I studied under Tim Birkhead (the lead author) at Sheffield Uni on his first lecturing post and I think he has a good point. All this is theoretical but I agree with an early statement in it, that when the egg is full, that is has albumen and yolk or a developing embryo inside, the point of balance is different from an empty egg. Therefore when it rolls it does not roll in a small circle to avoid falling off the nesting ledge. As an assistant ringer in the past I have attended nests and when eggs are full they roll very differently than empty shells. The small circular rolling was surmised from spinning empty Guillemot eggs, not full ones! Great link Nick, thanks