Population counts, time budgets, and disturbance factors of black brant (Branta bernicla nigricans) at Humboldt Bay, California

I examined population trends, spatial distributions, time budgets, disturbance factors, and behavioral responses of black brant (Branta bernicla nigricans) from January-May of 1996 and 1997 at Humboldt Bay, California. Brant counts differed little between years (P = 0.57), but distribution patterns varied substantially (X² = 119.94, df = 2, P 0.0001) among the three major areas within Humboldt Bay. South bay had the greatest number of brant. Time budgets were calculated from 2,146 5-minute focal observations. Brant spent 45% of their time moving, 36% feeding, 11% preening, 5% resting, and 2% each in alert and flight. Behavior was significantly affected by year, date, time of day, tide height, tide flow, and air temperature. Wind speed, wind direction, cloud cover, and flock size had no effects. When disturbances occurred, I recorded cause of disturbance, duration, and type of response. Overall, brant were disturbed 3.3% of the time. Small boats (27%), people (22%), and large boats (21%) accounted for the majority of disturbances. Brant usually (54%) responded to disturbances by taking flight. Humboldt Bay remains an important spring staging area for migrating black brant, but foraging opportunities may be limited by daily tidal cycles and human-caused disturbance.