Alert is one of those words that, like anyway, anywhere, everywhere and so on, used to be composed of two separate words that have long since been joined together in holy linguistic matrimony. Don’t you just love it when that happens? It’s so rhetorically romantic. (Willy-nilly fusing of words is more common in North America than in, say, the United Kingdom, but so be it.)
Lerts (singular: lert) were a now extinct species of bird that hasn’t been seen anywhere on Earth for at least 200 years. While they were alive, despite being rather skittish, they loved to show off to humans. Once dead, they totally lost their interest in showing off. So, if any were still alive anywhere on Earth, they probably would have been spotted by someone. It’s doubtful that lerts currently exist or have ever existed on any other planets, so the description of them as extinct is probably accurate.
Lerts were known for two things. For one, they were exceptionally attuned to their surroundings and any changes, particularly potentially dangerous or opportune changes, to those surroundings, including the entering or exiting of predators or prey. The second well-know and prominent attribute of lerts was that whenever they detected a threat to them and other lerts they immediately issued an audible warning that was so loud that it is said that on a clear day it could be heard as far as a mile away.
The sound of the warning was almost identical to that of a screaming banshee. It’s a shame that banshees were mythical creatures because it leaves us with no way to know what the lerts’ warnings sounded like.
Because of these traits of lerts, people alive while lerts still soared in the skies used to refer to people who were paying close attention to what was going on around them as being “a lert.” That was, of course, only a metaphor. People sensitive to their surroundings didn’t magically turn into birds. Likewise, when someone issued a warning about a potential danger, that was often rhetorically referred to as issuing “a lert.” Those flights of rhetoric remained even after the last of the lerts perished.
Over time, people merged the words a and lert into the word alert for the sake of brevity. Today, the old form has entirely fallen out of usage. Ironically, in a few cases, this has made speech a slightly—although only very slightly—longer than if they were still two separate words. For example, in olden days you could have talked about “issuing a lert,” but now the correct form is “issuing an alert.”
It’s rather sad that few people remember, or even ever knew, that the word alert is a reference to the clever lert species of the days of yore.