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The pattern of increased nest-defense effort over the course of a nesting season could result from 3 distinct (albeit non-exclusive) mechanisms: increased value of offspring to parents with progression towards independence (parental-investment theory), decreased opportunity for renesting (renesting-potential hypothesis), or decreased perceived costs of defense after repeated encounters with human observers (positive-reinforcement hypothesis). To gauge relative empirical support for each of these mechanisms, we disentangle these 3 often-confounded hypotheses using multi-model inference with mixed-model ordinal regression applied to an extensive red-winged blackbird (Agelaius phoeniceus) nesting data set (4,518 monitoring visits to 1,330 nests). Parent aggression was rated on an ordinal scale (0-4) during repeated monitoring visits. Additionally, we assessed clutch/brood size, nest density, time of day, and nest concealment effects on aggression. In a preliminary analysis, including all 3 major hypotheses, male and female nest defense was most strongly explained by parental investment (nest age). Positive-reinforcement (visit number) and renesting potential (Julian date), were also well-supported predictors in males. The interactions of decomposed nest age (within- and between individual centered) with Julian date were particularly important in the top male model. Additional factors, such as clutch/brood size, nest density, and nest concealment appeared to have larger predictive roles in explaining female aggression relative to males. These patterns are likely explained by different sexual reproductive roles within a polygynous mating system. Our study highlights the importance of interacting mechanisms involving parental investment theory and the use of within-individual standardization to help disentangle competing, and empirically confounded hypotheses.