[birding] Black swift puzzle

[Joel Geier]

Hi all,

Thanks to Dave Irons for a good account of the Black Swift situation in
spring migration, and suggestions on how & when to look. 

Now here's an algebra puzzle:

Alan McGie's phenology for Corvallis shows three local detections in 36+
years of spring migration records, with a mean of April 9 and an
earliest date of March 30. 

Without knowing the other two dates that contribute to the mean, one can
show (with just a bit of algebra), that at least one of the other two of
the Corvallis-area Black Swift detections had to be before April 15th,
and all three had to be by the end of April. 

It's curious that this should be so out of synch with the statewide
picture as described by Dave. A couple of ideas:

1) Were some or all of those March/April detections really Purple
Martins? Or

2) Is there a northern-nesting population of Black Swifts that might
pass through the Willamette Valley earlier than the coastal migrants or
the ones that nest at Salt Creek Falls -- but seldom detected?

Happy birding,
Joel

--
Joel Geier
Camp Adair area north of Corvallis