Re: Nest pic and question.
- From: Leon Fisk <lfisk@xxxxxxxxxxxxxxxxx>
- Date: Mon, 11 Jun 2007 13:22:56 -0400
On Sat, 09 Jun 2007 12:00:08 -0400, rossr35253@xxxxxxxxxxxx
wrote:
On Fri, 08 Jun 2007 14:25:55 -0400, Leon Fisk
<lfisk@xxxxxxxxxxxxxxxxx> wrote:
On Fri, 08 Jun 2007 13:11:42 -0400, rossr35253@xxxxxxxxxxxx
wrote:
Over in alt.binaries.birds, under the same subject, I posted the
following:
This is a Song Sparrow's nest, built inside a clump of oriental
poppies, less than 6 inches off the ground.
Reference books all say normally 3 to 5 eggs, rarely 2 to 6 eggs.
This nest has 7 eggs and I'm wondering if it's safe to assume that at
least one of the eggs is that of a Brown-Headed Cowbird.
Is there any way to definitely identify a cowbird egg? If it can be
identified, is it safe to try and remove it or does one just let
nature take its course?
Hi Ross,
I would think that a Cowbird egg would be noticeably larger
than a Song Sparrows.
That's what I thought as well, and everything I've found on the 'net
seems to agree. However, according to my copy of 'A Field Guide to the
Nests, Eggs and Nestlings of North American Birds', the Song Sparrow's
eggs are slightly larger.
Cowbird 21 x 16mm
Song Sparrow 22 x 17mm
Somebody has to be wrong.
I'll just keep watching the nest and try to keep a digital photo
record.
Hi Ross,
I took a minute (or two) and dug through all of my reference
material that might contain egg sizes. Only The Birders
Handbook had anything on sizes.
Song Sparrow 20mm
Cowbird 21mm
So I would say they are pretty close to being the same size.
The Song Sparrow is described as "dull pale greenish spotted
thickly or sparsely spotted or blotched with shades of
reddish or dark brown and lavender".
I have another book which is pretty much on just Cowbirds.
It notes that they have a thicker shell relative to its
volume than other (non-parasitic) icterines. Also compared
to non-parasitic passerines Brown-headed Cowbird eggs are
small relative to female size. They describe them as nearly
white to various shades of blue or bluish green. Maculated
with light to dark brown spots. Spots usually uniformly
distributed, but can be concentrated on one end, or the
spots maybe larger at one end.
There is a wide variation in egg size with the largest egg
measured being 1.63 times larger than the smallest egg. The
average female lays from 40 to 77 eggs per year. Pretty
close to one egg per day. Variation thought to be due to
host availability and nutrition among other things. Usually
only one egg per host nest, but more than one Cowbird could
be involved.
One other item I found really interesting is that the
average laying time is only 41 seconds. Other passerines
range from 20.7 to 103 minutes and six other icterines
ranging from 21.5 to 53.4 minutes. I double checked those
minute and second values too, they really are listed as just
41 seconds.
Just some info that I thought you may find interesting...
--
Leon Fisk
Grand Rapids MI/Zone 5b
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