Ornis Hungarica. vol.8 Suppl. 1.. (1998) p.79-86.
What controls the sex ratio of Goldcrests Regulus regulus?
The Goldcrest (Regulus regulus) is a partial migratory species. In both the resident populations wintering in Scandinavia and the migratory populations travelling to Southern Europe in autumn, the proportion of males is higher. In the wintering population, the survival of females is significantly lower. Our research was carried out between 1983-1994 in the Ã“csa Bird Ringing Station (Central Hungary) and between 1993-1994 in the Italian Alps near Bergamo (Ganda) . At Ã“csa, Goldcrests were ringed in both migratory seasons, while at Ganda only in the autumn. The birds were captured with mist-nets; biometric data were taken using the methods of Actio Hungarica. Sex ratio,, migration dynamics, the proportion of recaptured birds, the amount of reserved body fat of migrants and the changes of body mass of stopovering birds were examined. During autumn migration there were more males at both sites. The average sex ratio in 6csa was 20.2: 1, while in Ganda 20.9: 1. In spring, the sex ratio is reversed, but more balanced, 0.92:1. In autumn we found no difference between the migration dynamics of the two sexes at either sites. 50% of both males and females migrated in Ã“csa as well as in Ganda between 22-25. October. In spring, males migrated earlier: 50% of males arrived at 6csa on March 29, while that of females on April 3. There were no recaptures at Ganda. At Ã“csa, the proportion of recaptures was equal in both sexes (10-12%) in the autumn, whereas in spring twice as many females spent more than two days in the area than males (I 7-37%). The fat reserves of males and females during the autumn migration did not differ, but at Ganda the fat reserves of both sexes were significantly lower than at Ã“csa. During the spring migration the condition of females was much better than that of males. Body mass of the latter did not increase even during stopover. As a conclusion we can say that the earlier and faster migration of males is the factor which increases their mortality, reversing the sex ratio of Goldcrests before they reach their breeding site.