Successful reproduction for most birds requires them to have built 'good' nests. The remarkable diversity of nests across approximately 10 000 species of living birds suggests that 'good' nest design depends critically on a species' microhabitat, life history and behaviour. Unravelling the key drivers of nest diversity remains a key research priority-bolstered by renewed appreciation for nest museum collections and increasing correlational field and experimental laboratory data. Phylogenetic analyses-coupled with powerful datasets of nest traits-are increasingly shedding light on the evolution of nest morphology and there are functional questions yet to be addressed. For birds, at least, developmental and mechanistic analyses of building (behaviour, hormones, neuroscience) itself, rather than measurements and analyses of nest morphology, are already becoming the next major challenge. We are moving towards a holistic picture in which Tinbergen's four levels of explanation: evolution, function, development, and mechanism, are being used to explain variation and convergence in nest design-and, in turn, could shed light on the question of how birds know how to build 'good' nests. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.
Keywords: brain; building; development; evolution; hormones; nest.