My understanding is that once it's on the ABA list, they leave it to the observer to evaluate whether the individual is part of an established population or not and from that basis, whether or not to count it within ABA boundaries. As an example, introduced Red-whiskered Bulbul populations in NE Los Angeles / Pasadena area have significantly increased over the last 10 years, and I feel very comfortable counting it by this ABA standard. However, it was initially added to the list based on the bird's status in Florida, and California's state listing authority hasn't yet added it to their list as a state countable introduced species. So I'm left counting one more for ABA vs. CA on my official "countable" lists.I think its more that the ABA doesn't entangle themselves into debates on what and what isn't countable once that species is on the checklist. ABA's stance isn't "Now any Tricolored Munia seen in its boundaries is countable", it's "The Tricolored Munia is on the checklist...count it on your own discretion, as we do not evaluate all individual records of rare birds"
This is why I track 3 different numbers for my species counts (see signature): Overall count regardless of an area's "rules", "accepted" species within that area's rules, and non-introduced - this third number feels very clean and unencumbered by out-of-place exotics.
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