Avian Nest Defence: Theoretical Models and Evidence

Functional explanations for variations in levels of avian nest defence have been often based upon assumptions concerning optimal allocation of reproductive effort. Such a theoretical framework has led to the formulation of mathematical models aimed at predicting patterns of nest defence in the field. Here, a model is proposed that, starting from a general theoretical standpoint about optimal parental care, integrates previous models and generates new predictions concerning variables not previously included, namely predatory risk for both parents and offspring and age-dependent defensive tactics of nestlings. Optimal levels of parental defence are then expected to increase with (1) parent-offspring relatedness, (2) brood size, (3) offspring quality, (4) potential risk for the offspring, (5) offspring age, and (6) enhancement of offspring's self-defensive tactics while (7) parental residual reproductive value and (8) potential risk for parents are expected to exert a negative effect upon optimal levels of defence. Empirical data are reviewed for testing the model's predictions. Available data provide strong support for predictions concerning parameters 4, 5, 6, and 8. Failed tests concerning parameters 2 and 3 are shown to be flawed in the sense that we should expect defence levels to reflect intra-, instead of inter-individual variations in size and quality of the brood. A more detailed analysis is made concerning variations in residual reproductive value-correlated traits, such as brood number or time in the breeding season. It is argued that, when dealing with such traits, severe violations of basic assumptions (i.e. coeteris paribus statements or absence of an actual correlation between such traits and residual reproductive value) can occur, leading to unexpected results. It is concluded that life-history theory can account for most of the variability observed in nest defence patterns.