Birders have a lot of cliches. One you hear over and over again, in heated, sweaty-palmed and pained discussions on advanced bird ID (and occasionally, with regards to provenance) is "caution is warranted". What birder popularized this phrase? We may never know. One thing that is not uncertain is that it is good advice, albeit phrased in such a melodramatic and painfully nerdy manner that I wince when I try to say it.
An occasion in which a birder should not throw caution to the wind is when we don't get to see all we want of a bird, even when we think we do. Take the Black Tern above, for example. This is a pretty good look, right? It's almost a crush. But something is off...why does it have dark gray wing linings? It's not a shadow. What is wrong with it?! Call the bird police!
Oh, wait. That looks better. What happened?
What happened is angle. What happened is lighting. The effects even minute changes in angle and lighting can have on a bird's appearance is monumental. The above photo was taken less than a second after the top photo, and the tern looks like it changed wings in between frames. Sure it's hard to see birds when its dark, and obviously looking at backlit birds sucks, but I don't think birders fully appreciate how subtle changes in lighting and angle can fully transform a bird.
Cool. A quick glance at this photo may make a birder guess Caspian Tern. It's a good guess. That's a dark bill.
And here it is a second earlier. Obviously a Royal Tern. That bill is no-doubt-orange, not the slightest bit red.
Philadelphia Vireos are notorious. Notorious for being yellow. Not yellow like a Yellow-green Vireo, or a Yellow-throated Vireo...or a Thick-billed Vireo...but you know what I'm talking about. If you don't, you are probably unskilled at Philadelphia Vireo identification. Not that there is anything wrong with that...or so I'm told.
But I digress. Here is a vireo. Obviously a Philadelphia or Warbling. Yeah, the lores dark, but this thing is bland. Could it just be a funky Warbling? The head looks numbingly gray, the throat an unfathomable shade of drabness. Where is the yellow? Show me the yellow!
Oh. There it is. It obviously is saturated with yellow. That's funny. Do you see where I'm going with this? Not only am I trying to address problems of birding in the field, I'm talking about how misleading photos can be.
Of course, I can't talk about issues with lighting and bird ID without talking about gulls. Here is an ostensible Western Gull taking flight. Quite the dark mantle, eh? Looks pretty normal.
Oh. That looks pretty pale now....disturbingly so. Clearly not a pure Western Gull.
Oh wait. It's super dark. It looks like a Western Gull. What is going on? What is the real shade of its mantle? This is all the same bird, and the photos were all taken within ten seconds of each other. Questionable gulls (and even some unquestionable gulls) require a lot of study, and a large dose of caution. When you are trying to ID a gull based on someone else's photos of one (an increasingly common practice), I suggest even more caution. Huge amounts of it. You should be getting drunk on caution, because you didn't get to see all the angles and lighting on the bird in the field. This is one of the many reasons gulls suck and I hate looking at photos of "mystery gulls", addicting as they may be. And yes my nerds, the mild mottling on the nape is indicative of some Glaucous-wing ancestry.
Let us look at the gull in front (although you have the freedom to ponder the other one). It looks almost textbook for a Thayer's Gull. But why is the mantle so dark? Surely this is a dark-mantled species of some sort, or at least a hybrid of one, because look how pale the other bird's back is in comparison.
Here is the same bird. Why, it doesn't have a dark back at all. Suddenly, it becomes a pristine Thayer's Gull. The only difference is that the bird is turned a few degrees to the right, compared to the previous photo.
Despite the misleading bird-to-bird comparison I offered two photos above (that was more of a don't-ID-gulls-by-just-one-photo-sometimes kind of warning), comparing birds directly is the way to go. It can be gulls, it can be vireos, it can be various strolling Siberian ground-dwellers.
California birders still remember what happened last year...Savannah Sparrows were being misidentified time and time again as Red-throated Pipits. It was an outrage, a debacle of the highest order. GBRS ranks were in freefall all over the state. There was even a backlash counter-outrage to combat it, defending the rights of stringers everywhere. It was a sad state of affairs, and we may only be a few months away from history repeating itself. At any rate, when the two species are side by side, is it really that confusing? You can't even claim that they are both brown. The bird on the right almost looks like a fucking Prairie Warbler. This is a story of what happens when overenthusiastic birders don't bother comparing their bird of interest with what else is around...and when one Savannah Sparrow is around, there are more (that is a rule). Caution is hella warranted.
Remember my friends...angle and lighting make identification frightening. Take this to heart, and you will go far.