Iceland represents an essential part of the Brent's annual cycle, and not just a mere stop off on the way from Ireland to the Arctic. The vast majority of individuals spend a month (sometimes more) at various staging sites on the West Coast, building up energy reserves that must not only fuel the next stage of their epic migration, but also a large part of their breeding efforts as well - there is not much for them to eat in the Arctic at the start of June and they must start breeding almost immediately to be able to escape before the winter arrives again!
That means that improving their body condition as much as possible during their spring staging period is essential to being a successful breeder in the summer. As such, it is possible to watch (some) birds gain fat stores at an incredible rate as the spring progresses. Below is a graph that shows changes in the mean API (the method we use to give each bird a body condition score) for individuals recorded on each day of the study period as May goes by for two different "types" of individual. The green line is adults in family parties and the red line is for adults that are in pairs without any juveniles.
The difference between these two "types" of individuals is interesting. They seem to improve their body condition at a similar rate but birds in family parties arrive and leave Iceland in worse condition (the mean for the first day is best considered a blip as it is based on a very small number of individuals). This seems particularly surprising given that family groups are dominant in flocks and so should be able to compete most effectively for resources.
Fortunately, thanks to previous work - mainly by Rich - an explanation is on hand! The poorer condition of adults in family parties is best explained by the fact they have had to "care" for their useless young over the course of the winter. Juveniles are less experienced and struggle to exploit more nutritious intertidal resources as they become scarce during the winter. Families therefore spend more time feeding on terrestrial grassland and maintain worse body condition over the winter months.
The inability of adults to "catch up" in their energy stores means that they arrive at their breeding sites in worse condition and are less likely to be successful breeders. This highlights the huge importance of geese making the most of their springtime in Iceland!!
[Main reference is Inger, R. et al. 2010 Carry-Over Effects Reveal Reproductive Costs in a Long Distance Migrant. Journal of Animal Ecology, 79, 974-982 for anyone still reading...]