CORRELATION BETWEEN 2 COMPONENTS OF PARENTAL INVESTMENT - NEST DEFENSE INTENSITY AND NESTLING PROVISIONING EFFORT OF WILLOW TITS

Nest defence intensity and nestling provisioning effort of female will ow tits (Parus montanus) were significantly correlated at the end of n estling period: well-fed young were defended most intensely. Increased effort was rewarded, since broods with the highest female per-offspri ng provisioning rates were the most likely to produce local recruits. This suggests that the feeding ability is an important cue for parenta l investment decisions, at least in a species like the willow tit whic h has adopted the clutch adjustment strategy. Thus, the most valuable broods would not necessarily be the largest ones, but the ones in whic h the original number of young could be fed most adequately. However, no associations were found between the level of parental effort and of fspring weight, size or condition, nor did the broods producing recrui ts differ from other broods in timing of breeding or number and size o f offspring. The female behaviour may suggest that they invest the mos t time, energy and risk in the young whose chances of joining the wint er flock are the best. The first well-fed young also gain an advantage of prior residency in joining the hock. The first to join normally ob tain higher social status, and hence better winter survival, than late comers. The corresponding patterns in male parental investment behavio ur were weak or absent, which suggested that the male effort was affec ted by the female behaviour. Males seemed to invest in nestling provis ioning in such a way as to supplement the female effort. During nest d efence action males also seemed to invest in protection of females aga inst predation.