Causes and Effects of Divorce in the Blue Tit Parus caeruleus

Two hypotheses to explain why divorce in birds may be adaptive are the «incompatibility hypothesis» and the «better option hypothesis». At least two more, non-adaptive, hypotheses exist: the «accidental loss» and the «forced divorce» hypothesis. We propose a third non-adaptive hypothesis «the musical chairs hypothesis». 2. After making predictions to make it possible to distinguish between these hypotheses we analysed five blue tit Parus caeruleus populations. Divorce rates varied between 8 and 85%. 3. Birds that divorced laid significantly later and tended to have raised fewer young in the year before the divorce compared to pairs that stayed together. These latter did better than the population mean