Nature Nut: Why one bird sticks around while another passes through

March 30, 2019

It was a good day at the backwater cabin feeders, with lots of birds showing up. Besides the usual resident chickadees, woodpeckers, house sparrows, and white-breasted nuthatches, some interesting migrants also appeared.

First, there was a tree sparrow, not unusually odd, but a species that will probably not be around in a few weeks. And then, a rarer pair of red-breasted nuthatches, just over half the size of their white-breasted cousins.

But the prize was a bird I could tell right away I’d not seen before, so I needed to look closely. From the rear, it looked like a fairly common house wren, with an upright tail and similar colors, but somewhat larger. But, when it turned so I could see its face, with a large white eye-band I’d not seen before on a wren, I guessed it might be a Carolina wren. A quick look in my bird book confirmed my guess.

I remembered high school classmate, and “extreme” bird feeder, Leslie Kottke, who put out a heated tray of live mealworms in the winter, and was able to keep one of these rarities around the whole season. But, I only enjoyed mine for a few days, and it was gone when I returned earlier this week.

But, watching these birds, some usual, and some unusual, made me wonder why we are in the normal range for some and not for others. In the case of the tree sparrow and Carolina wren, the sparrow eats predominantly seeds, while the Carolina wrens prefer fruits and insects, thus the reason Leslie attracted one during numerous winters.

While watching the wren at my feeder, I noticed it was very picky, and I later figured it was eating only the pieces of already shelled sunflower hearts, rather than trying to break open the seed hulls with a beak not designed to do so. So, obviously, the feeding tools a bird has survived with over eons in large part dictates its range.

Presence of predators can also spell which birds might take up residence in a particular habitat. This could also be linked to how interactive or communal some species are, with those having a “herd” mentality perhaps able to better survive predator presence. When reading about this, I couldn’t help but thinking of recently watching crows feeding on garbage in the Silver Lake parking lot below my home. There was always at least one crow serving as a ‘watcher’ while others fed, even when only two crows were left.

I assumed, in some cases, climate is a significant factor, although when I look at the body size and insulating feathers of the Carolina wrens, it looks similar to juncos, which we have an abundance of around here in the winter. But, another factor I thought about, and read more on, was how humans have altered the ranges of birds.

One study, more than 50 years ago, noted many birds that live around water are now often found in human-created reservoirs, wetlands, or ponds. And, some birds, with cardinals being a good example, have expanded their range northward, possibly in response to climate changes, but also probably due to human feeding of birds that allows them to live in cooler climes.

But, even without the impact of more bird feeders, climate change is impacting bird ranges, with many expanding northward. And, some are losing substantial parts of their historic ranges due to unfavorable climatic changes.

So, with the pace of climate change seeming to be picking up, who knows what birds our children, and grandchildren, may be seeing around here in years to come.