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Thanks for your input Resin. I've not been able to find any egg identification information online. There are some photos at the Univ. of Cornell, but not eggs for all birds. I've tried looking up every bird I usually see around here and no luck so far, unless descriptions are just not accurate enough.
It looks similar to an egg photo I found for a type of Junco, but the color is wrong.
It looks like a house sparrow egg to me. However, there can be considerable variation in color. House Sparrows are notorious for taking over Bluebird nesting boxes. Cowbirds normally lay their parasitic eggs in open nests, of an other bird, rather than nesting boxes. Starling eggs are pale blue without any spots, however, I recently saw one reference book claiming they are spotted.
I wonder if it could be a house wren egg. Here in Minnesota, wrens & house sparrows are a bluebird's worst enemies. House wrens will poke holes or toss out ("turf") the bluebird's eggs, and then lay its own. They are extremely territorial. Your egg may be a bit too big to be a wren egg though, although the colors are right. It if IS a wren egg, you will want to consider moving your box as far away as possible from shrubby growth (the wren's hangout).
I found this website http://www.sialis.org/eggcompare.html with photos and sizes of various eggs. I'm thinking it's a Black Capped Chickadee egg and read that they, too, will take over a bluebird box. The descriptions I found elsewhere mentioned there being more color at the large end of the egg, too.
I'll have to keep a watchful eye on the box this year to see who is using it. I've had anywhere from 3 to 8 bluebirds around, but also have allot of trees they could be using to nest in.
Thanks for everyone's help. I will watch for those birds, too.
Definitely NOT a Chickadee egg!
The spots are the wrong color and it is to large to be a Chickadee egg. This photo is of a Black-capped Chickadee nest and eggs. Notice the reddish-chestnut colored spots. Most, but not all, spotted eggs have "more color at the large end of the egg."
Quoting:I wonder if it could be a house wren egg. Here in Minnesota, wrens & house sparrows are a bluebird's worst enemies. House wrens will poke holes or toss out ("turf") the bluebird's eggs, and then lay its own. ... Your egg may be a bit too big to be a wren egg though
Wow! Thanks Resin. The egg at the hyperlink looks exactly like it. The pictures I saw of House Wren eggs at other sites were solid more of a solid brown with lots of evenly spaced freckles. Only problem is, I don't think Carolina Wrens come all the way to Montana. Maybe I had a stray.
I just explored the hyperlink for sailis posted above. While the information is generally quite useful, the color balance of the photos is terrible. The photo for comparison of an American Robin and a European Starling was nowhere close to true color. A Starling's egg is a pale blue. A Robin's egg is a very deep rich shade of blue. But, the comparison photo shows the Robins egg as the paler blue. I suspect the photos were taken with old film photography, never captured the true colors, and faded with age.
Muddylou - I don't have any bluebirds here yet (south-central Montana zone 4 or 5). Although, I think I saw a female bluebird last week, I couldn't get the binoculars out fast enough.
I see at this page ( http://www.sialis.org/wrens.htm ) there are directions for a wren deterent on the bluebird house. I think I need to start looking into keeping other birds away. I've had the bluebird box up for 3 or 4 years because I'd see the birds along the roadside. So, I'm pretty new at this.
Well, I've not got two who say it's a House Sparrow! Gary (Grasmussen) & now I just got this reply from Bet at http://www.sialis.org about the egg. She says "It's a House Sparrow! BTW, the pictures and measurements REALLY help (sometimes I get emails from foreign countries saying they found a green egg with no more info and it's tough to help!)
In a nutshell, at a minimum remove all nests and eggs at least a couple of times a week. I trap and remove House Sparrows as they will peck eggs, kill nestlings, and decapitate adults if they can catch them inside of a box.
If you do get bluebirds using a box, I highly recommend sparrow spookers.
Looks like I've got some work to do to keep the "Bad" birds away.
I thought the information might be helpful to others out there.
YIkes! The dreaded HOSP! Well, there's a lot you can do to keep them at bay. If it turns out that your yard is too small or too wooded to provide the open space that bluebirds need (and that deters HOSP & wrens), you can always look into "borrowing" space at a local golf course or cemetery and start a trail! It's so worth the time & energy.