Grit Use by House Sparrows: Effects of Diet and Grit Size

Abstract

Free-ranging House Sparrows (Passer domesticus) were captured with mist nets in central Iowa from August through March, 1990-1993, and their gizzard contents were used to compare grit use by sex, season, and diet. Males and females did not differ in mean grit amounts or sizes (overall mean size = 0.5 mm) in their gizzards. Gizzards of birds captured during March and August contained more grit than those of 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). Aviary experiments then were conducted with captive House Sparrows to evaluate the effects of diet and grit size on grit choice and retention. When birds were given grit particles 0.2-1.4 mm in size and either soft animal food (canned dog food) or hard plant food (wild bird seed), grit in gizzards of birds on the two diets did not differ in mean number or size. When birds were given both animal and plant food and 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 five days. Grit replacement rates were unaffected by diet, but birds given only hard, plant food averaged more grit per gizzard than those given only soft, animal food (x¯ = 538 vs. 205). Gizzards of House Sparrows given only small grit consistently retained grit longer and contained more particles (x¯ = 853 vs. 174) than those of birds given only large grit.