Numerous 'decisions' face migratory birds. How much moult should they do on the fuelling grounds? How heavy should they be when they migrate? Do they fly in a few big flights or a series of small ones? If they are behind schedule, should they leave anyway, or wait until they are 'ready'? These are also the questions that fascinate biologists, though we are really interested in knowing what the consequences are of leaving at different times or in different condition. Ideally, we want to work in a system that has variation between individuals, but consistency within individuals.
Take Bar-tailed Godwits. Males moult into breeding plumage, but there is great variation between individuals. Figure 1 shows three males, all on or about to embark on migration. Why is there such variation? Some of this may be age-related - the pale left-hand bird was probably three years old, on its first northward migration. But some of the variation is consistent within individuals.
Figure 1. Examples of male godwits with very different breeding plumage condition while on migration. Left - a pale bird on migration in Japan (Photo Kouji Takenaka). Middle - a moderately red bird on stopover in South Korea (Photo Kim Hyun-Tae). Right - a stunning bird before departure from New Zealand (Photo Phil Battley).
Figure 2 shows the middle bird from Figure 1 again. In 2004 this bird was seen in South Korea on migration. It probably left the Firth of Thames on 26 March, and was seen and photographed by Kim Hyun-Tae at Seosan on 15 April (left-hand image). It was also photographed on 30 April (middle image). In 2004 this bird also probably left on 26 March, and it was, quite amazingly, seen and photographed in South Korea at the same site, by the same observer, in the same degree of breeding plumage, on almost the same day (13 April) (right-hand image). So godwits have what we are after, variation between individuals and some consistency within individuals.
Figure 2. The male godwit 2YWWB. Left - South Korea, 15 April 2004 (Photo Kim Hyun-Tae). Middle - South Korea, same site, 30 April 2004 (Photo Ham In-ja). Right - South Korea, same site, 13 April 2005 (Photo Kim Hyun-Tae).
At this point we have little information on what this variation means for different individuals. There is some evidence from godwits passing through The Netherlands that plumage reflects an individual's quality (parasite resistance, fuelling ability), but it could be that some of the variation in those studies incorporates age-related effects. An intriguing observation for those studying godwits in the EAAF is that biologists on the breeding grounds in Southwest Alaska never see birds as red as the darkest bird in Figure 1. Does this mean that there is variation between populations in the moult and plumage they breed in? Or does selection based on plumage happen only before birds reach the breeding grounds?
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