The Importance of Spring Staging: Why do female geese get fat faster?

The numbers on the y axis are body condition scores (our scale goes from 1-7). The start period was mean API measures for all individuals resighted between 01/05/2013 and 03/05/2013. The end  period was mean API measures for all individuals resighted between 26/05/2013 and 28/05/2013.

We talked quite a lot about measuring body condition towards the end of the time we were in Iceland. One thing that is particularly apparent towards the end of the spring is the big difference in API within each pair. As this graph shows this is because females are leaving in Iceland in much better condition than males. We've known this is the case for a while, but its nice to be able to show it with my data! Little results like this help to show that while you may have gone crazy whilst collecting all the data, you were doing a reasonable job still......

Clearly, females have to make more of an investment in reproduction once the birds arrive in the Arctic so have to leave their spring staging areas with more fat reserves. There is not much for a goose to eat when they first arrive in the far north, so all the energy reserves they can accumulate in Iceland are of vital importance!

Perhaps of more interest is trying to work out why the males do not end up equally fat...... It seems likely that there might be a cost to being fatter than you need to be, particularly if you take into account just how far the birds have to travel to reach their breeding areas. However, it is also very noticeable during our time in Iceland that there are differences between members of a pair in their levels of aggression and vigilance. It could well be that males are unable to accumulate the same energy reserves as they are spend more time engaging in these social behaviours to enable the female of the pair to forage more efficiently. This seems to be backed up by the fact that the Whitefronts that colleagues study just up the coast are very similar in this regard.

It is also very apparent that the birds left Iceland in much better condition this year than last spring - hopefully that bodes well for a successful breeding season!