The Black Robin Conundrum: What Happens When Humans Move Eggs in the Nest

The black robins on Chatham Islands were in trouble. By 1980 only five birds remained, including a single breeding pair. Introduced rats and cats had devoured the birds nearly to the point of extinction. Given the dire circumstances, researchers felt they had no choice but to step in and encourage the survivors to get busy.

Black robins are known to re-lay if a clutch is lost, so they removed the eggs laid by the last fertile female, “Old Blue,” from her nest, and placed them with birds of a related bird species to be hatched and raised. It worked; Old Blue quickly laid another clutch.

The meddling didn’t end there, though: Scientists also moved black robin eggs that were laid on the edges of nests, which usually don’t hatch, into the middle of the nest to increase their chances of survival. 

But the scientists’ well-intentioned intervention nearly backfired. Because they were moving the eggs to the center of the nest, the bad egg-laying trait was passed on to future generations. By 1989, 50 percent of all black robins were laying “rim eggs,” even though the species had begun to recover.

The black robin’s situation demonstrates how conservation interventions can have unforeseen, and potentially dangerous effects on the recovery of a species, researchers from Australia and New Zealand write in the latest issue of PLoS One. It’s a dilemma that conservationists face: Species have to recover quickly to avoid extinction, yet human efforts to help might, as the authors write, “unintentionally relax selection by allowing the ‘survival of the not-so-fit’.” 

So how did this bad habit get passed on? By reconstructing the bird’s lineage, the researchers found that the original reproductive male—the Adam of Chatham’s black robin population—was a silent carrier of the dominant allele, or genetic trait, for the rim-laying behavior. That means that half of his female offspring were likely to inherit the trait by laying eggs on the rim of the nest, and so on through the generations, and the other half inherited the recessive, or normal egg-laying, allele.  

Since 1989, the behavior has been naturally selected against, with females who lay eggs on the edge of their nest far less likely to see their eggs hatch. As a result, today only nine percent of females lay their egg on the rim of their nests. 

The Chatham Island black robin population has grown to 280 birds. It’s still not recovered, but it’s a move away from the edge.