Reprinted from: Update 10(3)
James R. Hill, III
Purple Martin Conservation Association
In the course
of a normal breeding season, Purple Martin landlords will typically encounter
many of the egg examples pictured here in and around their martin housing. Therefore,
it behooves each of us to be able to identify what we are seeing, and to correctly
interpret what each means. The text below explains what species of bird laid
each egg in the group photo and how to tell various species' eggs apart. Each
egg is shown at 100% actual size and are of the six species that commonly breed
in martin housing.
(a, b, c, d, j) Purple Martin: A Purple Martin egg is pure white and is 25mm x 17.5mm in length and width. Its shape is oval to long-oval. The shell is smooth with a slight gloss. Egg (a) is an addled Purple Martin egg, one that didn't hatch due to infertility or embryo death. Such an egg is normally white, but quickly becomes soiled brown by the fecal stains of nestling martins if it's not tossed out by the parents. Egg (b) is a "capped" martin egg, in which one or both of the hatched eggshell hemispheres (see images c), are not eaten or removed quickly enough from the nest by the martin parents and slip over the remaining unhatched eggs in the nests, often imprisoning (and killing) the unhatched nestling. Egg capping happens in about 5% of nests and capped eggs should be gently uncapped by landlords during their 5-day nest checks. Egg (j) is a hatching (pipping) martin egg.
(e) Tree Swallow: A Tree Swallow egg is pure white and typically 18.5mm x 13.0mm in length and width. Its shape is oval to long-oval. The shell is smooth without gloss.
(f) European Starling: A European Starling
egg is pale blue and is 29.5mm x 21.5mm in length and width. Its shape is oval
(g) Eastern Bluebird: An Eastern Bluebird egg is pale blue, (or pure white 5-10% of the time). It is 20.5mm x 16.5mm in length and width. Its shape is oval to short-oval. The shell is smooth and glossy.
(h) House Sparrow: A House Sparrow egg is white or greenish white, and dotted or spotted with grays or browns. It is 22.5 mm x 15.5mm in length and width. Its shape is oval to long-oval.
(i) House Wren: A House Wren egg is white, thickly speckled with small reddish or cinnamon-brown dots, with the deepest color at the larger end of the egg. It is 16.5mm x 12.5mm in length and width. Its shape is oval to short-oval. The shell is smooth with a slight gloss.
Great Crested Flycatcher (not shown): A Great Crested Flycatcher egg is yellowish or pinkish-white; scratched, lined, streaked, or blotched with dark brown or purple. It is 22.5mm x 17.0mm in length and width, with an oval to short-oval shape. The shell is smooth and slightly glossy.
House Finch (not shown): A House Finch egg is pale bluish-green; lightly spotted with black. It is oval and smooth with a slight gloss. It is about 19.0mm x 14.0mm in length and width.
The Peterson Field Guide to Birds' Nests, published by Houghton Mifflin Co., is a good reference for identifying eggs and nests.
Copyright 1998 by Purple Martin Conservation Association. All Rights Reserved.
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