Nutritional consequences of gastrolith ingestion in blue-winged teal: A test of the hard-seed-for-grit hypothesis

One benefit attributed to retention of mineral grit in avian gizzards is en hanced digestive efficiency which may be realized through higher metaboliza ble energy of hard seed diets. The hard-seed-for-grit hypothesis also sugge sts that hard seeds can substitute for grit in the gizzard as an aid to mec hanical digestion. However, neither of these presumed nutritional benefits of gastrolith ingestion have been quantified. We maintained captive blue-wi nged teal (Anas discors) on 4 experimental dietary supplements (none, grit, milo, and both grit and milo), and conducted true metabolizable energy (TM E) feeding trials to demonstrate the nutritional consequences of gastrolith consumption. We measured TME of 3 test diets: millet (Echinochloa rrus-gal li), milo, and smartweed (Polygonum pensylvanicum). Mean TME did not differ among treatments for ally of tile test diets, suggesting that pre trial ga strolith ingestion did not appreciably increase metabolizability of these f oods. There was little evidence that birds retained either pretrial seed su pplements or the experimental diets in the gizzard suggesting that seed sub stitution did not occur. Birds frequently regurgitated the test diet. Altho ugh we corrected food intake for regurgitated food mass, a significant posi tive relationship between net intake and TME occurred for smartweed. Howeve r, variation in net intake and occurrence of regurgitation did not appear t o alter the effects of gastroliths on TME. This study does not provide supp ort for either presumed nutritional benefit of gastrolith ingestion. These results may reflect the lack of energetic constraints on food aquisition in captive birds. A similar study of Canada geese (Branta canadensis) showed a significant effect of grit on TME of some fi,ods, suggesting th;lt digest ive responses may vary across a gradient in body size.