On Saturday I saw and photographed a Northern Harrier on Porter Road End. I've seen lots of Harriers, including many recently, and the bird sat still long enough for me to take several photos. And I saw a white rump as the bird flew off. So I had no doubts at all about the ID.
The trouble is, as one of my blog readers pointed out, the bird was not a Harrier. It was a White-tailed Hawk, a quite different and much more exciting bird.
How on earth did I manage to mis-indentify the bird in the field and even when I later looked at the photos?
Well, I suppose the two birds are both vaguely similar in size and appearance. However, I think the real reason for my mistake is that I was expecting to see a Harrier. There's usually a Harrier along Porter Road End and I had seen one there only a couple of days earlier. In fact, just seconds before the sighting, I was complaining to my wife about the absence of Harriers along the road. So when the hawk appeared, I didn't really examine it because I had already pre-identified it! All I worried about was trying to get good photos before it moved.
Of course, I should still have picked up on my mistake when I later sorted through and uploaded the photos. But I was so focused on cropping the photos and choosing between them that I never really looked at the bird itself.
I think this episode is interesting because it shows just how much what we see, in nature or in everyday life, is determined by what we expect to see. In this case, my determination to see a Harrier was such that I even persuaded myself that I had seen a white rump on a bird that actually has a white tail!