In May, an Amissville 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.
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.
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.
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 (tinyurl.com/ethology-eggs), 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.
The Danville nine-egg clutch was discovered by VBS Pittsylvania Count ycoordinator Vickie Fuquay during her monitoring of several bluebird trails in the city. “I knew something was up," she wrote in a VBS newsletter article. 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.”
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
Find more about nest parasitism and other aspects of nature, along with upcoming nature events, and view photos and slideshows, at Pam’s Wild Ideas blog.