Abstract
SUMMARY (1) Survival rates of Great Tits were studied using data collected in Wytham Wood, Oxfordshire. The breeding population was split into four groups by sex and status (born in nestboxes = residents, not known to be born in nestboxes = immigrants). Survival rates were only measured for breeding adults, the large majority of which were 1 year old when first found breeding. (2) Survival rates were estimated using a recent modification of Cormack's (1964) method. Relationships between these estimates and several environmental variables were investigated. (3) The comparison of two different estimates of capture rate showed that some birds seem to miss 1 year of breeding in nestboxes more often than others; these are more likely to be immigrants. (4) Age, sex, and immigration status of the birds have a strong influence on the survival rate. (5) Male survival rate is negatively related to the density of blue tits, while that of females is not. We suggested that this may be related to the cost of defending a territory. (6) Female survival rate is negatively related to the beech crop production of the previous year (BCC). BCC is probably an indirect measure of the number of resident males, so that the relationship reflects competition between males and females. (7) Survival rates of immigrant males and resident females are affected by different variables from those affecting resident males and immigrant females. Although it is difficult to explain these differences satisfactorily, they may be related to sex differences in dispersal behaviour. (8) The survival rates of the four groups seem to differ irrespective of the age of the birds. In any year females may survive better or worse than males and independently immigrants may survive better or worse than residents. These differences in survival between the four groups are not consistent in direction, although sometimes strongly significant.
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