Data from: Effects of immune status on stopover departure decisions are subordinate to those of condition, cloud cover and tailwind in autumn-migrating common blackbirds (Turdus merula)

Migratory birds encounter a large variety of parasites and pathogens en
route and invest in immune defences to limit the risk and fitness costs of
infection. Since both migration and immune defences carry costs,
individuals on tight budgets may face trade-offs between migratory
progress and immune status. Many species alternate legs of strenuous
migratory flight with stopovers during which birds refuel, rest, and
recover physiologically. Despite this, most time and energy consumed
during migration are actually spent on stopovers. As a result, identifying
what determines stopover duration is key in understanding how migratory
birds balance investments in immune defences and migration. Yet, it is
unknown under what conditions an individual’s immune status may affect
migratory progress through the duration of stopovers. We explored whether
immune status at arrival affects stopover duration by radio-tagging and
blood-sampling common blackbirds (Turdus merula) during autumn stopovers
on the Dutch island of Vlieland. To measure immune status, we quantified
levels of bacterial killing ability, natural antibodies, complement, and
haptoglobin, as well as heterophil-lymphocyte ratios. We show that
stopover departures peaked during periods with low cloud cover and strong
tailwinds. While lean birds prolonged stopovers, we only found a weak
tendency of prolongation in birds with elevated haptoglobin levels. We
conclude that effects of immune status on minimum stopover durations are
subordinate to those of condition, cloud cover, and tailwinds in
autumn-migrating common blackbirds. Hence, future studies on the link
between immune defences and stopover durations should take weather
conditions into account.