SEX-ROLE REVERSAL IN WILLOW TIT NEST DEFENSE

The roles of sexes in the nest defence behavior of monogamous willow t it (Parus montanus) were studied near Oulu, northern Finland, in 1988- 1990. The relative roles of the sexes changed during the breeding cycl e: females defended their nests more vigorously before hatching and ma les defended more after hatching. This sexual asymmetry was studied by means of the cost/benefit model of optimal parental investment (PI). Because of the monogamous breeding system, sexual differences in futur e benefits were not a likely explanation for the asymmetry. This was a lso supported by preliminary results of DNA-fingerprinting analyses. B ehavioral observations - sex-role reversal, high correlation between t he mates, and equal variations in male and female behavior - indicated the same. In the beginning of the breeding cycle, higher female defen ce was related to renesting potential. The extra costs of renesting ar e considerably higher for females than for males, and therefore, femal es appeared to benefit more than males in keeping the first clutch ali ve while renesting was still possible. Such a female strategy was poss ible in this northern population, since the time for renesting was rat her short and never lasted beyond the hatching of the first nests. Aft er this point, the male can only invest in the present clutch. By the end of the breeding cycle, increased defence of the nest by males may be a consequence of males being larger and/or in better condition than females. Therefore, in the latter half of the breeding cycle, the sex ual differences in nest defence did not indicate any sexual difference in PI. The asynchronous arrival of the parents at the nest also affec ted individual responses, indicating that defending the nest is a dyna mic process.