Supersized clutches of bluebird eggs found in nestboxes
Updated: Jul 27
In May, an Amissville, Virginia, man discovered nine eggs in one of the bluebird nesting boxes he'd maintained and monitored for years and wondered if this was a record for the species.
“I have been here for over 50 years and have about 20 bluebird houses scattered about the fields," he wrote (asking to remain anonymous) in an email exchange I had with him. He raises about 70 birds yearly, averaging four to six eggs per nest, he added, and "had never before seen nine eggs in one nest.”
Eastern Bluebirds normally lay three to seven eggs in a clutch, averaging four to five. In checking with the Virginia Bluebird Society (VBS) and some master naturalists who monitor bluebird “trails”—a series of nestboxes mounted on poles in open areas—I learned that eight eggs were found in one monitored nestbox this year, and nine in a Danville nestbox last year.
Michael Bishop, a master naturalist and member of the VBS board, suggested that these supersized clutches came from egg “dumping,” a behavior that was new to me. More formally known as intraspecific (or conspecific) brood parasitism, this is when a female bird lays her eggs in the nest of another bird of the same species. This differs from the more commonly known “obligate nest parasitism,” in which some species that don’t build nests of their own, such as cowbirds and cuckoos, always lay their eggs in the nests of other bird species.
CORRECTION (Nov. 30): As an astute reader pointed out, it is Old World cuckoos that are obligate nest parasites. "The North American Yellow-billed and Black-billed Cuckoos only rarely lay their eggs in the nests of other species, but occasionally lay some of their eggs in the nests of other members of their species," Paul R. Ehrlich and other scientists at Stanford University wrote in an article about cuckoo brood parasitism. "Our cuckoos usually build nests of their own and rear their own young. Only about 40 percent of cuckoo species worldwide are brood parasites, the rest care for their own eggs and young."
Although other bluebird monitors I contacted had not heard of egg dumping, ornithologist Roger Lederer, in an article for Ornithology.com, describes the behavior as “common” among eastern bluebirds. And this species is not alone — more than 200 bird species that normally build nests of their own dump eggs in another’s nest from time to time.
The prime benefit of dumping for the parasite species is saving resources—rather than spending them on defending nests, incubating eggs, and feeding young, they can lay more eggs. The downside is that they have no control over how the eggs, and young when they hatch out, will be treated by their foster parents (see more about nest parasitism below).
The evolutionary war between parasite and host species
Cowbirds and cuckoos are among the best-known and most-studied North America species that parasitize other bird species' nests. Researchers Rebecca Croston and Mark E. Hauber weighed the consequences of such parasitism in a 2010 article for Nature Education Knowledge:
In an escalating evolutionary arms race, the parasitizing species have developed strategies for outcompeting their hosts, the authors note. These strategies include mimicking the appearance of the host's eggs while having harder shells ("to impede rejection by puncture"). The parasite's eggs also require "slightly shorter incubation times." which gives the parasitic nestlings a size advantage once they hatch. Some parasitic young, including those of the Brown-headed Cowbird, also tend to grow faster, giving them a further advantage in outcompeting the hosts' young for food that the host pair bring to the nest, and some of the parasite's young will even push out the hosts' eggs or kill their hatchlings.
Nest owners who are victims of dumping are not totally defenseless against this behavior. According to Nature Education Knowledge, they can identify, discriminate against, and even reject foreign eggs or chicks on the basis of the way they look, sound, or a mix of such sensory cues. Lederer mentions that the bluebird mother of the parasitized nest, “apparently by recognizing the UV reflectance of her own eggs, moves the foreign eggs to the periphery of the nest where they will be incubated less efficiently.” In a study published in Ethology, researchers found that the host female bluebird can also aggressively protect her nest and brood from other females that come near, while the male tends to be more aggressive with other males, protecting the site and access to his mate.
While most host birds with healthy populations can survive some parasitizing, others are more vulnerable. The Brown-headed Cowbird, which is widespread and will parasitize the nests of several species, "poses a conservation threat to several of its North American passerine hosts," Croston and Hauber warn. (Passerine refers to the order Passeriformes, or perching birds, which includes more than half of all bird species.) The growing loss of habitat and other environmental pressures leave these host species more sensitive to parasitization, especially species that are already in decline. Of the examples the researchers give, only one is recorded in Virginia—the endangered Kirtland's Warbler (Dendroica kirtlandii), which migrates through the commonwealth's western counties. The impact on such endangered birds "can be devastating, and human control of cowbird population size may be necessary to prevent local extinctions," the researchers warn.
It's hard to blame some of these parasites for what may seem to be an arbitrary choice in reproduction. The Brown-headed Cowbird evolved on Midwest prairies, making its living through following nomadic American Bison herds. Being nomads themselves, they couldn't stop to raise their broods. And as humans cut down Eastern forests to make room for farming and homes, the cowbirds found their territory had expanded, with domestic cows taking the place of bison in most places—hence their common name.
The cowbird's relationship with the bison is known as "comensualism"—an association between two organisms in which one benefits and the other derives neither benefit nor harm. As the bison move across the plains, they disturb insects, which the cowbirds then devour. The birds will often work within inches of the massive ungulates to reap the rewards of their association with these mammals.
The Danville nine-egg clutch was discovered by VBS Pittsylvania County coordinator Vickie Fuquay during her monitoring of several bluebird trails in the city. “I knew something was up," she wrote in a VBS newsletter article. Bluebirds lay one egg a day, so the clutch kept growing over after Fuquay first found what seemed like it had reached the typical maximum. She thought that two females must have contributed to the oversized clutch. Bluebirds, like most birds, lay one egg a day. The day Fuquay opened the box and found the ninth egg, she got another surprise: “two mama blues sitting side by side in tight quarters,” incubating the eggs together.
These “sister wives,” as she called them, along with their mates and broods, “really were two families dwelling in one house!” When the chicks started to fledge, Fuquay was “dive bombed" by all four parents. So far, I haven’t been able to find any other accounts of two bluebird families raising clutches together in the same box, which if nothing else, would be a tight fit.
In an email exchange I had with Fuquay, she noted that four eggs in the Danville clutch seemed "slightly larger and darker than the other five," echoing Lederer’s comment on egg coloration as a possible cue for bluebird parents.
Egg dumping can occur for several reasons. “The dumping female may not have been able to find an appropriate nest site, may have lost her nest due to predation or storms, or is a young unmated bird,” Lederer explains. “But even mated females with their own nests sometimes dump additional eggs into other nests,” he adds. “Putting her eggs in another nest gives them some, if only slim, chance of survival.”
Eastern Bluebirds have two to three broods a year. Juveniles from an earlier brood in a year sometimes exhibit “helping” behavior in feeding siblings in a subsequent brood that year, although the quality of that help is questionable, according to some researchers (see below).
Juvenile bluebirds "help" feed siblings
Bluebirds, like many overwintering birds in Virginia, can have several clutches of eggs, and a juvenile from an early brood may try to help feed chicks in a later brood of its parents. NestWatch, a nationwide Cornell Lab of Ornithology monitoring program designed to track the status and trends in the reproductive biology of birds, reports that “in contrast to Western Bluebirds, Eastern Bluebirds rarely have helpers at the nest.” But this year, the nonprofit received “two independent, confirmed reports of juvenile Eastern Bluebirds feeding their (presumed) younger siblings.”
According to NestWatch, such "help" may not in fact be helpful. While research on this is "sparse," anecdotal reports suggest that juveniles may be more of a hindrance than a help: "Juvenile helpers can interfere with adults feeding the nestlings and they often do not adequately ‘prepare’ the foods, i.e., some insects are still wriggling upon delivery!”
Having watched parent songbirds “preparing” prey, mostly insect larvae, I've seen them thoroughly whack it against a branch or other hard surface before eating it or delivering it to their young. And according to Birds of the World, researchers found adult females do not tolerate fledglings around nest sites. Not only are the juveniles not good at preparing food for their younger siblings, they don't place food far enough down the throat of nestlings to make swallowing it easier for them.
According to Cornell Lab of Ornithology’s Birds of the World website, overall about 83 percent of Eastern Bluebird eggs hatch, 75–90 percent of hatchlings fledge, and the overall nest success (nests fledging young) is 55–84 percent. The highest failure is in larger clutches, and nest success rates decrease with decreasing latitude, with large clutches in the South being the most vulnerable to failure.
Fuquay reported one egg in the oversized clutch she found did not hatch and was subsequently removed from the nest by one of the adults, and one unhealthy-looking hatchling disappeared, probably also removed by one of the adults. The nest monitors of the two Rappahannock broods reported success for both: all eggs hatched, and all chicks fledged.
© 2020 Pam Owen