In migratory songbirds, older individuals are thought to be more efficient migrants than younger individuals. Age-specific differences in migratory efficiency have been reported mainly in respect of arrival timing, energy stores, rate of energy accumulation, departure behaviour, and departure direction. Yet, these traits were rarely assessed simultaneously in a single species. We further lack information whether age-specific differences in behavioural traits present in autumn still manifest to the same degree in spring. Here we used the northern wheatear (Oenanthe oenanthe), a long-distance nocturnal songbird migrant, and investigated age-specific differences in energy stores at capture (autumn: 1059 birds/spring: 803 birds), rate of energy accumulation (168/147), nocturnal departure timing (126/105), and departure direction (94/77) for both seasons. We found that in autumn, young northern wheatears departed significantly later in the night than older birds. This difference was not observed in spring. The resulting advance in nocturnal departure timing by young birds from autumn to spring may be due to learning based on experience gained during autumn, and/or to selective disappearance of those young individuals showing late departure times during autumn. We found no age-specific difference in any of the other migratory traits investigated. To get a better understanding of age effects in songbird migrants, we need to study the potential adjustments of migratory behaviour within the individual over its life time. By individually tracking songbirds throughout their lifetime, we could start estimating whether a certain migratory decision (fuelling, departure, orientation) translates into higher (or lower) fitness, and whether individuals adjust their migratory behaviour based on learning from “wrong decisions”.
data capture dates and energy stores
There are seven columns: 1. "energystores_capture": This is a numeric variable and it provides the individual energy stores at capture, estimated as detailed in eqn. 2 of the paper. 2. "Age.ad": This is a factor with two levels, i.e., "young" and "old". In autumn, "young" means 1st calendar year bird and "old" means that the individual was older than "young". In spring, "young" means 2nd calendar year bird and "old" means that the individual was older than "young". 3. "Sex": This is a factor with two levels. "1" says that the bird was a male and "2" that it was a female. 4. "subsp": This is a factor with two levels. "leu" says that the bird belonged to the leucorhoa subspecies of the northern wheatear and "oen" says that the bird belonged to the oenanthe subspecies of the northern wheatear. 5. “season”: This variable indicates whether the bird was caught in spring or autumn. 6. “year”: This variable indicates the year in which the bird was caught. 7. “jd”: This is the day of year of capture with 1 Jan is 1.
data departure direction
There are six columns: 1. “year”: This variable indicates the year in which the bird was caught. 2. “season”: This variable indicates whether the bird was caught in spring or autumn. 3. "subsp": This is a factor with two levels. "leu" says that the bird belonged to the leucorhoa subspecies of the northern wheatear and "oen" says that the bird belonged to the oenanthe subspecies of the northern wheatear. 4. "Age.ad": This is a factor with two levels, i.e., "young" and "old". In autumn, "young" means 1st calendar year bird and "old" means that the individual was older than "young". In spring, "young" means 2nd calendar year bird and "old" means that the individual was older than "young". 5. "Sex": This is a factor with two levels. "1" says that the bird was a male and "2" that it was a female. 6. “direction”: This is the departure direction in degree.
data nocturnal departure time and sun elevation
There are ten columns: 1. “dep.min.sunset”: This variable details the individual nocturnal departure timing in minutes after sunset. 2. “sunele.dep.2”: This variable details the sun’s elevation at the moment of the nocturnal departure in degrees below the horizon. 3. “Sex”: "Sex": This is a factor with two levels. "1" says that the bird was a male and "2" that it was a female. 4. "Age.ad": This is a factor with two levels, i.e., "young" and "old". In autumn, "young" means 1st calendar year bird and "old" means that the individual was older than "young". In spring, "young" means 2nd calendar year bird and "old" means that the individual was older than "young". 5. "subsp": This is a factor with two levels. "leu" says that the bird belonged to the leucorhoa subspecies of the northern wheatear and "oen" says that the bird belonged to the oenanthe subspecies of the northern wheatear. 6. “season”: This variable indicates whether the bird was caught in spring or autumn. 7. “energystores_release”: This is a numeric variable and it provides the individual energy stores at release, estimated as detailed in eqn. 2 of the paper. 8. “year”: This variable indicates the year in which the bird was caught. 9. “jd_arr”: This is the day of year of capture with 1 Jan is 1. 10. “rho.act.dep”: We converted departure direction (circular variable) into a measure of deviance between observed (radio-tracked) departure direction and seasonally appropriate migration direction which enabled the inclusion of directional information in subsequent multivariate analyses, cf. Müller et al. (2018, Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.12821 ). This measure of deviance, the resultant vector length ρ (rho; rho of departure direction), was calculated using the “circ.summary” function of the R package “circular”, for details see corresponding paper. It ranges from 1 (no deviance between observed departure direction and seasonally appropriate migration direction) to 0 (deviance of 180°).
data rates of energy accumulation
There are seven columns: 1. “subsp”: This is a factor with two levels. "leu" says that the bird belonged to the leucorhoa subspecies of the northern wheatear and "oen" says that the bird belonged to the oenanthe subspecies of the northern wheatear. 2. "Age.ad": This is a factor with two levels, i.e., "young" and "old". In autumn, "young" means 1st calendar year bird and "old" means that the individual was older than "young". In spring, "young" means 2nd calendar year bird and "old" means that the individual was older than "young". 3. “season”: This variable indicates whether the bird was caught in spring or autumn. 4. “change.energystores_average”: This numeric variable details the change in the energy stores as an average value over the bird’s duration of stay in a cage. This value was calculated, as detailed in eqn. 3 of the paper. In the paper, this variable is called “rate of energy accumulation”. 5. “nights_in_cage”: This variable states how many nights a bird was in a cage. 6. “year”: This variable indicates the year in which the bird was caught. 7. “Sex”: This is a factor with two levels. "1" says that the bird was a male and "2" that it was a female.
