Hi Tweets,

On at least five occasions, and about this time of year, I've seen massed
flights of---no kidding---Great Blue Herons in full soar in exactly the way
described by this observer. The largest grouping I ever saw was a large
kettle of about 70 herons over Jericho Park in Vancouver BC on a morning in
early April ; I estimated the lowest bird in the kettle was at least 300
feet up up, the highest birds close to a thousand, general direction
northward. And these birds were not in direct flight: they were soaring in
large circles in a generally cylindrical kettle, obviously working a thermal
or relief-caused updraft. Conditions in each sighting were consistent: late
March, April or early May, light warm southerly breeze with a high
stratus-sheet ceiling, indicating stable airflow with little turbulence.

Questions arise. Are they local birds in mass flight from local colonies (in
this case, UBC) to forage sites? Or, given the time of year these kettles
tend to occur, are they northbound migrants riding north on the warm-air
incursion from the south? And, why, and given that the question virtually
answers itself, given the vast efficiency of the blue heron wing in both
direct and soaring flight, don't they do this more? especially when soaring
is a proven efficient method of migration for birds whose wing-design
doesn't preclude it?

What other candidate species are there? American White Pelicans like to
soar, en masse, and sometimes there's been kettles of soaring Double-crested
Cormorants, both species reaching to a very high heaven. Sandhill Cranes
(and presumably any other type of crane) have been mentioned and eliminated.

Dunno, mate. Herons is my guess.

Michael Price
Vancouver BC Canada
loblollyboy at gmail.com

Every answer deepens the mystery.
- E.O. Wilson
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