Evaluation of factors influencing grit use by birds
Avian grit use was investigated by examining the gizzard contents of free-ranging birds and by conducting aviary experiments with captive birds. Free-ranging Iowa House Sparrows (Passer domesticus) were captured, and their gizzard grit was compared by sex, season, and diet. Males and females did not differ in mean grit amounts (overall x = 580 particles) or sizes, (overall x = 0.5 mm). Birds captured during March and August contained more grit than birds captured during September through February (x = 674 vs. 477). Gizzards containing >75% animal material (insects) had more grit than those containing, >75% plant food (x = 681 vs. 531). In aviary experiments, when House Sparrows were given either small (0.2-0.4 mm) or large (1.0-1.4 mm) grit, gizzards of birds consuming small grit contained 5 times more particles than those of birds consuming large grit (x = 275 vs. 51). In experiments evaluating grit retention, most grit in gizzards was replaced within 5 days. When captive House Sparrows and Northern Bobwhites (Colinus virginianus) were given mixtures of angular/oblong and rounded/spherical grit, most birds (24 of 30 sparrows and 21 of 26 bobwhites) had more angular/oblong and less rounded/spherical grit (P < 0.01) in their gizzards than predicted on the basis of availability. House Sparrows and Northern Bobwhites were offered grit mixtures consisting of equal amounts of 8 colors (red, brown, yellow, green, blue, black, white, clear), either on a light-brown or a dark-brown soil background. In gizzards of both species, yellow, green, and white particles represented the greatest proportions of colored grit. Birds generally used very little black and blue grit. Soil background color had only a slight influence on grit color selection, and only in House Sparrows. The experiments were repeated (on dark soil only) using birds maintained on food dyed to match 3 of the 8 grit colors (red, yellow, blue). House Sparrows preferred brown, yellow, and white grit, and Northern Bobwhites preferred yellow and green grit. Black again received little use. Food color affected grit color selection, but was not associated with major differences in the preference rankings of the 8 grit colors. Grit color use by males and females did not differ in either species in any of the colored grit experiments. A comprehensive synthesis of the available published information on avian grit use is presented.