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The challenges of first migration: juvenile vs adult ( Rotics et.al. 2016)
The challenges of first migration: juvenile vs adult ( Rotics et.al. 2016)
Summary
migratory juveniles exhibit lower survival
age-related difference in fall migration juvenile vs adult: ACC GPS
movement
behavior
energy
we found juveniles
flap 23% more
expend 14% more energy during flight
not increased refueling or resting to compensate flight cost
mostly behind adult in same flock
greater flight efficiency improvement
we demonstrate and suggest juveniles
inferior flight skills
urge to keep up with mixed flocks (together with Chernetsov, Berthold & Querner 2004)
:red_flag:flight costs are "
important proximate cause"
of juveniles mortality
Introduction
challenges in migration particular for juvenile
Previous studies juveniles
lower navigation capacity
higher sensitivity of adverse weather
shorter wings and higher wing-loading
:red_flag:
competitive foraging inferiority
spend more time at stopover
some species less efficient routs
multifaceted and unprecedented
behavior and ODBA
movement metrics, stopovers, behaviour during flight and ground
adult and juvenile leading position in flock
flight efficient flight (flapping/ gliding ratio and consequently higher fly energy)
forage less effectively and spend more time at stopover site
Materials and methods
tagging
sampling
estimating behavioural and ODBA: supervised machine-learning
atmospheric ENV-data
data set
unless specified, analyses were on flight day's data
departure date: first date southward
arrive date for winter/ end of migration: cross 17.5°N
stopover: daily displacement <30km between 50 °N to 17.4°N
Behavioral and movement parameters
flight records GPS >5m/s, ground records <2m/s
ACC: flapping ratio, relative foraing time , pecking ratio, relative preening time
field:, pecking success rate
flight parameters: flight speed, migration duration, flight height,geoid height, daily flight start or end(4 consecutive flight records with minimum displacement of 700m each), daily flight duration
parents offspring migration onset
Flight ODBA during migration (the improvement)
adult and juvenile in same flock
:warning:Juveniles mortality (but only compare the first third of fall migration)
unsuccessful (died during migration
n =4
and first month of winter Sep. to Oct
n=3
.)
Successfull
Statistical
random factors: id, year, family id,
movement and flight parameter: age, thermal uplift, tail and cross wind
flight ODBA in migration:age^travel distance+thermal+tail cross wind+random factors+ number of flight day
Results
energy and hehaviour
:red_flag:Juvenile had significantly higher flight ODBA and flapping ratio
ODBA and foraging on the ground not differ
juvenile had higher pecking ratio and lower success
juvenile less preening
juvenile movement
daily displacement distance shorter
average flight speed lower
number of flight days higher
no difference: daily flight duration/start time, daily straightness index, mead daily flight altitute
:red_flag:start migration earlier than parents
:red_flag:stopover date is
infrequent
and not different: 71% non-stop
atmospheric effects
regardless age, thermal expedited migration progress
Flight ODBA during migration
juvenile flight ODBA decreased faster
accumulated travel distance ↑ thermal ↑
:red_flag:strong interaction between accumulated migration distance and age on flight ODBA
joint migration(at same flock) juvenile
flew behind (16 cases, 4 with parent)
higher flight ODBA
average 5 days flew together, most case were left behind
:warning:juvenile mortality
first verify flight ODBA was a consistent trait (significant repeatablity)
then compare flight ODBA in succussful and unsuccessful
:red_flag:no different: thermals they encountered, migration onset time,
pre-migration flight experience
,
total flight hour from fledgling time to migration onset
Discussion
most prominent difference between adult and juvenile:
energy expenditure during flight
. but not spend more time at stopover
juvenile flight less efficiently, only 12 hours experience before departure, preening also less,
age-related flight skills can explain observed differences of migration behavior
food intake also less
social aspects: similar daily flight parameters support it (juvenile poor navigation capacity)
:star:
first study to elucidate trade-off for juveniles to follow experienced individuals and physiological drawbacks
:red_flag:
for white stork which need adult to guide, better leave with early-departing flocks (because they easy to left behind, and overall depart earlier than adult)
energy for flight decrease for both adult and juvenile(faster ,and this faster pattern can't explained by wind or thermal thus is learn)
:star:
Flight efficiency improvement" was crucial, linked to mortality
higher travel costs may one of the major factor of low juvenile survival
:red_flag:
"Bird mortality during migration is a large-scale phenomenon with implications on population regulation and presumably on evolutionary processes. "
Conclusions
higher flight energy expenditure
:red_flag:conflict between poor flight skill and urge to catch up
consequently lower survival