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 investmentand expected benefits to reproductive "decision making" by treeswallows (Tachycineta bicolor). Three experimental groups, balancedfor 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 chickfledging, and thethird had no brood or clutch reduction. All experimental groupswere tested for differences in 19 measures of parental defensebehavior at the same stage in the nesting cycle by exposingthem on successive days to two different predators, a ferretand a rat snake, at the nest box. There were no detectable effectsof experimental treatment on parental defense; however, therewere differences in abandonment frequency related to the severityof experimental clutch reduction. These results suggest thattree swallows respond to differences in expected benefits in"deciding" whether or not to abandon a nesting attempt. Becausepast investment can affect the prospective costs of abandonmentand renesting, the most natural currency for comparing costsand 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 reproductiveeffort influences the costs of future reproduction. Behav Ecol1991,2:133–142]