7. Norris, D.R., Marra, P.P., Kyser, T.K., Sherry, T.W., and Ratcliffe, L.M. (2004). Tropical winter habitat limits reproductive success on the temperate breeding grounds in a migratory bird. Proc. R. Soc. B 271, 59–64. 8. Reudink, M.W., Marra, P.P., Kyser, T.K., Boag, P.T., Langin, K.M., and Ratcliffe, L.M. (2009). Non-breeding season events influence sexual selection in a long-distance migratory bird. Proc. R. Soc. B 276, 1619–1626. 9. Møller, A.P., and Sze ´ p, T. (2005). Rapid evolutionary change in a secondary sexual character linked to climatic change. J. Evol. Biol. 18, 481–495. 10. Scordato, E.S.C., Bontrager, A.L., and Price, T.D. (2012). Cross-generational effects of climate change on expression of a sexually selected trait. Curr. Biol. 22, 78–82. 11. Marchetti, K. (1998). The evolution of multiple male traits in the yellow-browed leaf warbler. Anim. Behav. 55, 361–376. 12. Both, C., Artemyev, A.V., Blaauw, B., Cowie, R.J., Dekhuijzen, A.J., Eeva, T., Enemar, A., Gustafsson, L., Ivankina, E.V., Ja ¨ rvinen, A., et al. (2004). Large-scale geographic variation confirms that climate change causes birds to lay earlier. Proc. R. Soc. Lond. B 271, 1657–1662. 13. Verhulst, S., and Nilsson, J.-A. (2008). The timing of birds’ breeding seasons: a review of experiments that manipulated timing of breeding. Phil. Trans. R. Soc. B 363, 399–410. 14. Visser, M.E., Both, C., and Lambrechts, M.M. (2004). Global climate change leads to mistimed avian reproduction. Adv. Ecol. Res. 35, 89–110. 15. Thomas, D.W., Blondel, J., Perret, P., Lambrechts, M.M., and Speakman, J.R. (2001). Energetic and fitness costs of mismatching resource supply and demand in seasonally breeding birds. Science 291, 2598–2600. 16. Chaine, A.S., and Lyon, B.E. (2008). Adaptive plasticity in female mate choice dampens sexual selection on male ornaments in the lark bunting. Science 319, 459–462. 17. Cockburn, A., Osmond, H.L., and Double, M.C. (2008). Swingin’ in the rain: condition dependence and sexual selection in a capricious world. Proc. R. Soc. B 275, 605–612. 18. Slagsvold, T., Lifjeld, J.T., Stenmark, G., and Breiehagen, T. (1988). On the cost of searching for a mate in female pied flycatchers Ficedula hypoleuca. Anim. Behav. 36, 433–442. 19. Vitousek, M.N. (2009). Investment in mate choice depends on resource availability in female Galapagos marine iguanas (Amblyrhynchus cristatus). Behav. Ecol. Sociobiol. 64, 105–115. 20. Nussey, D.H., Postma, E., Gienapp, P., and Visser, M.E. (2005). Selection on heritable phenotypic plasticity in a wild bird population. Science 310, 304–306. Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, 80309, USA. *E-mail: maren.vitousek@colorado.edu DOI: 10.1016/j.cub.2011.12.024 Developmental Biology: Taking Flight Powered flight was first mastered by insects, many millions of years ago. Now, studies with the fruit fly Drosophila melanogaster reveal the critical role of a conserved transcription factor in programming the development of specialized flight muscles. Sudipto Roy 1 and K. VijayRaghavan 2 ‘‘God in his wisdom made the fly, And then forgot to tell us why.’’ It must have been an annoying buzzing that brought Ogden Nash to pen his famous poem. But the flight manoeuvres of insects, and in particular flies, are also sophisticated: they hover; rapidly change direction, dive and even fly backwards. Anatomically, insect flight probably first evolved by muscles inserting into the wing hinge: mayflies and dragonflies are extant examples where flight is powered by such ‘direct’ flight muscles. In most insects, however, flight is powered by controlling wing oscillation differently, namely through indirect flight muscles (Figure 1A). They are called ‘indirect’ because the muscles insert into the thoracic exoskeleton and produce high frequency wing vibrations by inducing cyclic deformations of the thoracic cuticle and of the wings as an indirect consequence. Indirect flight muscles also have an unusual physiology: the contraction of one set of muscles stretches another, which in turn causes contraction and stretching of the first set. This results in an oscillation of the thoracic box. The motor neuron’s role is to stimulate the muscle periodically, causing the release of Ca 2+ ions in the muscle, necessary to sustain contraction. The motor neuron firing frequency is asynchronous with indirect flight muscle contraction: the latter can be at several hundred to a 1000 Hz, while the former is usually tens of Hz (Figure 1C). Indirect flight muscles are thus stretch-activated and asynchronous, as distinct from other muscles such as those of the insect leg, which are activated by synchronous neuronal firing. The unusual physiology of the indirect flight muscles is made possible by their specialized structure in which the muscles are arranged in unaligned fibre bundles, hence the term ‘fibrillar muscle’, with the endoplasmic reticulum (ER) in the periphery. In contrast, other muscles, such as those of the insect leg, have a more distributed ER and myofibres aligned in a ‘tubular’ form [1]. While the physiology, ultrastructure and development of indirect flight muscles have been extensively investigated [2,3], the mechanism by which the fibrillar fate is instituted had remained unclear. In a recent paper, Schnorrer and co-authors [4] report that, in Drosophila, Spalt major (Salm), a zinc finger transcription factor, functions as a ‘master regulator’ driving muscle progenitors to differentiate into indirect flight muscles. An earlier indication for a role of the salm gene in indirect flight muscle formation came from a study that screened for genes regulating muscle development in Drosophila [5]. The new work [4] now suggests that Salm is a molecular switch that programs the distinctive properties of the indirect flight muscles. Flies deprived of salm function in muscle precursors form fewer and abnormal indirect flight muscles whose myofibrillar organization is shifted from fibrillar to tubular. The effect of salm was specific for the indirect flight muscles, and the formation and function of the tubular muscles, such as those in the leg, remained unaffected. In Drosophila, embryonic muscle precursors first assemble a set of body wall muscles that allow the larvae to crawl around. Then, during metamorphosis, larval muscles degenerate, and adult muscle precursors fuse and differentiate into new sets of muscles engineered for walking and flight [3]. Indirect flight muscles develop through a precisely choreographed series of events [2,3,6]. The development of one set of indirect flight muscles, the dorsal longitudinal muscles, is rather peculiar in that the adult muscle precursors fuse with three larval muscles that escape Dispatch R63