A sparrow has been so kind as to build a nest in the perfect place for
my family to observe the cycle of nature.  However, something is a bit
odd.  Here is what we have observed:  Day one - one egg slightly
smaller than a robin's egg.  White with brown/black spots.  Day 2 -
same egg as day one plus small blue egg about the size of a jelly bean
- has a few spots.  Day 3 - same egg as day one.  Little blue egg is
gone. I looked all around but cannot find it anywhere.  Day 4 - little
blue egg is back and same day one egg still hanging tough.  Day 5 -
two little blue eggs(!) and day one egg.  What is going on?!? (I have
my suspicions).  So specifically, I wonder if the disappearing blue
egg re-appeared, or is it a different egg, and I also wonder what kind
of ugly duckling will appear in the nest.  I do see the sparrow
sitting on the nest from time to time, but she does not seem diligent
(although if someone kept bringing extra kids into my house I might be
reluctant to come home too.)

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Request for Question Clarification by bobbie7-ga on 26 Apr 2006 16:34 PDT

The cowbird lay her eggs in other bird's nest, leaving these eggs to
be hatched by the another species of bird.


Nest Type
None. Lays eggs in nests of other bird species.

Egg Description
Whitish with brown or gray spots.

http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Brown-headed_Cowbird_dtl.html

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Request for Question Clarification by bobbie7-ga on 26 Apr 2006 16:36 PDT

This may explain what's going on.

"Brown-headed Cowbirds are brood parasites, that is, they have
completely abandoned the tasks of building nests, incubating eggs, and
caring for hatchlings. Instead, each female deposits as many as 40
eggs per year in nests that belong to other bird species. More than
100 other species have provided host nests for cowbird eggs. The
female cowbird finds these nests by watching patiently from an
observation post where she can look down upon grassland species, by
observing the nests of tree-nesting species while she walks quietly on
the forest floor, or by crashing noisily through shrubbery with
flapping wings to flush out potential victims. She typically chooses a
nest with eggs smaller than her own and lays a single egg quickly at
dawn once the host has also started laying eggs. Unlike the parasitic
European cuckoos, Brown-headed Cowbirds do not evict their nest-mates,
although the female may remove and sometimes eat eggs from the host
nest. Instead, cowbird nestlings typically out-compete their smaller
nest mates"

http://www.birds.cornell.edu/BOW/BNHCOW/

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Clarification of Question by nkamom-ga on 26 Apr 2006 19:29 PDT

Sounds like a pretty good likelihood that you have it figured out
bobbie7.  I will accept that as the answer.  Although I don't know how
a sparrow is going to manage to feed a cowbird and its own babies. 
Wonder why the cowbird did not pick the robin's nest - only a few feet
away and much roomier.

Dear Nkamom,

Tyhank you for accepting my comments as your official answer.


Here is some information about cowbirds laying eggs in song sparrow nests.


"Brown-headed Cowbird nest-parasitism is not a rare and insignificant
event. In one study in Ohio, between 24.6 and 77.7 percent of all Song
Sparrow nests contained Brown-headed Cowbird eggs or young. The
average nest size of 66 non-parasitized nests was 3.4 nestlings, while
the average size of 28 parasitized nests was 2.4 Song Sparrow
nestlings, plus the young Brown-headed Cowbird. Thus, each
Brown-headed Cowbird nestling was raised at the expense of exactly one
Song Sparrow.

As many as seven Brown-headed Cowbird eggs have been found in a single
Song Sparrow nest. Rarely, Song Sparrows recognize that something is
wrong, and build a new nest atop the old one, covering the
Brown-headed Cowbird egg so that it never hatches."
http://www.backyardnature.net/life_cc9.htm


Check out the third image. 
Cowbird Egg in Chipping Sparrow Nest
http://www.wbu.com/chipperwoods/photos/cowbird.htm


Best regards, 
Bobbie7

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Clarification of Answer by bobbie7-ga on 26 Apr 2006 19:54 PDT

"The most common hosts of Brown-headed Cowbirds are Yellow Warblers,
Song Sparrows, Red-eyed Vireos, Chipping Sparrows, Eastern Phoebes,
Eastern Towhees, Ovenbirds, and Common Yellowthroats."
http://www.shawcreekbirdsupply.com/brown_headed_cowbird_info.htm

A very interesting story. I had a nest full of robin eggs just on my
upstairs window ledge when I was a kid. Even though I was right there,
the window glass filled the mother with confidence. I got to watch the
whole program from start to finish.

   As for your hypothesis about a an intruder egg, you'll find out
soon enough empirically, unless the mom is spooked enough to forego
incubation.

Additional information:

Brown-headed Cowbird

"Females lay eggs in other birds' nests and leave the rearing to other
species. They find nests to parasitize by looking for birds building
nests, either by walking along the ground, perching quietly in shrubs
or trees, or making noisy flights back and forth, possibly to flush
potential hosts. The female generally chooses an open cup-nest to
parasitize, and usually lays one egg per nest. She waits to lay the
egg until the host bird has at least one egg in its nest, and often
removes one egg from the nest before laying her own."

 http://birdweb.org/birdweb/bird_details.aspx?id=446

Howdy nkamom-ga,

You state: "Although I don't know how a sparrow is going to manage to feed
a cowbird and its own babies."

Baby cowbirds have been known to push the other babies out of the nest so
they have less competition for the food the host mother supplies.

Looking Forward, denco-ga - Google Answers Researcher