Masters Thesis

Populations and behavior of black brant at Humboldt Bay, California

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 spring migration (69.0 percent), suggesting that adults lead the northward migration, with juveniles lagging behind. Use of specific areas by brant and daily flight activity were influenced by tidal level, food availability, time of day, and human activity, particularly hunting disturbance. Areas affected by human activity were used at lower densities than areas without these disturbances. Brant reacted to open-water hunting activity and, to a lesser extent, non-hunting activity by leaving the Bay and flying to the ocean where little food was available. Brant were particularly susceptible to disturbance by aircraft, especially helicopters; and flights below 300 meters often caused flocks to move to the ocean. Intense clamming activity frequently displaced brant from the tideflats to the ocean. Brant hunting at Humboldt Bay may influence a large percentage of the total North American Black brant population because the Bay lies at the northern end of the major brant distribution in California and Mexico. Hunting at locations farther south (central California and northern Baja California, Mexico) appears to precondition the brant to various degrees of hunting pressure. This conditioning causes the early spring migrants, primarily adults, to be quite wary of hunting activities. Brant engaged in some type of foraging behavior in the Bay during nearly two-thirds of the daylight hours. The rate was greater in spring (23 February to 21 April) than in fall and winter (1 November to 13 January). Denying the birds an undisturbed place in which to feed during the day could result in a loss of energy and a deterioration in body condition at a time when they need to prepare for northward migration and breeding. Brant were not observed to adjust their habits and feed at night as some other species of geese are known to do. Among other activities, feather maintenance occupied over 20 percent of the diurnal time budget, whereas swimming, flying, sanding, resting, alert behavior, and interacting with other birds occurred less frequently. The 37- to 40-day hunting season extended from 14 January to 22 February. An average of 43.1 hunters, representing 75.7 percent of the pressure, were afield on each of the last 20 days of the season, and over 44 percent of the pressure occurred in the last 10 days. Timing of arrival of spring migrants, weather, and regulations which restrict open-water hunting to specific days of the week had the greatest effect upon hunting pressure and harvest. The total observed legal hunting harvest in the Humboldt Bay area was 1211 brant in 1976, 368 in 1977, and 1375 in 1978. Over 94 percent of the harvest was in South Bay and more than 91 percent of the hunting mortality occurred in the last 20 days of the season, with over 62 percent in the final 10 days. Illegal kill ranged from 4 to 28. Crippling loss was estimated as 23.0 cripples per 100 birds killed in 1976, 14.0:100 in 1977, and 12.0:100 in 1978. Most hunters were relatively accurate when reporting cripples lost; this is probably due to the experience of local hunters and the fact that they knew they were being observed. The greatest crippling loss (33.6 cripples per 100 birds observed killed) resulted from pass-shooters along the Spit and the lowest (10.9:100) occurred among hunters in scull boats. After cripples and estimated unobserved kill are considered, the total estimated hunting mortality at Humboldt Bay was 1579 brant in 1976, 446 in 1977, and 1655 in 1978. The earlier arrival of spring migrants in 1976 and 1978 and periods of storm winds in 1976 and 1978 were thought to be the major factors contributing to the increased estimated harvest then, compared to the kill in 1977. Juveniles comprised 22.2 percent of the hunting harvest in 1976, 23.0 percent in 1977, and 17.9 percent in 1978. Hunting from stilt blinds with decoys was the most successful of the six methods employed on Humboldt Bay, averaging 1.5 brant per hunter-day. Use of scull boats ranked second in success and second in efficiency. Hunting from island blinds was the most efficient hunting technique used on South Bay, averaging one brant per 3.1 hunter-hours. Other methods (use of spit and floating blinds and pass-shooting) were relatively unsuccessful in take per hunter-day. Hunters in spit blinds with decoys comprised 63 percent of the hunting effort and averaged 49.9 percent of the total 3-year harvest at Humboldt Bay. Management recommendations include: (1) Set aside a portion of South Bay adjacent to the South Spit as a refuge from the beginning of the waterfowl hunting season to 30 April, (2) Continue to limit open-water hunting to Wednesdays, weekends and holidays, (3) Replace stilt blinds with portable floating blinds that are removed after each hunt, (4) Space shore blinds at a minimum interval of 275 meters to meet quality hunting standards, (5) Restrict or regulate pass-shooting from the west side of the South Spit Road, (6) Reinstate the fall brant hunting season once an adequate refuge is provided and brant use is established, (7) Prohibit all aircraft, especially helicopters, from flying below 600 meters over South Bay between 20 October and 30 April, (8) Avoid siltation, pollution, and bottom oyster culture and restrict channel dredging to the minimum necessary, and (9) Continue population and age-ratio counts during the spring to monitor population trends and juvenile survival rates.

Items in ScholarWorks are protected by copyright, with all rights reserved, unless otherwise indicated.