Over the past couple of decades my journal entries for Pine siskins go in a
pattern of none or low numbers for 2 years, high numbers one year, none to
low numbers the following year. For example



1992 None seen

1993 12 seen one day only

1994 Large flocks 100+ observed on several days

1995 None seen

1996 None seen

1997 Large flocks observed on several days



I wonder if this is just random or if the pattern is due to salmonella,
large numbers get knocked down by disease and take a bit to build up again,
or if it relates to cone production or some other factor?