Every once in a while, birding throws a curveball.

To keep the baseball analogy going, curveballs aren’t necessarily a bad thing. Once some batters learn how to recognize a curveball, they prefer them to fastballs. It’s just a matter of seeing enough curveballs and getting enough experience with them. You could say it’s a learning curve.

In the birding world, curveballs come in all shapes, colors and sizes. That’s why they are called curveballs. I would define a curveball in this regard as any bird that looks different from what a field guide says it looks like.

Leucism is a common curveball the birding world likes to throw. Leucism is similar to albinism in that the bird appears white, mostly white, or patchy white. Robins, juncos and red-tailed hawks are birds often seen with leucism. That is not to say these birds have a high percentage of individuals with leucism, but rather if a bird with leucism is spotted in New England, it’s often one of those birds.

Another example showed up at the feeder the other day. Having seen this particular curveball several times now, I knew what it was immediately. My son Andrew, who spotted the bird first, was fooled by the curveball.

“What is that bird, dad?” he asked.

“You know that bird. It’s just a slightly different color,” I answered in very dad-like fashion. I’m not just going to give him the answer.

“Never mind,” said Andrew, apparently not in the mood for one of my guessing games.

“It’s a house finch. It’s just yellow instead of red because of its diet,” I said quickly, not wanting him to walk away without knowing the answer. Andrew won that round.

This house finch, which was yellow where house finches are usually red or pink, had sat himself in the dish of a platform feeder and contentedly ate sunflower seeds. Other house finches perched nearby in trees and shrubs. If diet is responsible for the color difference, I wondered why only this bird was yellow and not the others. Assuming they travel together, wouldn’t they all be eating the same seeds and berries? Color variations in house finches often happen after a molt, so perhaps this bird molted (changed its feathers) at a different time than the rest.

Male house finches get their color from pigments, in this case carotenoids, in the food they eat. Carotenoids, not coincidentally, are usually red, orange or yellow, the same color variations that are common in house finches. It had been a while since I had seen a house finch that wasn’t red or pink. I think it goes back to my old house, which I sold more than three years ago. So, it is not common for house finches to be orange or yellow, but it happens.

Molt is responsible for another curveball often thrown at us. Every so often, a bald cardinal or blue jay will show up at the feeder. These birds look sick or downright goofy, but they are fine, and their head feathers will grow back in a matter of days or weeks. I’ve never seen a bald cardinal in a field guide, but they are out there in the real world.

Another type of curveball is when a bird appears in a geographic region unexpectedly. Range maps in field guides will tell you painted buntings, fork-tailed flycatchers and brown boobies live far, far away from New England, but I’ve seen each of those birds in our corner of the country. Birds sometimes get lost or blown off course and end up in strange places.

Curveballs are thrown to keep the batter off balance. It’s the same with birdwatching. They add to the challenge. Who wants a hobby where you see the same pitch over and over?