Náttúrufræðingurinn

Volume

Náttúrufræðingurinn - 1979, Page 97

Náttúrufræðingurinn - 1979, Page 97
lings of this species occurred iu 1917 or 1918 (Hachisuka 1927, Pálsson 1936). 8. Barrow’s goldeneyes that leave the Mý- vatn-Laxá mainly do so in late Octo- ber and at least through November. The main return movement takes place in April, beginning in late March. Adult males preceded females and juveniles on the Sog, sw. Ice- land, in the autumn. Females and ju- veniles are niore migratory than adult males; thus in Jan-Feb 1977 sex ratios (total females/100 adult males) were 68 (n 1124) in the Mývatn region (in- cluding the uppermost Laxá), 102 (n 451) on the lower Laxá and 104 (n 151) in the Ölfusá watershed; the corre- sponding ratios for juvenile males were 5 (n 702), 57 (n 349) and 38 (n 102). 9. Up to 1976, 156 Barrow’s goldeneyes (93 ad., 63 pull.) had been ringed in Iceland. Thirteen adult females ringed at Mývatn had been recovered, all but 1 which was shot at Lón 50 knr north were found dead at the ringing locality. Several recaptures liad also been made of fentales on their nests. l’he recoveries indicate an annual mortality rate of 0.3—0.4 which is higher than suggested by census estimates. In 1977 and 1978 94 (5 ad., 89 pull.) were ringed, but there have been no recoveries yet. 10. It is concluded that better estimates are needed of (1) the number of fe- males, (2) the number of breeders and (3) the number of fledged young. Further studies are required to show conclusively whether censuses in Ice- land account for the entire Icelandic population or whether there is a non-breeding segment in eastern Nortli America. To establish these points, censuses should be continued and ringing increased greatly. A more realistic population model may then be constructed, perliaps followed up by experimental manipulation of the population to test hypotheses of how it is regulated. 191
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