>
>In the Vancouver, BC area, since the mid-1960s, peak Snowy Owl
>numbers have occurred in the following winters: 1966-67,
>1970-71, 1973-74, 1977-78, 1980-81, 1984-85, 1992-93, 1996-97,
>and 2004-05. In some cases, there was an "echo" flight as large
>as, or nearly as large as, this peak flight the following year (e.g.
>1971-72, 1993-94, 2005-06). The expected invasions of 1988 and 2000
>did not materialize; there was a small flight in 2000, but no birds remained
>over the winter near Vancouver, although there were a few in the Puget
>Sound area.
>
>This looks suspiciously like a 3-4 year cycle to me.

With the salient exception of 1977, the numbers at Ladner/Vancouver
follow the trend that flights occurred in years when the Oceanic Nino
Index was dropping. 1977 was in the middle of an almost two year warm
period. I had overlooked 1977 in Mike's data.

>
>There is a tendency for every 3rd invasion or so to be much larger than the
>others. The biggest flights in the last 40 years were those of
>1973, 1984, and 1996. (No, 2005 does not compare with the
>numbers seen in those years.) Mike Patterson is correct when he says
>that he sees evidence of a 9- to 12-year cycle in numbers (not a lack
>of cycles, as Kerlinger et al. claim). However, part of the reason he
>fails to recognize the less dramatic 3-4 year cycle is that in these
>flight years, few or no birds get as far south as Oregon, and in
>some of them, none get south of the Canada/US border on the west coast.
>It may be that 1988 and 2000 were also flight years, but that in
>those years, few owls got even as far south as Vancouver.

I count nine possible episodes in 40 years in Mikes's data.

>Does the El Nino cycle (yes, a 3-4 year cycle, on
>average) affect weather in a predictable way as far north as the
>Arctic tundra? Perhaps. However, I'm not aware that anyone has
>shown convincingly that it does. Maybe someone will take up the
>challenge soon.

El Nino is just a part of the interconnected global weather
cycles--but the most prominent aspect. Furthermore the Index is a
measurement only. It just happens that in 8 out of 9 Snowy Owl years
the Index has the same pattern.


>
>Wayne C. Weber
>Delta, BC
>contopus at telus.net

--Jerry <tangren.family at verizon.net>