TO BREED OR NOT TO BREED - CAUSES AND IMPLICATIONS OF NON-BREEDING HABIT IN THE WILLOW TIT PARAS-MONTANUS

Causes and consequences of non-breeding in willow tits were studied in northern Finland during 1986-1992. The breeding status was sex and ag e biased; males and yearling birds were in excess among the non-reprod ucers. Due to sex bias in the population it appeared detrimental for m ales to lose a mate, especially shortly before breeding. Lack of a mat e was a more important factor for males not reproducing (37% of non-br eeding males) than for females (14%). Most of the non-breeding birds m aintained a pair bond which only rarely broke up for the next breeding season (divorce rate 5.5%). This implies that parental incompatibilit y is not a possible explanation for pairs not reproducing. Males that did not breed tended to survive better than reproducing ones, whereas such a relationship was not found for females. It is possible that thi s sex-related difference in survival cost is attributable to quality d ifferences among non-breeding individuals. It was especially low-quali ty yearling females, with low survival prospects, that were responsibl e for the discrepancy. The proportion of non-breeding females in the p opulation correlated highly with clutch size and subsequent juvenile s urvival. It is therefore suggested that for most of these females non- breeding is a phenotypic response to low offspring value in the prevai ling circumstances (inter-generational tradeoff). However, it is uncer tain whether willow tits in a northern population can use breeding den sity as an indicator of changing survival prospects of their descendan ts, as suggested by Ekman and Askenmo (1986) for southern Sweden.