Grit-site selection of Black Brant: Particle size or calcium content?

We examined selection of grit-ingestion sites by Black Brant (Branta bernicla nigricans) on South Humboldt Bay, California in relation to particle size and calcium content. We hypothesized that Brant site selection was dependent primarily upon calcium content and secondarily upon distribution of substrate particle size. We (1) mapped grit-ingestion sites, (2) ranked their importance by Brant abundance and individual movement probabilities between sites, (3) characterized Brant gizzard grit and compared it with grit available at ingestion sites, and (4) compared calcium content and particle-size distribution between ingestion sites and unused sites, and between primary and secondary ingestion sites. Brant repeatedly congregated at specific, discrete sites during the 2 years of observation. The distribution of gizzard-grit particle size was right-skewed toward larger particles (>0.5 mm) relative to the proportional availability of particle sizes in the substrate. We found no significant differences in calcium content or particle size between sites where grit was ingested and unused sites. Within used sites, the calcium content of substrates at the primary ingestion site was significantly higher than at the secondary ingestion sites, as ranked by Brant abundance and between-site movement probabilities. Our findings from the field corroborate previous laboratory results, and confirm that calcium is a significant ecological factor for this species.