The black robin (Petroica traversi) is one of the world’s most endangered birds – there are only around 250 or so in existence. But it’s also one of the success stories of NZ’s conservation efforts – brought back from the brink of extinction. However, this has come at a genetic cost to these little black birds.
Black robins are found only on Mangere & South East Islands, in the Chathams. Once widespread on the Chathams, by 1976 there were only 7 birds left (2 females & 5 males) on Little Mangere. (How did things get to this state? The usual story: habitat destruction & the introduction of new predators: rats & cats.) NZ Wildlife Service workers – led by Don Merton – moved this last remnant of the species to Mangere Island, which had been extensively replanted. But by 1979 the population was down to 5 (2 pairs plus a lone male) - & one of those pairs was effectively infertile: all their offspring died.
Time for desperate measures – in 1980 conservation workers began a cross-fostering program, placing robin eggs in warbler nests. Unfortunately the warblers couldn’t raise robin chicks successfully, so in 1981 eggs from the only breeding female robin – ‘Old Blue’ – were placed in the nests of tomtits (Petroica macrocephala chathamensis) on South East Island. This worked, & since Old Blue produced second clutches to replace those fostered with the tomtits, the black robin population began to grow, to the point where cross-fostering stopped in 1990.
However, there is a genetic cost to this. The robins have gone through a severe bottleneck & are now highly inbred, as shown by their DNA profiles. And in addition, fostering has had its own impact. When the fostering program began, scientists recognised the possibility that the robin chicks would imprint on their foster parents, the tomtits, rather than recognising their own species when it came to mating. And this did happen – but wrongly-imprinted black robins were taken to Mangere Island, which at the time had no tomtits. But once cross-fostering stopped, and it was thought that most of the wrongly-imprinted robins had died out, tomtits were re-introduced to Mangere. Nonetheless, a male robin did form a breeding pair with a female tomtit, successfully producing offspring. Ma & Lambert (1997) undertook a DNA profiling study of this hybrid family, to see if it was possible to find obvious genetic markers for hybridisation between these closely related species. They also studied other individuals from both species, to assess the level of genetic variation in the populations.
Their findings: both robins and tomtits have a very high degree of genetic similarity between individuals, but there are also microsatellite DNA sequences that are different between the species (found in all tomtits but absent from robins, for example). As for the tomtit/robin family – the ‘black robin’ father turned out to have some of those unique tomtit markers: he was probably the result of an earlier robin/tomtit mating. Ma & Lambert concluded that Despite the fact that this black robin recovery program has been extremely successful, our result, that hybridisation is apparently occurring as a result of cross-fostering, will be of importance to others conducting similar programs. Why? Because of its implications for the gene pool of a highly-endangered species.
W. Ma & D. Lambert (1997) Minisatellite DNA markers reveal hybridisation between the endangered black robin and tomtit. Electrophoresis 18: 1682-1687