Black robins and the problem of inbreeding - December 15th

One of the main reasons why Melanie is interested in studying black robins isn’t because they are very friendly and produce super-cute chicks, but because they are terribly rare. Today, there are only about 160 black robins alive, of those 120 or so live on Rangatira Island and about 40 live on Mangere Island. All of the black robins are descendents of Old Blue, who we talked about a few posts ago, and her mate Old Yellow. In the early 1980s, when Old Blue, Old Yellow and three other robins (named Old Green, Old White and Old Red ) were the only black robins alive at that time, they were forced to breed with close relatives because there were no other robins around. This is why all black robins alive today are very closely related to each other (they are one BIG family, really), although there have been several generations of robins since Old Blue and Old Yellow were a happy couple.

Newly hatched black robin chick:

When all remaining individuals of a population are close relatives, as it is the case in the black robins, biologists like Melanie talk about an ‘inbred’ population. The problem with inbred populations is that they might not be able to withstand environmental changes or an outbreak of an infectious disease. And here is why:
A group of related individuals usually share a lot of their traits, such as eye colour, big ears, immunity against a disease. Ever heard your Auntie say: “you have the same nose as your grandfather Edgar”? Of course, you have the same nose as grandpa Edgar because you are related to grandpa Edgar and as a matter of fact you share about a quarter of your genes with him (and within the quarter of shared genes is the shape of your nose!). Maybe you think that the shape of your nose is not a big issue, but some traits can increase your survival chances.

For example, one family of birds may have better eye sight than another. Bad eye sight might not matter when there are plenty of big worms and bugs around, but let’s say that conditions change and for some reasons there are fewer and only smaller worms and bugs around to eat. In this case, the eagle-eyed birds will do much better than the others and will find more food for themselves and their chicks. Some of the chicks will inherit the good eye sight and again will raise more chicks than birds who can’t see that well. So over time more and more birds will have the good-eye-sight-trait.

In a big, healthy population there are lots of different versions for all kinds of traits around – good eye-sight, small beaks, long tongues. These get mixed and matched in every new generation, with chicks getting one set of genes from mom and another from dad. If the conditions change, chances are good that some individuals have the right traits to survive. Maybe you can already see the issues that arise in populations that are inbred.

Weighing a black robin chick:

Firstly, since there are only few individuals around and they are all related, there is no great variability among the traits they have. To stick with our example, if no bird has the eye sight required to find enough small worms and bugs, it could lead to the extinction of the species. Another problem comes from the fact that sometimes genes don’t work properly. Fortunately, since there are 2 copies of genes, the second copy might be enough to get the right function. But when the parents are closely related, they might pass on the same faulty version of a gene and there is no functioning version around. In bad cases this can lead to deformities or death, which is often observed in small and inbred populations.

The black robins today are the most inbred a population can become, because all are descendents from one happy couple, Old Blue and Old Yellow. And this is why the black robins are the perfect study object for Melanie to find out how they are dealing with environmental changes today.