Náttúrufræðingurinn

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Náttúrufræðingurinn - 1979, Side 96

Náttúrufræðingurinn - 1979, Side 96
scanty), and there are old breeding records from the Sog and from Lón. Outside tliese areas there are records of passage migrants and occasional wintering birds (Eyjafjördur, n. Ice- land; Straumur and the Reykjavík area, sw. fceland), and vagrants in a few other localities. Census results are summarized in Table 3. Gensuses in May 1976 (total 1796) and Jan-Feb 1977 (2023) pre- sumably accounted for the entire po- pulation. The males moulted on the Mývatn-Laxá where they were censu- sed in 1975 (1022), 1976 (1026) and 1978 (1273). Census figures and ob- served age ratios in spring indicated a mean apparent annual survival rate of 0.94 for full-grown males. The real survival rate may well be lower, if some juveniles migrate out o[ Ice- land and return as adults. It is more diflicult to estimate fernale survival as only two complete censuses were made (May 1976, Jan-Feb 1977) and censuses restricted to the Mývatn — upper Laxá may be biased towards females, making observed sex ratios doubtfully applicable. A niean appa- rent annual survival rate ol' 0.88 is suggested for the females, again this may be liigher than real survival if immigration/emmigration occurs. A model of the Icelandic population in April 1975—78 (Table 4) suggests a total population of about 1800 in spring 1975 and 1976, about 2000 in 1977 and about 2100 in 1978. Ghanges in the adult population were closely correlated with the nuntbers of ju- veniles in the preceding spring in males, but not in females, suggesting that migration or dispersion was im- portant in regulating female numbers on the breeding grounds. The agc of first breeding in the wild is not known. l’robably no males breed in their 2nd yr and certainly at least some 2nd yr females do not lay. The number of females older than 2 yr (i. e. the potential breeding popula- tion) was estimated at about 550—570 each spring 1975—77 and perhaps up to 640 in 1978. 6. l’rocluction of young to the fledging stage (Table 5) was estimated from young/fentale ratios in August 1976— 78. The young occurred mainly on the upper Laxá, where tliey could all be censused, and on Lake Mývatn where census efficiency was presuma- bly much lower. The proportion of young on the lake and river varied be- iween years and was associated witlt changes in the food supply in these two habitats. Age ratios of Barrow’s goldeneye were about 1.9 young/9 in 1976 and 1977 and about 1.0 in 1978. Age ratios of liarlequin duck (Hislri- onicus histrionicus) varied similarly: 1.7 (1975), 2.4. (1976), 1.3 (1977) and 0.6 (1978). Low production in both species in 1978 was associated wilh a reduction in the blackfly (Simulium vittatum) populalion of the river (Table 2). Food conditions in Mý- vatn were unusually poor in 1975 and 1976 and then improved greatly. This was reflected in production of diving ducks on the lake; the age ratios of scaup (AytHya marila) and tufted duck (A. fuligula) combined were: 0.5 (1975), 2.0 (1976), 3.6 (1977) and about 3 (1978). 'l'hus food and not weather probably determined the production of young of these íour species. Production in the Barrow’s goldeneye may be more even between years than in the other species because of its ability to use the resources of Lake Mývatn when those of the Laxá fail and vice versa. 7. Prior estimates of the Barrow’s golden- eye population of the Mývatn region are briefly reviewed, these are more fully dealt with elsewhere (Gardars- son 1979, Gudmundsson 1979) This population has remained approxi- mately stable since 1960 and probably since abput 1920. During 1900—15 it may have been larger, at least at the lake itself. Mass mortality of duck- 190
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