[SBB] American Dipper Temporal Distribution

[Bill Bousman <[[email protected]]>]

Folks:

This is an interesting thread--thanks, David, for your valuable comments on 
dipper temporal distribution in Santa Cruz County.

The county notebooks have 25 years of data.  To look at the temporal 
distribution I make plots of sightings per year and sightings per 
week.  Then, with plots in hand, I get out my ouija board and interpret 
what I see.  The plot of sightings per year is a way of looking at 
long-term temporal changes (and observer bias, that is, there are more 
active birders here in 2005 than there were in 1985).  Interestingly, 
although there is a lot of year-to-year variance, there does not seem to be 
any long-term trend in the data, once processed on the ouija board.

The sightings by week plot assumes that I break each month up into a "week" 
of either 7 or 8 days--that way I always get four weeks in a month.  If one 
bird is seen four weeks in a row, then this counts as four 
"sigthings".  Looking at this temporal distribution there is not very much 
variation over the year, although there appears to be a downturn from June 
to early September.  This sort of distribution seems to my ouija board to 
be typical for residents.  That is, we detect a lot of birds in the spring 
and early summer months as they are nesting, but once they are over nesting 
and they are starting their prebasic molt, they tend to become secretive 
and quiet.

There are a number of factors that make it difficult to know the temporal 
status of American Dipper in Santa Clara County with accuracy.  First, most 
of the streams they use in the county are not easily accessible (good for 
dippers, by the way), so it is difficult to census them.  Second, breeding 
pairs tend to be noisy, but when breeding is over, pairs do not share 
territories, so one or both move on.  Three, they are rare.

Some example summer records that may be of interest, from that intrepid 
pair of hikers, Jim Yurchenco and Amy Lauterbach:
        22 Jul 1984     8, Arroyo Hondo, near Joseph Grant CP
        25 Aug 1985     7, Smiths Creek, near Joseph Grant CP
It looks like this drainage, in the 1980s, had substantial numbers, all 
spaced out, of course.

Bill Bousman
Santa Clara County records compiler 

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