To breed or not to breed: causes and implications of non-breeding habit in the willow tit Parus montanus.

Causes and consequences of non-breeding in willow tits were studied in northern Finland during 1986-1992. The breeding status was sex and age biased; males and yearling birds were in excess among the non-reproducers. Due to sex bias in the population it appeared detrimental for males to lose a mate, especially shortly before breeding. Lack of a mate was a important factor for males not reproducing (37% of non-breeding males) than for females (14%). Most of the non-breeding birds maintained a pair bond which only rarely broke up for the next breeding season (divorce rate 5.5%). This implies that parental incompatibility 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 this sex-related difference in survival cost is attributable to quality differences among non-breeding individuals. It was especially low-quality yearling females, with low survival prospects, that were responsible for the discrepancy. The proportion of non-breeding females in the population correlated highly with clutch size and subsequent juvenile survival. It is therefore suggested that for most of these females non-breeding is a phenotypic response to low offspring value in the prevailing circumstances (inter-generational tradeoff). However, it is uncertain whether willow tits in a northern population can use breeding density as an indicator of changing survival prospects of their descendants, as suggested by Ekman and Askenmo (1986) for southern Sweden.