How often do we stop and think about how amazing the bird egg is? It is such a familiar object via the breakfast meal that we do not consider how remarkable it is that a complex organism can develop within such a closed structure. Indeed the evolution of the “cleidoic” egg, which exchanges only gases with the surroundings, was one of the developments that enabled the full colonization of dry land by early vertebrates.
Aside from the their remarkable physiology, bird eggs and nests are simply beautiful expressions of the adaptations of each species. We have the opportunity to enjoy them by observing them in our yards, being very careful to minimize disturbance. One advantage of locating nests is to subsequently protect them from undue disturbance. By watching the movements of birds within a 100 foot radius of our house and examining all dense bushes and small trees I have been able to find nests of 16 species (Baltimore and orchard orioles, mourning dove, house finch, barn and tree swallows, phoebe, robin, Carolina and house wrens, bluebird, mockingbird, catbird, willow flycatcher, blue grosbeak and redwing). Certainly I have missed some (for example yellow throated vireo) that are too high to see and some that are in dense grasses (song, savannah and grasshopper sparrows).
The colors of eggs in our yard can be roughly divided into those that are all white, all blue, and a mixture of speckles and streaks with variable ground colors. In some cases the colors would seem to enhance camouflage and in other cases there is no obvious reason for the color and pattern. For example white eggs which would seem to be the most obvious to predators are found both in mourning doves, which have a simple open nest of sticks and grass, and tree swallows which nest in dark, protected tree holes (or nest boxes).
Eggs that are streaked or speckled would seem to be the best camouflaged and are found in the open nests of redwings, willow flycatchers, and mockingbirds. But Carolina wrens which nest in less exposed situations also have speckled eggs.
Sky blue eggs are typical of robins and catbirds which both make open nests of grasses. What would be the purpose of this color? Thrushes (robins and bluebirds) tend to have all blue eggs; the mimid catbird is similar whereas the mockingbird is bluish with dark speckles.
Truly bird eggs are beautiful, fascinating and enigmatic! So enjoy the hunt for bird nests in your yard, protect them from disturbance, and thrill in the remarkable process of avian reproduction. Life is certainly grand, but it can use a hand from us in ensuring that our birds and their progeny will return next year.