Males and females often disagree over reproduction. Females produce few but large gametes (eggs), whereas males produce many small gametes (sperm). Females therefore limit the reproductive rate in sexually reproducing species. As a consequence, the number of offspring a female can have depends on the number of eggs that she can produce, but the number of offspring a male can sire depends on how many females he can fertilize. This divergence of optimal mating strategies is the basis of sexual conflict. Adaptations that enhance the reproductive potential of one sex may be detrimental to the other sex, and are expected to drive counter-adaptations by the disadvantaged sex. The battle of the sexes is a pervasive evolutionary force. Recent studies suggest that sexual conflict results in disagreements over mating optima that trigger a chase-away battle of the sexes. This battle can drive the rapid evolution of morphological and behavioural traits, and consequently result in diversification in reproductive behaviour and anatomy. The arms race between the sexes may also incur costs that may compromise their ability to adapt to environmental changes. Thus, sexual conflict has major implications for breeding strategies, speciation and extinction. The conflict of sexes, however, may not end at fertilisation, since parental care is costly so that parents often do better by making their mate do the hard work of raising young. The evolutionary consequences of parental conflict, however, have been largely neglected. Recent advances in statistical comparisons of species and in phylogenetic reconstructions (akin to family trees), and the rapid accumulation of data on natural behaviour and ecology of birds now allow us to test predictions of sexual conflict theory using powerful statistical analyses. We will test four major hypotheses across and within major bird lineages. 1. What factors limit parental conflict? The parents are expected to cooperate in raising young if the demands of the young are high, or the mating opportunities of both parents are limited. We will test these predictions by comparing variation in the outcomes of parental conflict in groups of birds that have different demands of the young and mating opportunities. 2. Do males and females exhibit an evolutionary tug-of-war over care? If the male deserts the brood, does the female compensate and intensify her care to meet the shortfall, or do the offspring lose out? We will also test whether males or females drive the battle of the sexes using a recent statistical approach that can detect whether changes in one trait were more likely to occur before changes in a different trait over evolutionary time. 3. Does sexual conflict result in rapid evolution? We will test whether variation in the intensity of sexual conflict is associated with mating behaviours (song and male display flights), morphology (plumage colour, ornaments and sexual size dimorphism), and life-histories (reproductive rate and survival). 4. Finally, we will test the consequences of parental conflict on species richness and extinction. Is intense sexual conflict over care associated with bird families that harbour unusually high numbers of species? Does the battle of the sexes increase the risk of extinction or population declines? This proposal represents the first (to our knowledge) major phylogenetic initiative to unravel the influence of parental conflict as a potentially pervasive evolutionary force. We anticipate that the outcomes of our research will identify the effects of sexual conflict on mating strategies and rates of trait evolution, and determine whether conflict over care drives species richness and risk of extinction. Thus, our work will not only contribute to a core topic in evolutionary ecology (sexual conflict theory) but will also advance our understanding of social behaviour, life-history theory and conservation biology.