Risks taken by female red-winged blackbirds (Agelaius phoeniceus) in defence of their young were recorded for individuals with differing past investment but identical expected benefits. Investment was manipulated by switching clutches of equal size between nests differing in initiation date, thereby achieving incubation periods as brief as 6 days and as long as 17 days in addition to control nests with normal 11–12 day incubation. While this manipulation had no apparent effect on the females' ability to rear their offspring, during the late nestling stage (6–11 day old nestlings) there was a significant correlation between defence response and length of incubation for 1 of 2 measures used. If females use their cumulative time of investment to assess offspring age, the strong correlation between these two parameters under natural conditions would mean that the females would normally behave in an economically sound fashion. Some evidence is presented which supports this interpretation while contradictory evidence is also discussed. The general conclusion drawn from this study is that the validity of the assumption that animals invest in offspring in an economically optimal fashion remains to be adequately empirically demonstrated.