James West wrote:

> .... a single good (i.e.
> unequivocally accepted) record of a species far from its home
> territory establishes the _possibility_ of its occurrence in the location
> in question, which surely has two important implications:
>
> - It will lend credence to subsequent one-person or less clearly
> identified sightings, which might otherwise be dismissed, giving
> a false picture of the vagrancy pattern of the bird in question;
>
> - Let's not kid ourselves that every vagrant that makes it to a given
> region is seen. Each record is only a statistical event, posited
> the number of actual vagrants in the area and the probability of
> their being observed. There are obviously quite a few Black-throated
> Blue Warblers in Washington for every one that actually shows up in
> the field of vision of the minute proportion of the population that
> knows a warbler from a canary. This makes single records inherently
> significant.
>


Some points:

Vagrancy is by definition an abherent event. The circumstances by which a
bird appears far from it's native range are many and varied. Sometimes it
might give clues to their migration strategy - east coast birds geting caught
up in fast moving storms and being transported across the Atlantic to Europe,
but for the most part very little information other can be obtained. The bird
might be ship-assisted, mis-identifed, released from captivity, etc.
Collecting it is an option but it offers less information per sample than a
bird collected in its native range.

The key is to establish, as is possibly the case with black-throated blue
warblers, that "sampling error" is the reason a bird is scarce/rare. In my
experience birds tend to get found when people are aware of the possibilities
of their existence. I would not be surprised if black-throated blue's got more
common over the next few years.

As far a credence to subsequent records is concerned. I would guard against
this, every unusual occurrence should really be judged in isolation. It is
relatively common (certainly in the UK) that birders will claim to have seen a
bird in order to make a name for themselves - better to choose a bird that
has only been seen a few times rather than a first occurrence ;-).


Anyway the upshot of all this is that rare birds sightings are so full of
unknowns to be pretty much useless, except that seeing one is really cool,
finding one is even better and it gives lots of people a really good trip.


Stuart MacKay