How the effects of latitude on daylight availability may have influenced the evolution of migration and photoperiodism

Abstract

Date and latitude interact to determine photoperiod, the daily period of daylight. This interaction has important implications for latitudinal migrants for whom daylight may be a resource or for whom photoperiod regulates annual transitions in life‐history stages (i.e. birds). Using an established formula, we developed user‐interactive, animated models that enable the visualization of how latitude and date determine photoperiod for latitudinal migrants. We also calculated the photoperiodic schedules for a broad range of hypothetical migratory programmes and real migratory programmes newly available through the proliferation of citizen‐science data. This enabled us to infer the limitations some migratory programmes place on mechanisms for photoperiodic regulation of annual breeding. In the vast majority of cases, the act of migrating elevates annual daylight exposure. This raises the hypothesis that daylight availability selects for latitudinal migration, potentially contributing to its evolution in animals such as diurnal birds with limited time during the spring and summer to feed young. However, photoperiodic mechanisms regulating annual cycles could constrain the evolution of such migrations, depending on how they affect photoperiodic schedules. Most migratory programmes are consistent with known mechanisms of avian photoperiodism, but the range of feasible mechanisms declines for transequatorial migrants, which experience semi‐annual, 180°‐phase‐shifts in their photoperiodic cycles. Understanding photoperiodic constraints on migration is particularly important in this age of changing latitudinal distributions and phenologies driven by climate change. A free Plain Language Summary can be found within the Supporting Information of this article.

Longer days enable higher diurnal activity for migratory birds

“…stay in middle latitudes. Therefore, our study covers a range of behavioural features (flight/ground foragers, arctic/temperate breeders, Figure 1; Table 1), providing us with a diverse set of parameters to explore our hypothesis that the activity of arctic migrants will increase stronger than that of mid-latitude migrants (Sockman & Hurlbert, 2020).…”

Circadian and Seasonal Patterns of Body Temperature in Arctic Migratory and Temperate Non-migratory Geese

“…It has been hypothesized that, for diurnal animals, the available daylight can be a limiting resource that may have favoured the evolution of latitudinal migration. Latitudinal migrants can benefit from a relatively longer photoperiod at higher latitudes, allowing them to prolong their daily activity and foraging time (Sockman and Hurlbert, 2020). In line with this hypothesis are observations of a strong positive relationship between daylight availability and activity in various species of latitudinal migrants (Pokrovsky et al, 2021), however, in those studies, no distinction was made between foraging and non-foraging activities.…”

Chasing the sun: When to migrate, when to breed

“…Sockman and Hurlbert () 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.…”

“…Sockman and Hurlbert () 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).…”

“…Finally, in a section entitled, ‘Limitations of Migration on Photoperiodic Regulation of Annual Reproduction (and vice versa)’, Sockman and Hurlbert () 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.…”