Jeff,

Cowbirds often remove a host egg from a nest before they parasitize it. They scout out the nests and may remove an egg on days prior so that they can then go directly to the victim's nest at sunrise the next day and lay their egg in it. The egg removal and eating have been documented before. I've seen them do this a couple of times as part of my graduate research, and also seen the parastitic act by staking out likely nests and watching them at sunrise. the cowbirds can be brutal - if the host bird is on the nest when the cowbird comes to lay her egg she (the cowbird) will shove the host bird off the nest by pecking it on the head (with Plumbeous Vireos) or, if it is small enough (Yellow Warbler), picking it up by the scruff of the neck and throwing it off.

Back to egg removal - cowbirds will grasp small eggs between their mandibles, but will lift larger eggs by puncturing them with one mandible just as you describe. Eating the contents provides valuable nutrients which the cowbird needs - she will lay from 25-40 eggs a season!

Cowbirds, gotta love 'em (a great Ameican success story!)

Dave

David Curson, PhD
Director of Bird Conservation,
Audubon MD-DC,
2437 Eastern Avenue,
Baltimore  MD  21224
Tel: (410) 558 2473
E-mail: 





-----Original Message-----
From: Maryland Birds & Birding [mailto:[log in to unmask]]On
Behalf Of Jeff Shenot
Sent: Tuesday, May 06, 2008 8:47 PM
To: 
Subject: [MDOSPREY] Jug Bay marsh birds


This eve I heard my first Least Bittern for the year, and there have been
Soras calling in the evening since Sunday.

I also saw something odd- a female cowbird doing something with an egg on
the ground.  I came up on her as she was apparently trying to lift it.  The egg
appeared to have a small hole in it and she very gently lifted it by placing her
lower bill inside the egg through the hole, and her upper bill on the outside
outside.  I did not see her poke the hole, it was there when I first noticed.  I
couldn't tell where the egg was from (no visible nest nearby); it was a white
with only a few light brown speckles.  It was larger than a Bluebird egg and
smaller than a Robin egg.  I hoped she would leave it so I could see what kind
it was, but she managed to get a hold of it and carry (fly with) it a short
distance (~25 feet) near the ground away from the trail I was on.  Then to
my shock she ate it -- egg, shell, and all!  When she finished she flew away
and did not return to the area.  I looked at the place she had been standing
at but there was nothing left.  I looked around to see where she came from
but could not find a nest.

Anyone know if cowbird parasitism includes eating a host bird's egg?  I have
seen grackles eat bird eggs (as scavengers, not as parasites), but never saw
a cowbird do this.

Cheers-
Jeff Shenot
Croom MD