Do savannah sparrows commit the concorde fallacy?

1. The nest defense behavior of a tundra population of savannah sparrows (Passerculus sandwichensis) was examined to determine the relative importance of past investment and future prospects in determining the observed patterns. 2. A comparison of birds initiating nests at various times of the breeding season indicated that the change in renesting potential within a breeding season had little influence on the birds' behavior. 3. For a monomorphic, monogamous passerine the prospective reproductive success for males and females is expected to be quite similar. However, the respective patterns of nest defense behavior differed considerably (Figs. 1 and 2) and most closely reflected past investment. These results are discussed in light of parental investment theory.