas 1980, Stokes 1987). Moas and other Pleistocene birds in New Zealand (Forbes 1892, Young 1967, and Pleistocene, Holocene, and Recent birds in Alaska (Hoskin et al 1970) also carried stones in their stomachs. The gizzard-like stomachs of modem crocodilians (Darby andOjakangas 1980, Siegel-Causey 1990) often contain stones.…”