Bird migration has always been fascinating. Where do birds go in winter? How fast do they travel? Do they fly non-stop or break their journeys into segments? And what can possibly favour travelling thousands of kilometres to stay 6 weeks at high latitude to raise offspring and then return to the tropics for the rest of the year? Many of these questions were unanswerable until recently when we entered the golden age of bio-logging in which we attach tags (light level geo-locators, satellite transmitters) to birds and have them report back to us where they have been (Mckinnon & Love, 2018). And once again birds prove to be remarkable. In the extreme, the great snipe makes long and fast nonstop flights (4,300–6,800 km in 48–96 hr), and the common cuckoo can travel 7,500 km in 7 days (Cuckoo Tracking Project, 2020; Klaassen, Alerstam, Carlsson, Fox, & Lindström, 2011). Over the same time we learn that birds are in steep decline. North America has seen the loss of nearly 3 billion birds over the past 50 years (Rosenberg et al., 2019) and uncounted birds around the globe. Migratory birds, especially aerial insectivores and grassland birds, are among those declining at the fastest rate. Cats, collisions and habitat loss are believed to be major causes, along with climate change. A much-studied topic is the impact of climate on phenology and the potential for mismatch between timing of migration and reproduction in relation to the seasonal appearance of supportive resource pulses (Visser & Both, 2005; Wann et al., 2019). The pressure is on to advance understanding of the determinants of phenology and the limits to its plasticity and adaptive scope. Birds monitor many environmental factors to prepare for and conduct their migrations (Cornelius, Boswell, Jenni-Eiermann, Breuner, & Ramenofsky, 2013), but changes in day length are critical. In nature, resident birds in mid-latitudes prepare to breed as days increase gradually from 12 to 14 hr between March and June. But a migratory bird flying from the equator to the Arctic Circle might experience an increase from 12 to 20 hr. And, except for the equinoxes, a bird that migrates from pole to pole like the arctic tern can live in days longer than 12 hr all year, chasing the sun. Such birds fly into longer days after both equinoxes, so what keeps them from reproducing in both hemispheres? Sockman and Hurlbert (2020) present a model that allows readers to visualize how changes in day length accompany migration across dates and latitudes. The programme takes the reader to an interactive web site where you enter a species' breeding latitude, wintering latitude, and spring and autumn departure and arrival dates. The output is the associated number of daylight hours (daylight exposure) by season, making it possible to compare exposure across populations and species. Unlike the static graphics that relate date and latitude to day length, which can bewilder, this programme is engaging and informative and definitely something to include in a classroom setting. Sockman and Hurlbert (2020) also develop the hypothesis that daylight is a resource whose abundance can act as a selective force favouring latitudinal migration. Greater day light exposure means more time to feed young, which may compensate for the shorter season and fewer thermally favourable days found at higher latitudes, and perhaps accounting for the well-documented pattern where clutch size increases with latitude (Griebler, Caprano, & Bohning-Gaese, 2010). An increase in nestling growth rate may also allow for a shorter nestling period and reduced risk of predation. The hypothesis is novel in its emphasis and deserves testing. Finally, in a section entitled, ‘Limitations of Migration on Photoperiodic Regulation of Annual Reproduction (and vice versa)’, Sockman and Hurlbert (2020) describe how mechanisms that regulate annual life-history transitions may act as constraints on a species' migratory schedule and potential geographic range. This is the most novel aspect of the paper and (warning to reader) the most challenging to understand. In order for a bird species to take advantage of the opportunity provided by longer days at higher latitudes, it must employ mechanisms of phenology that can function adaptively over the range of day lengths the animal encounters, sometimes while day lengths are changing rapidly owing to the animal's own movements. It is a long-standing observation that whether or not an animal migrates, day lengths that are stimulatory in spring are not stimulatory when experienced again in autumn. For example, in spring, as days grow longer than 12 hr (give or take), gonads will grow, but when an individual next encounters 12 hr in the autumn, that same day length is no longer stimulatory, and the animal has become photorefractory (Dawson, 2015). The annual cycle is thus divided into phases—phase 1 is when lengthening days are stimulatory; and phase 2 is when days of the same length or longer are not. We know much of what we know because of experiments that have exposed birds to fixed day lengths. If birds are held on a constant light to dark ratio of 12L:12D, they will cycle though the phases of the annual cycle, migratory restlessness, gonadal growth and molt, being first stimulated, then suppressed, then stimulated again in a circannual pattern (Åkesson & Helm, 2020). But if they are held at 16L:8D, they will grow their gonads that later collapse and remain collapsed indefinitely because the bird remains in a constant photorefractory state (Dawson, 2015). Fewer experiments on annual rhythms have exposed captive birds to naturally changing day lengths, but a true knowledge gap relates to studies that simulate the rapid changes in day length that accompany a migration northward (or southward in the southern hemisphere) after the spring equinox. The authors of this article summarize this gap as follows: ‘The proximate effects of photoperiod on migration are well studied, with decades of research on mechanisms…. But the proximate effects of migration on photoperiod (i.e. daylight exposure), and its ultimate effects on photoperiodism are not’. They make the case why it matters. According to Sockman and Hurlbert, the timing and duration of the photorefractory phase are key to determining where and when a bird can breed and whether it can benefit from chasing the sun. They say that a bird living in two hemispheres must have some mechanism that prevents breeding in both, and without such a mechanism its geographic distribution may be confined by photoperiodism. A deep strength of this article is that it addresses the function of migration but also the interplay between function and the mechanisms that determine thresholds for stimulation and time to onset of photorefractoriness. It asks whether these mechanisms may constrain a species' potential geographic distribution, which is of particular interest as species' distributions are changing in response to environmental change. The authors invite more research on the impact of light schedules that simulate movements and that these studies be integrated with newer methods that can be applied in the field such as stable isotopes to determine latitudes where molt has taken place and bio logging to reveal actual rates of migration and consequent daylight exposure. Are there limits to the range changes that are the possible set by the mechanisms that underlie photoperiodism? This article forced me to reconsider what I thought I already knew and raised doubts that altered my thinking. Now is an exciting and critical time to be studying migration, and this article will surely stimulate new research directions. I am grateful for discussion with Ketterson laboratory members and with Adam Fudickar and Tom Hahn.