In this thesis, the reproductive strategies of male and female wood warblers are studied. Males which arrived early to the breeding grounds had higher reproductive success than later males. This was due to early males more often mating polygynously than late males. Females preferred early males and by settling in attractive territories they had a relatively high reproductive success because of low rates of nest predation. Males in monogamous pairs were the principal providers in feeding the nestlings, especially during early nestling stage. Polygynous males assisted both females in feeding the young, each female receving less assistance than in monogamous pairs. Females of polygynous males then had to increase their feeding rates to compensate for reduced male assistance. In cases of nest predation for one of a polygynous males two females, males gave full assistance to the female that did not suffer from nest predation. On average, however, females of polygynous males had a cost in compensating for relatively little assistance by their males. It has been suggested that females could use behavioural cues to separate already mated males singing to attract additional females from unmated males. Mated males sang less frequently and spent significantly less time in their secondary territories than unmated males were in their territories, since mated males frequently visited their female in the primary territory. The variation was, however, large with some unmated males also leaving their territories. This variation makes territory presence an unreliable cue for females in assessing pairing status of males. When males returned to their females in their primary territories they often flew close to the nest and sang a short song. To test if this song function as an "all-clear" signal to the female, a stuffed marten (Martes martes) was shown close to the to the incubating female. All males stopped singing and uttered warning calls. There was a difference between monoterritorial and polyterritorial males in the trade-off between helping the primary female and trying to attract a second female.