The Chatham Island Black Robin
Back from the brink of extinction
One of the joys of a walk through the New Zealand bush is to be ‘found’ by a robin. These confident, inquisitive little birds will often accompany walkers along the track, feasting on bugs in the disturbed leaf litter and flitting from side to side only metres away from their human companions. Sadly, this cheeky confidence, as endearing as it is, also makes the birds vulnerable to predation from introduced species such as cats, rats, and stoats. Today, New Zealand robins only survive in areas that have intensive pest control measures in place.
By far the most endangered species is the black robin, or kakaruia (Petroica traversi). These beautiful, rotund little birds are endemic to the remote, windswept Chatham Islands, located 800 km (500 mi) off the east coast of New Zealand’s South Island. In 1980, the Chatham Island black robin was the world’s rarest bird with only five individuals left. These included just one breeding pair, a female affectionately known as Old Blue and her mate Old Yellow. The only known extant species of black robin was on the verge of extinction.1
A unique conservation success story
That they have survived to this day is due to the efforts of a small, dedicated Wildlife Service team led by Dr Don Merton (1939–2011). Dr Merton and his team decided on a last-ditch attempt to save the species. They removed the first clutch of eggs laid by Old Blue and placed them in the nest of a closely related species, the Chatham Island tomtit (Petroica macrocephala chathamensis). This technique, known as cross-fostering, proved successful. The tomtits raised the first clutch, and Old Blue, having lost her eggs, laid and raised a second brood. Numbers started to recover, and the fostering program was extended to subsequent breeding pairs.
This was continued until 1990, at which stage the population was deemed to be self-sustaining. By 2021, the number of black robins was estimated to be in the region of 300 birds and slowly growing. Old Blue was last seen alive in December 1983. She had lived to the grand old age of at least 13 years, almost three times the average black robin lifespan.2
Success ‘against the odds’
When a breeding population is substantially reduced in size, it loses variety in the genetic raw material available for future generations. This is because it no longer contains all the versions of genes (alleles) present in the initial, larger population. Such small populations are also more susceptible to losing further versions of genes due to ongoing, random processes (genetic drift).3 With less variety in the genes, a population experiencing such a genetic ‘bottleneck’ will have less variability, i.e., a reduced ability to vary (adapt) in response to future changes in the environment.
With a much smaller population, there is also an increased chance of closely related individuals interbreeding. This heightens the risk of harmful recessive mutations being expressed.
Today, all living black robins are descended from Old Blue and Old Yellow. This was a revelation to conservationists around the world. It had been assumed that the smallest starting population required to rebuild a species was in the region of 50 individuals. This was the number presumed necessary to ensure the genetic health of future populations and to prevent inbreeding depression (the reduced survival and fertility of offspring of related individuals). Chatham Island black robins have passed through the most extreme population bottleneck possible, yet numbers have recovered to levels not seen for more than 100 years.
How do biologists explain this surprising result? One report suggests that the species may have been subject to several previous population bottlenecks resulting in the loss of genes that might cause deleterious inbreeding effects. So, the ‘solution’ to one bottleneck puzzle is … just add more (unobserved) bottlenecks!4 This is, of course, just speculation in the absence of observational evidence.
A lesson for Flood deniers
One of the most often repeated challenges to the Bible is the fact that just one breeding pair of most kinds of air-breathing animal were saved from the floodwaters on Noah’s Ark. This is, of course, an extreme population bottleneck. We are told that its genetic effect is an insurmountable scientific problem for those who hold to a plain reading of the biblical text.
The Chatham Island black robin provides clear observational scientific evidence that this is not the case. Even some 4,500 years and many thousands of generations after the Flood, a single breeding pair can still have sufficient genetic variety to produce a viable population.
Over this time, much speciation has occurred as the Ark kinds have spread out across the earth and adapted to new and diverse environments. This speciation inevitably leads to an increasing loss of gene variety in each of the populations. At the same time many harmful mutations would inevitably have accumulated in the DNA. For these reasons alone, the pairs that came off the Ark would have been in a far better position to propagate genetically healthy populations than any pair of their kind today.
In addition to this, however, the founding pair of black robins were ‘chosen’ at random simply because there were no other options. The pairs chosen by God to represent their kinds on the Ark (Genesis 6:20) were likely chosen for maximum genetic health and genetic variety in order to ensure that their descendants could ‘fill the earth’ in a post-Flood world.
That these birds have survived—despite such disadvantages compared to their Ark ancestors—is a heart-warming story of human ingenuity, dedication, and perseverance in the face of overwhelming odds. It is also testament to the creative genius of our Lord and Saviour, Jesus Christ, and the authenticity of His Word in Genesis
The culling effect of natural selection—a cautionary tale
In 1984, it was noticed that some black robin females were laying eggs on the rim of the nest, rather than in the nest itself. If left on the rim, these eggs would not hatch. So, with the best of intentions, conservationists repositioned the eggs within the nest. Over the next few years, rim laying increased significantly and by 1989 over 50% of all females were exhibiting the behaviour. At this point, repositioning was stopped and by 2011 just 9% of females laid rim eggs.
A subsequent study found there was a genetic basis to rim-laying, resulting in substantially reduced fitness.5 Under normal circumstances, females with the trait incubated significantly fewer eggs, resulting in lower breeding success than females without the trait. Natural selection ensured that genes for the maladaptive trait were less likely to be passed on to future generations.
By repositioning the eggs, scientists had reduced this selection pressure, allowing the trait to become more widespread in the population. Had the repositioning continued, the species may well have become entirely dependent on human intervention for its future survival.
References and notes
- NZ Dept of Conservation, Karure / Kakaruia / Chatham Island black robin, doc.govt.nz, acc. 21 Jan 2023. Return to text.
- Kliman, R., Sheehy, B., and Schultz, J., Genetic drift and effective population size, Nature Education 1(3):3, 2008; nature.com. Return to text.
- Guthrie, K., Remembering Don Merton and a bird called ‘Old Blue’, predatorfreenz.org, 10 Feb 2020. Return to text.
- Team Beauty of Birds, Black Robins or Chatham Island Robins, beautyofbirds.com, 16 Sep 2021. Return to text.
- Massaro, M. and 5 others, Human-assisted spread of a maladaptive behavior in a critically endangered bird, journals.plos.org, 9 Dec 2013. Return to text.
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