On Thu, 2007-06-21 at 12:20 -0400, Janet Millenson wrote:
> Unlike the other mimids, our breeding pair of Brown Thrashers skulks around 
> utterly silently. Earlier in the season, the male sang for what seemed like 
> only a couple of days after returning from migration. Previous years, same 
> thing. Of course many species (e.g., crows) get very quiet during nesting 
> season, but the catbirds in our yard sing constantly, and the neighborhood 
> mockingbirds proclaim from the chimney-tops.
> 
> Did this thrasher hear no other males of his kind nearby, and therefore 
> decide his territory was safe from rivals? Was he only singing until his 
> mate showed up? What's typical for this species?

Janet,

I think you are exactly correct.  Often birds do not sing if there is no
need.  Another example: Prothonotary Warblers are not particularly
common around here but there are a few disjoint territories.  Some years
the male will sing all summer and there will be no other evidence of
nesting.  Other years, I'll only hear a few songs from a territory in
May but this time of year I'll find a pair quietly tending to their
young.  This year my thrashers are silent but I see them on occasion.
Last year there were four males singing at each other along a two block
section of this street.  They sang much of the summer.

One of the dirty little secrets of the Breeding Bird Atlas is that it is
often only the unpaired males that sing.  Sure, rival males can sing
non-stop in areas where the species is common and they have to defend
the boundaries of their territory but in those areas it's also pretty
easy to find other, more substantial evidence of nesting.  Where the
species is few and far between, using song as evidence of a male
defending its territory is *truly* suspect.

I know, the pat answer is simple.  A bird persistently singing from an
area is *evidence* of appropriate *habitat* for that species.  Even
though the bird may not be nesting when I hear him, the habitat was good
enough to draw him in this year so they *probably* nest there some
years.  After all, that's why it's given a *probable* code in the atlas.

If the habitats were left unchanged over the last 20 years, comparing
the results of the two atlases would be fairly simple.  My concern is
due to the habitat becoming more and more fragmented.  The good
territories are less likely to have neighbors so the birds are less
likely to sing and atlasers are more likely to miss them in the areas
where they still nest all the time.  Birds of the same species are more
likely so sing in worse habitat where it's harder to find a mate and
atlasers are more likely to count them in places where they do not
currently nest.  Depending on the species, we could either greatly
overestimate or underestimate their decline.

To state that a different way, the probability of finding a bird and the
probability that the bird is there are two entirely different things.
We measure one because we want to know the other.  Since the ratio
varies not only between species but also for the same species between
areas and between times, interpreting what the atlas is telling us may
be a difficult task indeed.  I can imagine scenarios where the
probability of detecting a species increases as the habitat (and number
of true breeders) decreases.

Dave
-- 
David Mozurkewich
Seabrook, PG MD
mozurk at bell atlantic dot net