Abstract
In the polygynous Fan-tailed Warbler Cistcola junicidis, complete clutches and nestlings were frequently deserted by parent females late in the breeding stage.
1) Desertion by females inevitably means loss of offspring, because the eggs and young are left without parental care.
2) The mortality rate caused by desertion was 9.3% of eggs incubated and 15.2% of nestlings.
3) The desertion showed a strong seasonal pattern. Twelve (40%) out of 30 nests during and after August, were deserted in contrast to only three (1.9%) out of 160 nests before August.
4) Some proximate factors favoring such desertion are discussed. Self maintenance against delay of moult and migration seems to be important, although it is costly for females to desert grown offspring.
