DOI: 10.5061/dryad.7n53r7f
title: Data from: Potential age differences in the migratory behaviour of a nocturnal songbird migrant during autumn and spring

description: 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”.
description: data capture dates and
 energy storesThere 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
 directionThere 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 elevationThere 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
 accumulationThere 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.