Cliff swallows, like the more familiar tree and barn swallows, are small experts at catching and eating insects on the wing.

At 5¼ inches long and just three-quarter ounce, cliff swallows have the speed and maneuverability to dine well wherever insects swarm.

There are few cliffs in flat and sandy South Jersey, so we don’t have the namesake nesting habitat for this bird. The nearest big colony of cliff swallows I’ve run across was under a bridge over the Delaware River near Trenton.

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But cliff swallows are regularly seen in the area, typically starting in March each year in the Cape May area, where the intensive birding quickly finds them.

Cliff swallows could be used as a harbinger of spring, and the famous swallows that used to return to San Juan Capistrano in southern California were another kind of cliff swallow. More appropriate for our area, though, is the laughing gull, which many have accepted as our signature bird of spring.

Migrating cliff swallows, more common in the fall, are easily identified if you can get your binoculars on them as they speedily scoop up bugs.

A robust, short-tailed swallow, cliff swallows have a bright white forehead. Around here, only the rarer cave swallow looks similar, and its forehead is chestnut when mature.

In nature, cliff swallows build rounded nests of mud on rock walls, often beneath an overhang for protection.

By nesting together, they help each other find and follow the large swarms of insects that make feeding youngsters easier. And if a female produces too many eggs for her nest, she might lay one in another nest nearby or even pick one up in her bill and move it there, the Cornell Lab of Ornithology says.

Lots of places good for insects — ponds, marshes and fields — don’t have suitable cliff nesting ground nearby. Fortunately for cliff swallows, people have provided a lot of artificial “cliff” surfaces by building road bridges.

Unfortunately, road bridges come with speeding vehicles.

Cars kill a lot of wildlife in America, hundreds of millions of animals a year, of which about 15 percent are birds, studies suggest.

Many times we see animals at risk or already hit, and we wish that they would learn to stay off the road – but learning is what we do, not the animal way.

Animals pursue their lives and depend on natural selection to slowly equip them with whatever attributes and instincts they need to survive in a changing world.

That’s exactly what seems to be happening with cliff swallows.

Charles and Mary Brown, of the Universities of Tulsa and Nebraska-Lincoln, have studied cliff swallows for 30 years and found that the number of nests in their area have more than doubled in that time.

Part of the reason, the Browns think, is that the cliff swallows are evolving slightly shorter wings — 2 millimeters shorter out of about 13 inches wide.

The Browns found that birds killed by cars, often after landing on the road, have longer wings than the general population. Shorter wings enable birds to take off vertically more quickly and get out of the way of approaching vehicles.

Assuming the research is solid, cliff swallows are a rare example of evolution in action — the kind usually seen among bacteria and viruses becoming resistant to our drugs.

Nice to see a beneficial example of it.

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