Masters Thesis

Optimal grit : investigating grit acquisition and site use by black brant

Arctic geese arriving at spring stopover areas are under strict time constraints to build-up endogenous fat and nutrient stores before migrating to their northern breeding grounds. Gizzard grit facilitates the mechanical breakdown of ingested food plants, aiding efficient digestion and potentially influencing daily energy budget decisions made by birds. Grit can also be a significant source of important minerals such as calcium. I investigated whether black brant (Branta bernicla nigricans) in Humboldt Bay, California used a traditional gritting site in a manner that facilitated “optimal grit” ingestion. Based on past grit studies and my own comparison of grit in brant gizzards to grit available on the traditional gritting site, I defined optimal brant grit as large grit with a potential to provide supplemental calcium. I found that: (1) brant were present at the grit site at lower tides when the best grit became available, despite eelgrass (the birds’ primary food source) also becoming available at that time; (2) brant flocks were positioned over sections of the gritting site with the best grit; and (3) grit ingestion behavior increased when the best grit was available. Data suggest that some brant staging in Humboldt Bay strive to balance trade-offs between departing to feeding areas and selectively ingesting better grit to meet specific physiological needs.