. The Canadian field-naturalist. 170 The Canadian Field-Naturalist Vol. 107 35 30 S 25 (0 CO H Not aged m Adults â¡ Immatures â First-year birds Glaucous-winged Gull. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3. Age composition of Glaucous-winged Gulls found in each month in beached-bird surveys. The immature cate- gory includes 2- and 3-year olds, as well as some incompletely classified first-year birds. Some birds classified as adults might have been 3-year olds. west coast of Vancouver Island and in the White Rock-Boundary Bay area, which had the highest overall densities. Ther
. The Canadian field-naturalist. 170 The Canadian Field-Naturalist Vol. 107 35 30 S 25 (0 CO H Not aged m Adults â¡ Immatures â First-year birds Glaucous-winged Gull. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3. Age composition of Glaucous-winged Gulls found in each month in beached-bird surveys. The immature cate- gory includes 2- and 3-year olds, as well as some incompletely classified first-year birds. Some birds classified as adults might have been 3-year olds. west coast of Vancouver Island and in the White Rock-Boundary Bay area, which had the highest overall densities. There are several reasons for the seasonal peak in beached-bird depositions. First, all of the coastal areas sampled here experi- ence a massive influx of birds in late summer and fall. Many, such as shorebirds or California Gulls, Larus californicus, move away as winter sets in, but huge populations of loons, grebes and waterfowl overwinter in British Columbia's coastal waters (Vermeer et al. 1983, 1992; Vermeer and Butler 1989; Campbell et al. 1990). This cannot explain all of the fall and winter mortality, however. Some species, such as Glaucous-winged Gulls, Common Murres, Cassin's Auklets, and Rhinoceros Auklets are present year-round and population densities at sea are not necessarily highest in fall and winter (Morgan et al. 1991). A second reason for the sea- sonal peak is that high mortality of newly-fledged juveniles occurs at this time and, in the case of the gulls, murres and Rhinoceros Auklets, contributes substantially to the peak. Adult birds, which might be stressed after breeding or while moulting, are more likely to die at this time too. A third reason for the fall-winter peak is that car- casses are more likely to be deposited on beaches by storms in those seasons than in spring or summer. Again this doesn't fully explain the pattern because days on which many carcasses were found did not always follow stormy weather, and conversely, many intense storms did no
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