Parental investment decision rules in tree swallows: parental defense, abandonment, and the so-called Concorde Fallacy

I conducted an experiment to detect the importance of past investment and expected benefits to reproductive "decision making" by tree swallows (Tachycineta bicolor). Three experimental groups, balanced for measurable indicators of parental quality, were created: one had its clutch reduced in size near the beginning of incubation, the second had its brood reduced near chick-fledging, and the third had no brood or clutch reduction. All experimental groups were tested for differences in 19 measures of parental defense behavior at the same stage in the nesting cycle by exposing them on successive days to two different predators, a ferret and a rat snake, at the nest box. There were no detectable effects of experimental treatment on parental defense; however, there were differences in abandonment frequency related to the severity of experimental clutch reduction. These results suggest that tree swallows respond to differences in expected benefits in "deciding" whether or not to abandon a nesting attempt. Because past investment can affect the prospective costs of abandonment and renesting, the most natural currency for comparing costs and benefits is prospective. Adopting such a currency for parental "decisions" lessens concern over the so-called Concorde Fallacy, but it highlights our relative ignorance of how past reproductive effort influences the costs of future reproduction. [Behav Ecol 1991,2:133-142]