The European Starling (Sturnus vulgaris) is a globally widespread and highly adaptable avian colonizer, common in North America. These glossy, dark-feathered birds occupy habitats ranging from urban centers to agricultural landscapes. Their breeding habits, particularly concerning nesting in relation to other birds, are often misunderstood. The question of whether starlings deposit eggs in the nests of other species involves a unique reproductive strategy. Understanding this requires examining the specific type of nesting behavior starlings exhibit and the biological reasons driving it.
Intraspecific Brood Parasitism
The direct answer is yes, but with a critical nuance: starlings primarily lay eggs in the nests of other starlings. This behavior is scientifically termed intraspecific brood parasitism, meaning the parasitic act occurs within the same species. This differs from interspecific brood parasitism, used by birds like the Brown-headed Cowbird, which involves laying eggs in the nests of different species.
The European Starling is a facultative brood parasite, meaning the behavior is optional and occurs only under certain circumstances. Studies show this parasitic behavior is not rare, though rates vary significantly by location and year. In some populations initiating their first clutches early in the spring, the rate of parasitism can affect between 15% and 37% of nests.
The frequency of parasitic events is highest during the first wave of nesting, declining dramatically for subsequent clutches later in the season. A female starling performing this act enters an unattended nest cavity and deposits a single egg before departing. This strategy allows the female to increase her reproductive output without investing the time or energy required for incubation or raising the young herself.
Why Starlings Lay Eggs in Other Nests
The motivations behind this strategy are rooted in maximizing a female’s reproductive output and mitigating nesting risks.
Optimization of Clutch Size
One primary driver is optimizing clutch size. If her own nest is nearing capacity, a female may lay an egg in a neighbor’s nest. This allows her to pass on more genetic material than she could physically raise, effectively outsourcing care for the additional offspring.
Insurance Against Failure
The behavior also serves as an insurance policy against reproductive failure. Younger or unpaired females, often called “floaters,” may not secure a quality nest site or mate early in the season. These floaters engage in parasitic laying as a hedge against having no successful nest of their own.
Risk Mitigation
Laying an egg parasitically also acts as risk mitigation against the potential failure of the starling’s primary nest due to predation or abandonment. Distributing eggs across multiple nests avoids putting all reproductive investment into one vulnerable location. Parasitic eggs are laid early, coinciding with the host female’s laying period, which maximizes the chance of successful incubation.
Host Recognition and Survival Rate
Once a parasitic egg is laid, the host starling must detect and decide whether to reject the foreign egg. Unlike victims of interspecific parasitism, starling hosts have difficulty distinguishing between their own eggs and those of a conspecific parasite. The eggs are nearly identical in color and pattern, meaning the host’s ability to recognize the foreign egg is low. Furthermore, the risk of accidentally rejecting one of her own eggs often outweighs the cost of raising the parasite.
Host starlings are not entirely without anti-parasitism tactics, however. Some host females eject foreign eggs from their nest, often discriminating based on subtle physical differences. Starlings are more likely to eject an egg if it is noticeably smaller than their own clutch, using egg volume as a key recognition cue.
The fate of the parasitic chick is often less successful than that of the host’s biological offspring. Nests with parasitic eggs fledge fewer total young compared to non-parasitized nests, according to genetic analysis. This reduced success is likely due to increased competition for food and resources among a larger brood. Parasitic young that survive and fledge were typically laid synchronously with the host’s eggs, allowing them to compete effectively.