The topic of what makes a recognized species is extremely complicated, and involves as much human preference and culture as it does biological data. As other contributors have pointed out, there are many books on the subject, and they don’t generally agree with each other. In terms of birds, both North America and Europe have ornithological societies that have committees that review the bird list for each place, examine any evidence suggesting changes to the lists of recognized species, and distribute to their members and the public information on their updates. For example, when the Western Scrub-Jay was split into California Scrub-Jay and Woodhouse’s Scrub-Jay a few years back, a committee voted to accept the published evidence that these are two separate species. They were one species when I was a grad student in California, I moved away for some years, and came back, and now they are two. There is nothing eternal or fundamental about many of the species lines that are drawn, they are just useful labels.
Also welcome to the wonderful and confusing world of taxonomy!
Not sure if this applies to this exact example, but you might also look into convergent evolution. There, you may be surprised by the number of “nearly identical species” that aren’t that closely related.
Yeah, conservatism in morphology and convergence, two very different selective forces in evolution that make systematics and taxonomy even more complicated.
Appearances can be deceiving! You can have two almost identical critters that are not related all that closely (I think of ant mimics in the spiders) and you can have two very different critters that are in fact the exact same species (orbweavers, sexual dimorphism, lifestages, etc)
and here is a katydid that thinks it is an ant
https://www.inaturalist.org/observations/66039526
On the other hand, there are many species that are identical. The most obvious ones are domesticated animals, cats, dogs, cows, sheep. But there are a lot of other animals that were carried between contintinerts either accidentally (like the Brown Rat, that spread from Asia to every inhabited continent) or on purpose (like the North American Racoon was brought to Germany as for its fur and escaped to infest large areas in Germany).
I live in a relatively “wild” area. On the hillside behind my house, over half of the biomass of grasses and herbs are plants from other parts of the world that naturalized here. Some are landscaping plants or fruits that escaped cultivation (like figs. The seeds are spread by birds). Some are just nasty plants, like yellow star thistle. No one is sure how it got here from the Mediterranean areas of Europe, but it is here and takes over entire fields. Some of the exact same species of plants are likely in your area too.
… with some moths, it is getting even more weird! Examples:
(1) North American scientists decide a certain moth belongs to genus A, whereas European scientists put it in genus B. A genus is not as “well defined” as a species. What would now be necessary is, to investigate all of them on a worldwide basis. It is now quite random, which groups have been scientifically revised or not. Look at Agrochola / Sunira, for example.
(2) In the tropics, I get the impression that certain genera are like “waste bins”! Some species can clearly be allocated to a genus, some cannot. When scientists describe a new species, they somehow spontaneously decide what genus it should belong to. That sometimes leads to a genus which is full of different looking, and perhaps not even related species. See genus Eois (South America).
I guess, if you are a hobby iNatter, you’d simply have to go with the current taxonomy. Luckily, on iNat it is flexible and will be changed by the experts according to latest publications. This is so much better, as compared to all the books (field guides etc) or for my hand written own diary … where names will not be adjusted.
As a computer nerd, I would of course propose to abandon Linnaeus’ system, at least for faunistics / hobby field entomologists, and completely re-do animals and plants with a new set of identifiers, which would be AI based… (taxonomic relationship is not at all correlated to species identification, and it’s rubbish to combine it. Linnaeus made a mistake. He couldn’t foresee the “inflation” of species caused by (over)ambitious scientists)
And in some cases you can’t quite trust the species name either. At least in Lepidoptera (butterflies and moths) some countries don’t follow the gender agreement and some do. So for example Hellinsia osteodactylus is the same as Hellinsia osteodactyla. Fortunately that source of variation is easy to learn and ignore.
Linnaeus didn’t know about evolution and the underlying degrees of relatedness of species based on their evolutionary histories. The Linnaean taxonomic system was adapted to evolutionary systematics and, despite some inadequacies, has continued to work. At least no one has come up with a better system that biologists are willing to switch to. That doesn’t mean that identification keys can’t be devised to make IDing easier, without relying on taxonomic relatedness. If two species are not closely related but look a lot alike, there’s no reason an ID key shouldn’t place them near each other.
How would that not exclude even more people than Linnaean binomials already do?
If you want to assign blame for the overwhelming diversity of species, you can blame the process of evolution and speciation! 
… no, I am referring to the many species described as new, only to be synonymised a few years later. The list of synonyms is much longer than the list of valid species! And the worst: a handful similar species swapped around back and forth! For example what is now Pseudoips prasinana and Bena bicolorana (prasinana=fagana=sylvana and bicolorana=prasinana=quercana, some well known authors even swapped the 2 genus names!)… or the 2 European Spilosoma species S.lubricipeda and S.lutea (which I knew as S.menthastri and S.lubricipeda, but lubricipeda = menthastri and lutea = lubricipeda) …a perfect mess with historic faunistic records… if you haven’t come across this, you will 
I’m more of a gardener than a naturalist. For my normal purposes, it is more important to know how a plant looks and is shaped than it is to know what it is evolutionarily related to. Local nurseries sell Ceanothus that is a tree, shrub, or ground cover, all the same species. If I was looking for a ground cover, and they were didn’t have the low growing ceanothus, I would find a substitute in a completely different family, not a tree ceanothus.
I get frustrated occasionally because gardening books put in too much botany and not enough gardening.
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