Drake Ruddy Ducks in breeding plumage are pretty conspicuous, most birders would agree, but one spent a fair portion of June at the Cape May Point State Park this year and I never saw it. I knew exactly which pond it preferred, and checked about four times, but the bird just wasn’t visible.
The reason was obvious—just because a bird is present doesn’t mean you can see it! The ponds at the state park, like ponds everywhere, are rimmed with cattails and reeds, and have little coves and peninsulas where a bird can hide. Plus Ruddy Ducks dive, so it would be completely possible (with my luck) for it to be down when I scanned the pond. If you wanted to see every bird on a particular pond, you would have to walk the entire perimeter (impossible without lots of bushwhacking or wading), scanning the pond from multiple angles.
Though I didn’t see this particular bird, I do take the angle factor into account whenever I’m birding. Just because you’ve scanned a spot from “here” doesn’t mean you won’t find something different from “over there”. I suppose a geometry professor could team up with a statistician to figure out exactly the right number of spots to scan from, and where they would be, to maximize the chances of finding every bird in a pond, or a field. Until they do, just remember to check such areas from several places or angles.
I also try to remember to make my second and third scans, made from different vantages, just as thorough as my first. For me this takes a conscious effort—I tend to figure that since I scanned the area once, I’m not too apt to find anything different on succeeding tries, and this then becomes a self-fulfilling prophesy unless I remember to keep scanning carefully.