Today I had the opportunity to see a 2007 and a 2008 bird side by side. I had seen the 2007 bird earlier in the day but had not recognised it as one of the older birds. I was trying with some difficulty to see which of the 2008 radio frequencies it matched, the problem was of course it was a 2007 bird. As the the bird was face on and I couldn't see it's wing tags I drove to a different postion and tried to creep up on it. Just as I reached my intended vantage point the bird flew off, I followed it in my bins and watched it land in the distance, on a fence post next to another kite.
I made my way back to the van and got my scope out to see if I could read the tags. I still thought both birds were 2008 birds. Seperating adult and juvenile kites isn't too difficult according to the books, but in fairness these are our first adult kites. Side by side I could see that there were differences, but as both birds were again face on the differences were kind of subtle. I couldn't figure out at that point whether what I was seeing was just individial variation of the plumage or age related difference. Both birds were left wing to me, so they were both blue tags, there is a colour bar at the bottom to indicate year, but a small purple bar and a small black bar look very similar at a distance.
The deciding factor that one bird was a 2007 bird was the eye colour. Even at 300+ meters, through the scope the paler eye colour of the older bird was apparent, its head also looked somewhat bigger and more contoured. The bird from this years release had a much blacker and to appearance "beadier" eye! As I walked closer and closer to get better views, the birds eventually flew off and then the difference in plumage is readily apparent. The 2007 bird, now sporting its second tail, has a beautiful "rooster red" tail, while the first year birds have a more subtle brown tail.
I guess the long and the short of it is if you want to age kites look at their tails, not their eyes.