For the past couple weeks I have found myself grumbling during my mornings at the kitchen window.

For some reason my yard has become popular with a flock of house sparrows (Passer domesticus) and I am not at all pleased. The house sparrow is an invasive species that was intentionally introduced to this continent by well-meaning, but ignorant people. They are a menace to many a native species and I am particularly concerned about my eastern bluebirds and tree swallows. All three species like to nest in the boxes that I have provided, but the sparrows are larger, stronger and more aggressive than the other birds. What to do about them is a topic for another day.

Because the house sparrows are so numerous they have come quite close to me while I sit at my kitchen window with my camera. Although I was initially loathe to “waste” a photo on these birds, I finally relented because the details in their feathers were just too nice to pass up on. So I took a selection of photos, repressed the urge to take a shower (because I felt that I needed to cleanse myself of guilt by association) and headed up to my computer where I could download the images. Once that was accomplished I sorted out the good ones, and set about the task of logging them into the computer.

It was the same day that I decided to finally try to put a dent in the backlog of photos that I had taken during the summer. Next up on my list was the wide variety of photos that I had taken in Sicily and among them were some photos of the house sparrow’s close cousin, the Spanish sparrow (Passer hispaniolensis). All at once I was transported back to the hot, arid conditions of a drought-afflicted Sicilian countryside and my amazement and excitement at spotting a bird that was completely new for my life list. Even then, however, I thought about the difficulty that taxonomists must face when attempting to identify all of the species in the world. Sifting through the closely related species must be a nightmare!

So I thought it might be interesting to conduct a side-by-side comparison of the two birds to look for physical differences that a birdwatcher might depend on for identification. The important thing to keep in mind is the fact that modern taxonomists rely on DNA comparisons to discriminate between species, but since the average person with a pair of binoculars cannot see the genetics of a bird that is being observed, I’ll just stick to the differences in phenotype.

First off you will notice that these two birds look very similar. Both are males, but the house sparrow was photographed in the winter, while the Spanish sparrow was photographed in the summer. The winter plumage of the male house sparrow features a diminished black “bib” that extends down the throat. In the summer, the throat feathers darken and the most senior males will have a most extensive swath of black to indicate their rank.

What I want you to look at, however, is the head of each bird. Note that the Spanish sparrow has a solid brown cap on its head, while the house sparrow has a pronounced patch of gray feathers on top of the head. That is the first giveaway. Second, take a look at the “cheeks” of each bird. On the house sparrow these feathers are, again, gray. In contrast, the cheeks of the Spanish sparrow are bright white.

This is a subtle difference, but still a good field mark for an experienced birder. And finally I want you to examine the breast feathers of each bird. Note that the house sparrow has a “clear” breast, while the Spanish sparrow has some black streaks (especially along the flanks).

The question that really burns in my brain is this: If the Spanish sparrow had been introduced to North America instead of the house sparrow, would the Spanish sparrow have become the same environmental nightmare? These birds are very similar in appearance, but is there some difference in their behavior that makes one “worse” than the other when introduced to new places? Might we have only had a small population of these birds in the U.S., rather than a continent-wide infestation?

Interesting to think about, but impossible to answer without a time machine and an unlimited supply of fuel for my Mr. Fusion.

Bill Danielson has been a professional writer and nature photographer for 25 years. He has worked for the National Park Service, the US Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more in formation visit his website at www.speakingofnature.com, or head over to Speaking of Nature on Facebook.

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