Red breasts as honest signals of migratory quality in a long-distance migrant, the Bar-tailed Godwit

Bar-tailed Godwits (Limosa lapponica) partially molt contour feathers before flying to their subarctic breeding grounds. This molt starts in February on the West African wintering grounds, is suspended in late April before the flight to the staging sites in western Europe, and is continued by more than half of the birds during their stage in May in the Dutch Wadden Sea. During molt the appearance of the plumage changes from the dull grey of a winter plumage to an intense rusty-red in males and a beige-rufous in females with a complete breeding plumage. The total mass of the plumage remains the same. Molt is scheduled earlier and is more intense in males than in females. The duration of contour feather growth on the Dutch staging area is estimated to be 12 days, leading to an estimated daily dry feather production of 0.09-0.13 g/day. On the Dutch staging area molting individuals of either sex had a more complete breeding plumage than non-molting birds and were always heavier. Very few birds started molt in the last 12 days before northward departure indicating that time provides a critical constraint on the occurrence of molt. Although the extra costs of molt are estimated at only 7% of the costs of the simultaneous energy storage, the site-related difference in the mass of molting individuals suggests that the decision to molt depends also on nutritional factors. We argue that in view of the tight time/energy schedules faced by these long-distance migrant godwits, only individuals which have sufficient energy/nutrient reserves for the time of the year due to better feeding or flying performances, can afford to upgrade their plumage on the staging areas. The extent and quality of the breeding plumage could thus provide males as well as females with an indicator of the migratory, and possible concomitant, qualities of potential mates.