The breeding season is well underway here on Metinic Island. We’ve completed breeding censuses for common eiders, and herring and great black-backed gulls. We are also monitoring breeding passerines, black guillemots, Leach’s storm-petrels, and common and arctic terns. There is an incredible diversity in eggs, both between and within species. Eggs come in a variety of shapes and colors, which are related to the ecology of the species, nesting behavior and depredation risk. For example, owls lay spherical to elliptical eggs that are whitish to cream colored because they are top predators and mostly nest in cavities. Owls do not need to disguise their eggs as much as say towhees, which nest on or near the ground.
In avian reproduction, the formation of the shell and addition of pigment occurs just prior to laying. Pigments in the egg shell strengthen it, as well as provide camouflage and other signals. While females of a species typically produce similar eggs, subtle differences between individuals can cause unusual patterns or pigments in eggs.
Mallard Nest (CA 2018)
Common Eider Nest (ME 2018)
Prairie Warbler Nest (MA 2013)
Many species within a family produce similar eggs either in color, pattern or both. For example, most duck eggs are relatively plain and white to pale green or blue, while the basic pattern in warblers is cream to white with brown speckling. However, other families, such as gulls and terns, show substantial variation between individuals and within a clutch (the group of eggs that a female lays in a single nest). Gull and tern eggs can be pale brown to blue or green, or darker brown or green. There are brown and gray speckles, squiggles or splotches (all very technical terms) that both strengthen the eggs and distinguish them from other eggs.
We’ve seen a huge variety of color and pattern in herring gulls especially. Most are some shade of brown from pale cream to dark brown. Some are delicately speckled while others have splotches of dark browns. At least one egg was found that had dark speckles around the wide part, and was plain everywhere else. It was like a printer that runs out of ink while printing a picture. Tern eggs can be pale to dark green as well, but new this year was an almost teal arctic tern egg. The base color with brown speckles make the egg look like a scoop of mint chocolate chip ice cream, but why that egg is so brightly colored compared to other tern eggs.
Herring Gull Nest (Metinic 2018)
Herring Gull Nest (Metinic 2018)
Herring Gull Nest (Metinic 2018)
Herring Gull Egg. Female may have had limited resources and could not produce pigments.
Arctic Tern Nest (Metinic 2018)
Arctic Tern Egg, resembling mint chocolate chip ice cream.
These eggs show some of the possible variation in bird eggs. Some research has been conducted to understand what directs the shape, coloration and pattern of eggshells. Research suggests that the individual variation may improve nest identification for colonially nesting species, or eggshells may be signals. For example, the depth of blue-green in eggshells may be a result of a higher antioxidant level in females, signaling greater health of both the female and offspring.
Swainson’s Thrush Nest (MA 2013)
Eastern Towhee Nest (MA 2013)
Brown Thrasher Nest (MA 2013)
Eggs are just one of the many fascinating aspects of avian biology that we get to experience out here. It’s an egg-cellent way to study individual variation, health and reproduction in birds.
~Nick and Nora
