REVIEW Adaptation and evolution of photoperiod response systems in birds Scott A. MacDougall-Shackleton Æ Thomas P. Hahn Received: 30 January 2007 / Revised: 6 June 2007 / Accepted: 7 June 2007 / Published online: 20 July 2007 Ó Dt. Ornithologen-Gesellschaft e.V. 2007 Abstract The reproductive cycle of most birds is driven by the annual change in photoperiod, with birds cycling between the physiological states of photosensitivity, phot- ostimulation and photorefractoriness. Comparative studies show that variation in breeding schedules is often corre- lated with variation in photoperiod response systems. We caution, however, that before adaptive specialization of photoperiod response systems can be concluded, the effects of conditional plasticity and phylogenetic history need to be considered. Conditional plasticity can result in birds with identical response systems displaying different breeding schedules at different latitudes. Consideration of phylogeny can reveal whether parameters of response systems are derived adaptations or ancestral traits. Com- parative data on photorefractoriness suggests that one cri- terion for absolute photorefractoriness—spontaneous regression of the gonads on constant long days—is ancestral in the songbirds. Only four species lack this form of photorefractoriness and all of them are opportunistic breeders. A second criterion for absolute photorefractori- ness is insensitivity to even 24 h light when birds are refractory. In contrast to spontaneous regression of the gonads, the distribution of this second criterion across species does not support an adaptive hypothesis. In cardueline finches, a lack of photorefractoriness by this second criterion is widespread and is present even among strictly seasonal breeders. Further exploration of the evo- lution and adaptation of photoperiod responses will require measuring the response of each species to a range of photoperiods and interpreting results within a phylogenetic context. Keywords Photoperiodism Á Reproduction Á Seasonality Introduction About eight decades ago it was discovered that photoperiod plays an important role in organizing birds’ annual change in reproductive physiology and behavior (Rowan 1925, 1926, 1929). The last century saw intense research activity directed at understanding the nature of photoperiodic re- sponses in birds (e.g., Wolfson 1959; Lofts and Murton 1968; Farner et al. 1983) and the termination of repro- duction by photorefractoriness (Nicholls et al. 1988). More recently, advances have been made in our understanding of photoperiodism in birds on two fronts (Goodson et al. 2005). First, important progress towards understanding the physiological mechanisms underpinning photoperiodic re- sponses has been made (reviewed in Goodson et al. 2005). Second, data from a variety of bird species have accrued allowing comparative approaches to understand the evo- lution of a diversity of photoperiod response mechanisms (e.g., MacDougall-Shackleton et al. 2005, 2006; Hahn and MacDougall-Shackleton 2007). In this mini-review, we will briefly highlight some of the important advances made towards understanding the physiological mechanisms underlying photoperiod responses in birds, followed by a review of the diversity and evolution of photoperiod re- sponses in songbirds. We highlight the importance of Communicated by F. Bairlein. S. A. MacDougall-Shackleton (&) Department of Psychology, University of Western Ontario, London, ON, Canada N6A 5C2 e-mail: smacdou2@uwo.ca T. P. Hahn Section of Neurobiology, Physiology and Behavior, University of California-Davis, Davis, USA 123 J Ornithol (2007) 148 (Suppl 2):S219–S224 DOI 10.1007/s10336-007-0172-y