Barbara M. Tomotani, Florian T. Muijres, Julia Koelman, Stefania Casagrande and Marcel E. Visser
Pied flycatchers are migratory birds that breed in Europe and Asia and spend the winter in sub-Saharan Africa. They are extremely time-constrained in their activities during their summer stay in Europe, because they have just a few months (around four in the Netherlands) for a large suite of activities. They need to find a breeding site, find a partner, lay eggs, take care of the chicks, replace their whole plumage – a process called moult – and then migrate back to Africa. This time constraint may force them to perform two activities at the same time, such as breeding and replacing the feathers (moulting). This so-called moult-breeding overlap is regarded as costly for the birds because it results in feather gaps on their wings during a phase when they should fly a lot, that is, while they are gathering extra food to feed their chicks.
We studied the consequences of this strategy by simulating moult: we produced gaps on the wings of male pied flycatchers that were rearing their young, by removing two feathers just as it would happen naturally in early moult stages. Then, we measured parental care and monitored body condition of males, females and chicks. Finally, we tested how these simulated moult gaps affected escape flight performance, by filming upward escape flight manoeuvres of these males using high-speed cameras.
We found out that males with feather gaps had, as expected, a reduced flight performance compared to those with intact wings. However, there was no change in their overall body condition, nor in the body condition of their offspring. Males with feather gaps, however, visited their nests less often than males without gaps. On the other hand, the females that were paired with these gap-bearing males visited their nest more often than the control birds. Thus, the females fully compensate for the low visiting rate of their males. Therefore, males may be able to gain time by overlapping breeding and moult, but female partners end up paying the costs.
Image caption: Flight performance set-up, inserts show two birds in flight, control (above) and with wing gaps (below). Photo credits: Killu Timm (flight set-up), images of birds in flight extracted from