Age-dependent timing and routes demonstrate developmental plasticity in a long-distance migratory bird

1. Longitudinal tracking studies have revealed consistent differences in
the migration patterns of individuals from the same populations. The
sources or processes causing this individual variation are largely
unresolved. As a result, it is mostly unknown how much, how fast, and when
animals can adjust their migrations to changing environments. 2. We
studied the ontogeny of migration in a long-distance migratory shorebird,
the black-tailed godwit (Limosa limosa limosa), a species known to exhibit
marked individuality in the migratory routines of adults. By observing how
and when these individual differences arise, we aimed to elucidate whether
individual differences in migratory behaviour are inherited or emerge as a
result of developmental plasticity. 3. We simultaneously tracked juvenile
and adult godwits from the same breeding area on their south- and
northward migrations. To determine how and when individual differences
begin to arise, we related juvenile migration routes, timing, and
mortality rates to hatch date and year of birth. Then, we compared adult
and juvenile migration patterns to identify potential age-dependent
differences. 4. In juveniles, the timing of their first southward
departure was related to hatch date. However, their subsequent migration
routes, orientation, destination, migratory duration, and likelihood of
mortality were unrelated to the year or timing of migration, or their sex.
Juveniles left the Netherlands after all tracked adults. They then flew
non-stop to West Africa more often and incurred higher mortality rates
than adults. Some juveniles also took routes and visited stopover sites
far outside the well-documented adult migratory corridor. Such juveniles,
however, were not more likely to die. 5. We found that juveniles exhibited
different migratory patterns than adults, but no evidence that these
behaviours are under natural selection. We thus eliminate the possibility
that the individual differences observed among adult godwits are present
at birth or during their first migration. This adds to the mounting
evidence that animals possess the developmental plasticity to change their
migration later in life in response to environmental conditions as those
conditions are experienced.