The three North American bluebird species (Eastern, Western, and Mountain) are members of the thrush family. These birds are known for laying distinctive, unspotted blue eggs, but their clutches are often mistaken for those of other blue-egg-laying species. Understanding the precise physical characteristics of bluebird eggs is necessary for accurate identification, especially when examining a nest found in the wild or a man-made nest box. This information provides the key characteristics needed to differentiate a genuine bluebird clutch from its common look-alikes.
The Distinctive Appearance of Bluebird Eggs
The most recognized attribute of a bluebird egg is its uniform color, which typically ranges from pale blue to deep turquoise. This coloration comes from the pigment biliverdin deposited during eggshell formation. While blue is standard, a small percentage of female bluebirds (two to nine percent) will lay pure white eggs due to a genetic variation.
The shell surface is smooth and generally matte or non-glossy. Bluebird eggs are almost always unspotted, lacking any discernible markings or speckles. They possess a delicate, non-reflective texture sometimes described as porcelain-like.
Bluebird eggs are relatively small, measuring approximately 0.75 to 0.85 inches (19 to 21.5 millimeters) in length. Their width is typically around 0.6 inches (15 millimeters), giving them a slightly elongated oval shape. This combination of specific size and uniform, unspotted color is crucial for distinguishing them from other species.
Distinguishing Bluebird Eggs by Species
Although all three species lay predominantly unspotted blue eggs, subtle variations in color saturation exist. These differences can offer clues to the species when observing eggs in their respective geographical ranges.
Eastern Bluebird eggs often exhibit the deepest and most saturated blue color. Western Bluebird eggs usually fall in the middle of the saturation range, appearing as a standard sky blue. Mountain Bluebird eggs tend to be the palest blue, sometimes appearing so light that they are nearly white.
Where Bluebirds Lay Their Eggs (Nesting Habits)
Bluebirds are secondary cavity nesters, relying on existing holes in wood rather than excavating their own sites. They prefer open areas, such as orchards, pastures, and open woodlands, where they can easily spot insects from a perch. Suitable nesting locations include natural tree cavities, abandoned woodpecker holes, and man-made nesting boxes.
The nest itself is a neat, cup-shaped structure built almost entirely by the female bird. It is loosely woven from fine, dry materials like grasses, pine needles, and plant fibers. This simple, grass-based construction contrasts sharply with the nests of other species that may use mud or trash.
A typical clutch size ranges from four to six eggs. The female lays one egg each day until the clutch is complete, after which she begins incubation.
Look-Alikes: Blue Eggs That Aren’t Bluebird Eggs
Identifying a bluebird egg requires ruling out other common North American birds that also lay blue or blue-green eggs in similar environments. The American Robin is perhaps the most famous look-alike, but its eggs are typically a brighter, more saturated sky blue and are noticeably larger than those of the bluebird. The robin builds an open, cup-shaped nest reinforced with a layer of mud, and it places this nest in trees or shrubs, not inside a cavity.
Another common cavity-nesting competitor is the European Starling, an invasive species that frequently occupies bluebird boxes. Starling eggs are usually a pale, slightly greenish-blue color, and their shell surface is often lightly spotted or speckled. The presence of any speckling or green tint immediately rules out a bluebird egg, as does the starling’s messy nest construction, which often includes trash or coarse material.
The Tree Swallow is a native bird that competes directly with bluebirds for nest boxes, but its eggs are easily distinguishable by color. Tree Swallows lay pure white eggs that are slightly more elongated and pointy than the bluebird’s oval shape. The presence of white eggs in a bluebird box, especially if the nest is heavily lined with feathers, suggests a Tree Swallow clutch.