Individual variation in reproductive strategy and partner negotiation in the great tit

Individual variation within populations is the driving force behind selection, but has largely been ignored in the search for population-wide selection. In biparental care, individual variation in reproductive strategy has important consequences for cooperation and conflict. Nesting birds are ideal for studying reproductive strategies because investment decisions in eggs and chick provisioning, and responsiveness to partner work rate can be easily quantified. Here I show that, not only do individual female great tits (Parus major) vary in their reproductive strategy, but that such strategies are revealed in their plumage. A four-year cross-fostering experiment showed that, in some years, females with small plumage stripes invested more in egg laying and less in brood provisioning while the reverse was true for females with large stripes. In other years these relationships were reversed. Females with large or small plumage stripes, and therefore different dominance and aggression levels appear to be making different decisions over pre and post hatching investment between years. Individual optimisation according to the fluctuating resources available may therefore be maintaining this striking variation in strategy and phenotype between individuals. Variation in reproductive strategy has implications for partner compatibility and responsiveness. Partners varied their responsiveness to changes in work rate by partners with different plumage ornaments. How parents respond to, and protect themselves from manipulation by, partners with varying strategies will be discussed. Individual variation within populations is the driving force behind selection, but has largely been ignored in the search for population-wide selection. In biparental care, individual variation in reproductive strategy has important consequences for cooperation and conflict. Nesting birds are ideal for studying reproductive strategies because investment decisions in eggs and chick provisioning, and responsiveness to partner work rate can be easily quantified. Here I show that, not only do individual female great tits (Parus major) vary in their reproductive strategy, but that such strategies are revealed in their plumage. A four-year cross-fostering experiment showed that, in some years, females with small plumage stripes invested more in egg laying and less in brood provisioning while the reverse was true for females with large stripes. In other years these relationships were reversed. Females with large or small plumage stripes, and therefore different dominance and aggression levels appear to be making different decisions over pre and post hatching investment between years. Individual optimisation according to the fluctuating resources available may therefore be maintaining this striking variation in strategy and phenotype between individuals. Variation in reproductive strategy has implications for partner compatibility and responsiveness. Partners varied their responsiveness to changes in work rate by partners with different plumage ornaments. How parents respond to, and protect themselves from manipulation by, partners with varying strategies will be discussed.