European starlings are small, stout songbirds that are native to Europe, Asia, and North Africa. However, they have been introduced to many other parts of the world, including North America, South Africa, Australia, and New Zealand. European starlings build nests in cavities in trees, cliffs, or building structures. Their nests are made up of grasses, twigs, feathers, and other materials.
Appearance
European starling eggs are oval or elongated oval in shape. They are smooth and slightly glossy. The background color of European starling eggs can range from very pale blue or green to a deeper turquoise blue. The eggs often have reddish-brown spotting, speckling, streaking, or blotching on them. However, some eggs can be immaculate with no markings at all. The markings help camouflage the eggs when the adult is away from the nest.
In terms of size, European starling eggs typically measure 21-25 mm long by 16-18 mm wide. They weigh around 5 grams on average. The eggs of females that nest early in the season tend to be larger than those of females that nest later in the season.
Color Variation
As mentioned, European starling eggs can vary quite a bit in background color and markings:
- Pale blue or bluish-green eggs with reddish-brown spotting
- Bright blue eggs with heavy dark brown streaking or blotching
- Turquoise or greenish-blue eggs with reddish-brown speckling
- Plain, immaculate eggs with no markings at all
This color variation helps make each clutch somewhat unique. It can also aid bird watchers and researchers in identifying individual nests.
Clutch Size
The clutch size of European starlings ranges from 3-7 eggs, with 5 being typical. The eggs are laid at a rate of one per day. If a clutch is lost early in incubation, the female may lay a replacement clutch.
Here is a table summarizing information on European starling clutch size:
| Clutch Size | Frequency |
|---|---|
| 3 eggs | Uncommon |
| 4 eggs | Common |
| 5 eggs | Most common |
| 6-7 eggs | Less common |
Incubation and Hatching
Incubation of European starling eggs begins as soon as the first egg is laid. Both the male and female share incubation duties, which last 10-13 days. The eggs typically all hatch within a day of each other. The chicks are altricial, meaning they hatch helpless and require extensive parental care.
Here are some key facts about European starling egg incubation and hatching:
- Incubation period: 10-13 days
- Incubation shared by both parents
- Eggs hatch asynchronously over 1-2 days
- Hatchlings are altricial (helpless, requiring feeding)
Variation Between Populations
Some subtle geographic variation in European starling eggs and breeding does exist between different populations and subspecies. Here are a few key differences:
- Largest eggs and clutches found in the nominate population in Europe
- Smallest eggs and clutches found in North African populations
- Background egg color darkest in nominate population, palest in Far East populations
- Highest hatching success in nominate population, lowest in North America
These variations may be adaptations to local conditions like climate, predation levels, and availability of food resources.
Role of Eggshell Pigments
The blue-green background color of European starling eggs is produced by the pigment biliverdin. This is the same pigment that gives a bruise its blue-green color as it heals.
Researchers believe biliverdin in eggshells may have the following beneficial effects for the embryos inside:
- Acts as an antioxidant, protecting embryos from harmful molecules
- Aids in calcium absorption during embryonic bone development
- Helps regulate fluid levels in the egg
The protoporphyrin pigment produces the reddish-brown speckling on European starling eggs. This is thought to strengthen and reinforce the shell.
Mimicry of Eggs of Other Species
The coloration and patterning of European starling eggs shows some adaptations for mimicry and camouflage amongst the eggs of other cavity nesting species. For example:
- Resembles tree swallow eggs when in a nest box colony
- Mimics house sparrow eggs when nesting in urban areas
- Blends in with rock pigeon and jackdaw eggs when nesting on cliffs or buildings
This mimicry likely provides protective benefits by preventing rejection of the odd-looking egg by the parents and reducing detection of the clutch by predators.
Threats and Predators
European starling eggs face threats from a number of natural predators. Common nest predators include jays, crows, snakes, cats, martens, raccoons, and opossums. Sharp-shinned and Cooper’s hawks also frequently prey on adults, eggs, and nestlings.
In addition to natural predators, European starlings face threats from human activities. Disturbance of nesting sites, removal of nesting habitat, and agricultural pesticides have impacted some populations. Their habit of nesting in cavities in human structures like vents or roofs also brings them into conflict with humans.
Conclusion
In summary, European starling eggs are oval-shaped with variable blue-green base coloration and reddish-brown spotting or streaking. Clutch sizes range from 3-7 eggs, with 5 being most common. Both parents share incubation duties for 10-13 days until hatching. Some variation in egg size, color, and breeding success exists between different geographic populations and subspecies. The pigments producing the egg coloration play important roles in embryo development and egg camouflage. While adapted to cavity nesting, European starling eggs and nestlings face threats from predators and human activity in many parts of their range.
