Redhead reproductive strategy choices: a dynamic state variable model

Female redhead ducks (Aythya americana) exhibit one of the highest frequencies of facultative parasitic egg laying, extending reproductive choices within a season beyond nesting only. The occurrence of alternative strategies on a population level within and among years and the factors that influence choices are not well documented or understood. We developed a dynamic state variable model to predict reproductive strategy choice and the influence of female age, body mass, food availability, and host availability on strategy choice. The model predicts a general distribution of strategy choice by body mass and a strong influence of both age and host availability on strategy choice. As body mass increases, females choose more costly reproductive strategies from nonbreeding to parasitizing to nesting to a dual strategy, which is defined as a parasitically laid clutch of eggs followed by another clutch laid in the females' own nest. Comparatively, food availability only influenced strategy choice by slightly increasing the use of more costly strategies. Predictions of strategy choice by body mass reflect relationships similar to those proposed by others. Previous studies of the influence of food availability on observed parasitic frequencies produced mixed and often conflicting results. We propose that female redheads are assessing the host environment before making reproductive choices and food availability functions to fine tune this assessment by encouraging or discouraging more costly strategies at a lower body mass.