When to include vagrants

As I was compiling species for the book I am working, what I realised is that there is a multitude of vagrant species. Some (for example, European golden plover) were recorded several times while others only once, which I thought was a simple rule. However, I do not want to include species that were recorded once 30 years ago however I have seen few records of a species that may establish itself in the future, or simply overlooked due to how poorly-studied the particular habitat in the region is. Do you have any criteria on what I should follow, I do not want to include every species that was ever recorded but at the same time I do not want to overlook significant species that might be encountered if the region was studied better or the birdwatcher was just especially lucky

You might want to examine a number of different books and regional checklists on avian distribution. I suggest that records of “vagrants”, “accidentals”, or “rarities” ought to be included but they deserve special mention or separate treatment, e.g. with a special annotation or in an appendix. You’ll have to decide where your cut-off might be for something that is simply rare (but expected to occur again) and what might be considered true vagrants (not expected to recur). These criteria differ from place to place. For a well-covered area, those criteria can be rather specific and detailed (see example below). For poorly-covered areas, the concept of a “rarity” runs into the issue of observer coverage and becomes hard to define. But here’s an example from the well-covered area in/around Austin, Texas:
After the main body of our regional checklist, which covers all regularly occurring species, there are three extra lists:
“Accidental Species: Species recorded [in the previous 10 years] that have averaged less than two records per decade since 1978.”
“Historical Species: Species not record [in the previous 10 years]. Date of last known occurrence is listed.”
“Hypothetical Species: Probable, but insufficiently documented, species; or those documented within 10 miles of the checklist circle.”
You could modify your criteria to fit your particular geographic region and focus.

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Whatever criteria you use, briefly explain it somewhere, probably in the introduction.

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The regional bird lists or books I’ve seen typically address any and all species that were confirmed in the area – even if only once as a vagrant – and also those species that are now extinct/extirpated in that area, for the sake of completeness. If that approach complicates the information you want to focus on (those species of more regular occurrence), you could always include those rare vagrant/extirpated outliers in an appendix.

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It’s interesting how official bird lists treat different occurrences in the region of interest. For example, in my state (New Mexico, USA) our list still includes two native grouse species that have been long extirpated from the state. And it includes one-time-only occurrences of very rare vagrants that got here “naturally” (presumably no human help). But it doesn’t include Whooping Crane, which I and others have actually seen in the wild here in the past, because that species only occurred as the result of a failed effort in cross-fostering with Sandhill Crane parents in the 20th century (so it involved human intervention). As already said, whatever criteria you use, you have to explain it.

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I’ve decided to write it in the opening pages, which are
Vagrants with several records in a short time (white-tailed sea eagle)
Vagrants with few records, but have potential to return to the country or establish themselves in it due to a similar habitat (lappet faced vulture)
Vagrants with few records but due to poorly studied habitats may be encountered more frequently (Persian shearwater, black legged kittiwake)

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This seems like a good solution, although I might move them to the back instead of the front. I would say the breadth you should include depends on the type and target audience of book/field guide you are creating. If it is for casual birdwatchers, my experience is that most readers ignore distribution/abundance notes, so I would only include species that are regularly encountered. If you are trying to create a definitive birding resource (e.g. Sibley’s for US, Collins for Europe), I would include every species with probable occurrence records and include detailed notes on said records under the species account.

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