It's night on Te Hauturu o Toi, Little Barrier Island, and a half moon glows softly behind the clouds. A movement off to our left alerts us and we dive into the brush and extract our prize - a baby North Island brown kiwi (Apteryx mantelli).

Beautiful unfeather-like feathers

Kiwi are renowned for their soft brown plumage that is most unfeather like and prized by Māori for making traditional cloaks (Kahu Kiwi). This little bird is no exception, but is even more spectacular as it is covered not in brown, but beautiful white feathers! Such all-white kiwi are rare and the hatching in 2015 of three all-white kiwi chicks at the Pukaha Mount Bruce National Wildlife Centre has been cause for much celebration and excitement.

The recipe for colour

White colouration in the feathers occurs in many bird species and in the Land Vertebrate Collection at Auckland Museum we have a whole flock from kingfishers to penguins clothed in white. The loss of colour in the feathers is caused by a break down in the bodies' machinery (or melanocyte cells if you like scientific jargon) that puts melanin pigments into growing feathers to provide black and brown colouration.

But how you say? Well if we go back to a freshly fertilised kiwi egg, it will receive two sections of DNA, called genes, which provide the recipe for normal kiwi brown feather colouration, one from mum and one from dad. Let's call these brown alleles (alleles are different forms of the same gene).

Now here's the rub. If the embryo inherits one brown allele and one white allele (a name we might give to a genetic mutation that causes the white feathers), the resulting chick will still have brown feathers. However, very rarely a baby bird will inherit two white alleles and this baby bird's feathers will be all white! Scientists call this a recessive trait because it sits in the background, can be carried in the genes of a population of animals, and is often not revealed until at some stage bang! The trait turns up.

Key point is that some of these recessive traits, and there are many, are not so good for the individual that is unlucky enough to inherit them. Let's take white feathers for example. Pretty safe if you are a baby kiwi safe on predator free Hauturu or nice and snug at Pukaha. But if you were a white kiwi born, say one thousand years ago, well this would be more of a problem.

Evading the visual hunters

Why? Well at that time New Zealand had a number of predators that are now extinct but would have been happy to make a meal of a kiwi including the Laughing owl Sceloglaux albifacies (much larger than the Morepork) a giant harrier hawk, Eyles' harrier Circus teauteensis, weighing 5 kg and a giant 20 kg rail called an Adzebill Atornis sp., with a bill design for bludgeoning its prey to death. All these predators were visual hunters and white kiwi hunting near dusk, but lacking the dull brown camouflage of its mates, would have made a tasty morsel.

The problems of declining genetic diversity

In general genetic mutations that are detrimental to a species tend to be limited in a large populations because animals that have them either die (remember the Adzebill bludgeoning), don't breed well or have evolved adaptions to minimise their impact.

In particular, animals generally avoid inbreeding with close relatives that might share the same genes. This strategy works just fine in large populations with plenty of space. However when population become small its genetic diversity is also reduced and the chance of close relatives breeding with each other, and recessive traits being expressed, moves from remote to much more likely.

When the trait being expressed is bad, scientists call this inbreeding depression. Animal breeders have taken advantage of inbreeding for centuries, pairing up related animals with a desired traits to create French poodles, or even white tigers. Yes that's right white tigers are not a distinct species, rather Bengal tigers Panthera tigris tigris subjected to decades of inbreeding for genes found in a single white male tiger named Mohan captured from the wild in 1951.

Today there are hundreds of Mohan descendants in captivity for our amusement; though the chances of a white tiger being born in the wild is only 1 in 10000 births. Unfortunately thanks to this inbreeding pure-bred dogs and tigers suffer from a range of unpleasant conditions including shortened leg tendons, club feet, kidney problems, crooked backbones and reduced fertility and miscarriages.

Manufacturing a founder effect

You can see there are genetic consequences for animal populations that are reduced to a fraction of their former size. But what has this got to do with Hauturu kiwi you might ask? We must travel back to Little Barrier Island in the early 1900s, where we would find that kiwi were purposefully moved to the island to secure the species following rapid declines on the New Zealand mainland.

Importantly some these birds were selected for their colouration being either all or partially white. As a result we manufactured a perfect small population founder effect, a small number of individuals with genes for white feathers common in the population. It was from this population that 28 kiwi were translocated in 2010 to the Pukaha to begin a breeding programme now famous by its white offspring.

Caring for endangered species in a world in pieces

Now it's time to confess, and you may be on to me, but this story is not really all about white feathered birds, French poodles and tigers. Rather it's a rather protracted and sneaky hook to get us into world of conservation genetics without putting you to sleep. The real point is, from the perspective of wildlife, we humans have created a world in pieces.

We have cut and drained and slashed and burned and hunted and tar sealed and concreted to such an extent that much of the life that shares the planet with us now does so in fragments of habitat, a jigsaw puzzle of isolated smaller populations, each as a result, more susceptible to negative genetic consequences.

Ironically attractive traits such as white plumage in Hauturu kiwi demonstrate nicely the issues managers of kiwi and many other endangered species must be aware of and try to combat through the active management of populations in an increasingly fragmented world.

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