While it may seem obvious that a disease that blinds infected House Finches
will cause House Finches to die, scientists still have to answer these
apparently obvious questions sometimes. Further, we are interested not only in
whether House Finches have died but in what proportion of birds have died.
Studies on captive House Finches suggested that mortality was almost certain,
which would mean there should be essentially no House Finches left in
eastern North America. These laboratory results did not agree with what
FeederWatchers were telling us, and with what we were seeing out our own windows.
A rigorous look at the data showed that our general impressions were not wrong:
This graph shows how abundance of House Finches declined
starting from one year before the disease arrived ("Year -1" on the horizontal
axis of the graph) to five years after the disease first arrived. The information
is a summary of events in 29 different regions in eastern North America, with
the horizontal red bars indicating how a typical population declined, and the
boxes and "whiskers" above and below indicating how much variation existed
among regions. Declines are presented as percentage decreases in numbers of
House Finches relative to the number that would have been present (which differs
among regions) had there been no disease (the "100%" level on the vertical
axis).
Conclusion: Mortality was far from rapid and universal after the disease
appeared in a region. Instead, two or three years passed before House Finches
reached their lowest abundance in a region. Somewhat surprisingly, House
Finches have not rebounded in numbers after reaching their low, which may
indicate that mycoplasmal conjunctivitis is continuing to regulate how many
House Finches cna occur in eastern North America.