Reconsidering the role of photoperiod in relation to effects of precipitation and food availability on spring departure of a migratory bird

Along with many current studies investigating the impact of climatic change on the behavioural patterns of migratory birds, the paper by Studds & Mara [1] demonstrates relationships between environmental conditions and timing of departure from an overwintering site—Font Hill Nature Preserve in southwestern Jamaica—of the American Redstarts (Setophaga ruticilla), a Neotropical–Nearctic migrant. This study adds new information of how the specific conditions of rainfall and food availability—insect abundance— experienced on the tropical non-breeding grounds can modify the endogenous timing mechanism regulating departure at the time of spring migration. The authors point out that studies investigating the role of climate change in the tropics and migratory schedules have heretofore received little attention because of the insinuation in the literature that circannual rhythms require no environmental input [2,3]. However, endogenous oscillators are entrained by the Zeitgeber of seasonal photoperiod that regulates onset of the vernal migratory programme [4]. Given this as background and recent studies identifying climatic conditions, namely rainfall in the tropics are related to migrant arrival on breeding grounds in spring [5,6], the authors have gone ahead to investigate the role of environmental parameters at departure from the wintering grounds. The authors’ claim their results challenge the hypothesis that photoperiod alone regulates the onset of migration. However, few, if any, have actually made such a statement. As stated, this point is misleading. Granted the authors are using this premise as a straw man; yet the discussion as presented suggests a lack of understanding of how photoperiod, and other cues, influence vernal migration. It should be noted that photoperiod plays an instrumental role both as a driver and a Zeitgeber for endogenous circannual rhythms initiating the developmental phase of vernal migration and possibly that of autumn for birds [7–12] as well as the timing of migration and oestrous cycling in an arctic mammal, the reindeer (Rangifer tarandus) [13]. For long-distance, transequatorial migrants, the most convincing data for the role of circannual oscillators are from earlier studies [4,14], who expose captive birds to a 12 L : 12 D photocycle and demonstrate continuous cycles of gonadal growth, molt and migratory restlessness or Zugunruhe (nocturnal locomotor activity that represents migratory flight). These studies point to the fact that for