Populations and behavior of black brant at Humboldt Bay, California

• Main Author: Henry, William G.
• Other Authors: Harris, Stanley
• Format: Master Thesis
• Language: English
• Published: California State Polytechnic University, Humboldt 1980
• Subjects: Humboldt State University > Theses > Forestry; Brant; California; Bird populations; Humboldt Bay; South Bay; Branta bernicla
• Online Access: http://hdl.handle.net/2148/874

Although Humboldt Bay was considered to be a major wintering area for black brant (Branta bernicla nigricans) in the 1890's, the evidence suggests that since at least 1930 it has not supported a large wintering brant population, but instead serves primarily as a major concentration area for northbound migrants. The majority of birds now bypass California bays during the fall, and the population peaks in the spring. A study to determine present brant populations, behavior, and environmental relationships was conducted at Humboldt Bay, California between 15 January 1975 and 10 June 1978. Findings showed that the first groups of fall migrants arrived at South Humboldt Bay during the third week of October. The fall population peaked at 236 on 7 November 1975, 600 on 1 November 1976, and 220 on 11 November 1977. The present wintering period, involving only low numbers of brant, extended from 15 November to 30 January. Spring migrants began to appear in early February. Recorded peak spring populations on Humboldt Bay ranged from 20,000 on 16 March 1977 to 37,500 on 26 March 1975. Principal northward migration occurred between 16 March and 21 April. Eighty-seven percent of brant use was in South Humboldt Bay. A marked decline in the total number of brant-use days was noted on South Bay from over 1,007,740 in 1975 to a low of 595,245 in 1977-78, while the overall black brant population count was increasing from 123,000 to 162,000. Reasons for the decline are debatable but are concluded to be due primarily to the continued intense human activity on and around the Bay. Stands of eelgrass in the Bay were considered to be more than adequate to sustain the brant present. During the fall and winter, 1 November to 13 January, juveniles comprised 41.8,percent of the population. This soon changed with the buildup of spring migrants to 15.6 percent between 23 February and 3 April. As spring migration progressed from 4 April to 23 April, the proportion of juveniles rose to 41.2 percent. This continued to increase during late ...