Nov 172013

On July 4th I discovered an Abert’s Towhee nest with five eggs.  The next day the towhee had laid another egg.  Altogether there are:

2 Abert’s Towhee eggs

1 Bronzed Cowbird egg large

1 Brown-headed Cowbird egg

2 Bronzed Cowbird eggs small

Because Brown-headed Cowbirds eject the eggs of their host, there is no way of knowing how many eggs were actually laid by the towhee.

Note in several shots you can see one of the eggs has a peck mark on it, very likely the work of a Bronzed Cowbird.

The first Bronzed Cowbird’s egg is the single largest all white egg.

The small dark heavily speckled egg belongs to the Brown-headed Cowbird.

The Abert’s Towhee eggs are the two large eggs with black speckles on the end.

The two small white eggs with few spots on them are from a second Bronzed Cowbird.

On the 15th one of the two small eggs was pushed out of the nest.  Here is  a different angle of the 7/15 nest.

I was interested in the treatment of the eggs by the host.  Here is a graphic.

DENT and NODT are the two small mostly white eggs.

The graphics directly follow the photographs above.

It can’t be determined whether the ABTO eggs got preferential treatment and based on what I have read the eggs are turned multiple times a day.

Although the photo of July 16th shows just one chick, actually two hatched, the BHCO in the morning and the first ABTO in the afternoon.

The two small Bronzed Cowbird eggs have raised gray calcium bumps on them. The bumps are a sign of a young bird, similar to calcium bumps on eggs from young chickens.

Bronzed Cowbird eggs

Bronzed Cowbird eggs

I recovered two of the three failed eggs and candled them. Both are from Bronzed Cowbirds. Note the difference in size, which is common.

candling failed eggs

candling failed eggs

Abert’s Towhee — Chapter 1

Brown-headed Cowbird — Chapter 2

Bronzed Cowbird — Chapter 3

Hatchlings — Chapter 5

  2 Responses to “Brood Parasites
Chapter 4 — Eggs in Parasitized Nest”

  1. I have identified what appears to be an Abert’s Towhee that made a nest in our yard with 3 eggs…. since yesterday one of the eggs is now turning very dark and has a bad odor. What does that indicate? Thank you for your help.

    • Sorry for the late response. The parents will eject the egg sometime through the incubation period. That is what my towhee did with three eggs.

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