Absolute Consistency: Individual versus Population Variation in Annual-Cycle Schedules of a Long-Distance Migrant Bird

Abstract

Flexibility in scheduling varies throughout an organism’s annual cycle, reflecting relative temporal constraints and fitness consequences among life-history stages. Time-selection can act at different scales, either by limiting the range of alternative strategies in the population, or by increasing the precision of individual performance. We tracked individual bar-tailed godwits Limosa lapponica baueri for two full years (including direct observation during non-breeding seasons in New Zealand and geolocator tracking of round-trip migrations to Alaska) to present a full annual-cycle view of molt, breeding, and migration schedules. At both population and individual scales, temporal variation was greater in post-breeding than pre-breeding stages, and greater in molts than in movements, but schedules did not tighten across successive stages of migration toward the breeding grounds. In general, individual godwits were quite consistent in timing of events throughout the year, and repeatability of pre-breeding movements was particularly high (r = 0.82–0.92). However, we demonstrate that r values misrepresent absolute consistency by confounding inter- and intra-individual variation; the biological significance of r values can only be understood when these are considered separately. By doing so, we show that some stages have considerable tolerance for alternative strategies within the population, whereas scheduling of northbound migratory movements was similar for all individuals. How time-selection simultaneously shapes both individual and population variation is central to understanding and predicting adaptive phenological responses to environmental change.

Two sides of a coin: ecological and chronobiological perspectives of timing in the wild

“…Ecological studies of migratory birds also echo the findings on chronotypes described above for chronobiology (figure 3). Studies of repeated journeys of individuals in some species have revealed between-individual variation in combination with high individual consistency [16,[93][94][95][96]. Similar findings have been reported for daily behaviours of animals, where chronotype has become an increasingly popular measure [55,[97][98][99][100][101].…”

“…[150]). It can be estimated as the proportion of variance within individuals relative to the overall variance measured within a population (ranging from 1 ¼ fully repeatable to 0 ¼ not repeatable; but see [95] for caveats). Traits that show high repeatability, and whose repeatability is to a large extent genetic or epigenetic, are labile to evolutionary changes to the mechanisms that determine their expression.…”

Repeatability in Spring Arrival Dates in Pied Flycatchers Varies Among Years and Sexes

“…Repeatability is the fraction of total phenotypic variance that could be attributed to the individual, and thus can be affected by how consistent individuals are and the amount of phenotypic variance present (see e.g. Conklin et al 2013). For females, repeatability between combinations of consecutive years was largely explained by how consistent arrival was in individuals, suggesting that under some circumstances individuals could not express their normal individual schedule, resulting in low repeatability.…”

“…Repeatability describes how consistently individuals differ from each other (Conklin et al 2013), which depends on how consistent individuals are, and how large differences are between individuals (total phenotypic variation). Low repeatabilities could thus be a result of low consistency within individuals, or alternatively through low variation among individuals.…”

Individuality in northern lapwing migration and its link to timing of breeding

“…Furthermore, the choice of a certain wintering area and migratory strategy may affect annual schedules including timing of events at breeding area (Marra et al 1998), which demonstrates the importance to follow individuals throughout the annual cycle. Knowledge of variation in migratory routines within and between individuals of a population is also important to understanding and predicting the ability of species responses to environmental change, including climate change (Conklin et al 2013).We employed GLS (Global Location Sensing, also called 'light-level geolocation' or just 'geolocation') technique based on the analysis of diurnal changes in light levels to track annual movements of northern lapwings (Vanellus vanellus; here synonymously termed 'lapwing(s)'). Archival tags ('geolocators') record light intensities to determine dusk and dawn times from which geographical positions (two fixes daily) are calculated; day (night) length determines the latitude and time of local midday (midnight) the longitude (Hill 1994).…”