Foraging and energy acquisition by black brant (Branta bernicla nicricans) on South Humboldt Bay, California

• Main Author: Elkinton, Elizabeth
• Other Authors: Black, Jeffrey M.
• Format: Master Thesis
• Language: English
• Published: Humboldt State University 2013
• Subjects: Behavior; Brant; Branta bernicla nigricans; Eelgrass; Foraging; Geese; Humboldt Bay; Zostera marina; Arctic; Branta bernicla
• Online Access: http://hdl.handle.net/2148/1648

Thesis (M.S.)--Humboldt State University, Natural Resources: Wildlife, 2013 Black brant (Branta bernicla nigricans) breed in the Arctic and sub-Arctic in spring, a time when food resources are limited. They rely largely on energy stores, acquired at staging sites, for migration and to support egg laying and incubation. In California, Humboldt Bay is the most important staging site for black brant due to abundant eelgrass (Zostera marina). Previously, it was thought that the birds??? access to eelgrass was limited to periods of low-tide when plants were within reach. I tested this assumption by quantifying foraging behavior and intake rates throughout the tidal cycle. I also examined factors contributing to the occurrence of detached floating eelgrass leaves that became available to the birds at higher tides. During winter and spring periods (January - April 2011) black brant at Humboldt Bay foraged directly on eelgrass beds during low tides and continued feeding on drifting leaves during higher tides. They switched from bed-feeding to drift-feeding at a tide height of approximately 0.9 m. Forty-eight percent of flock members typically fed when eelgrass beds were reachable during low tides and 24% of birds fed on drifting eelgrass during higher tides. The proportion of birds bed-feeding was not influence by tide height or month, whereas the proportion of drift-feeding individuals decreased significantly with increasing tide heights in January and February but increased with increasing tide heights in March and April. The amount of drifting eelgrass increased significantly with higher maximum wind speeds, greater tidal range during the previous 24 hrs, and a greater number of brant present on the bay. Intake rates were significantly higher when the birds fed directly on eelgrass beds (0.21 g min-1) than on drifting eelgrass (0.16 g min-1). Mean intake rate achieved during bed-feeding was highest in January and February combined (0.23 g min-1) and lowest in April (0.16 g min-1). Intake rates achieved on drifting eelgrass decreased as tide height increased beyond 0.9 m. Paired adults had the lowest intake rates during drift-feeding (0.15 g min-1) and unpaired adults had the highest intake rates (0.18 g min-1). This study will inform management of spring staging sites and future projections of the carrying capacity of Humboldt Bay for staging brant.