CAUSES OF DIVORCE IN THE MONOGAMOUS WILLOW TIT, PARUS-MONTANUS, AND CONSEQUENCES FOR REPRODUCTIVE SUCCESS

Divorce rates were measured in a population of willow tits, in which t he size of some broods was manipulated. The population, with annual su rvival rates of ca 63 and 64% for females and males, respectively, had a relatively low divorce rate of 12.0%. The experiments on altered br eeding success did not support the hypothesis that poor success is lik ely to initiate divorce. Most variation in reproductive success of the pairs differing in breeding history was attributable to the age of th e female, yearling females being least successful. Yearling birds spli t up more often than older birds. Divorced females usually paired with older, and divorced males with younger, mates than their previous one s. With a new mate, divorced females bred successfully and advanced th eir laying date more than widowed females indicating that divorce was beneficial. The success of divorced males was low and some even remain ed unpaired in the season after splitting up. Divorce thus seems to be a strategy chosen by females to increase future reproductive success. The decision seems to be initiated when a female can obtain a higher- ranked mate among the birds in her own or in a neighbouring winter flo ck.