The extent of parental care should usually increase with the benefits expected in tenns of reproductive success. In monogamous birds, parental care should therefore increase with brood size. Some recent studies failed to show such a relationship, and we wondered if this may be due to phenomena like extra-pair copulations and/or intraspecific brood parasitism, that could lead to nestlings which are unrelated to one or both putative parents. Thus, measuring the expected benefits by counting the nestlings may be misleading. In our study on parental care in the great tit (Parus major) we detennine parentage via multilocus DNA fingerprinting and show that parental care (measured as anti-predator nest defence) seems to be adjusted to the number of offspring fathered by the resident male rather than to the total number of nestlings.