Runner Duck Eggs
Mike Ashton asks: what colour should they be?
‘The Runner cannot help but lay large, white and green shelled
eggs of a most delicious flavour.’ Levi D. Yoder, Natural and
Artificial Indian Runner Duck Culture (Pennsylvania, 1910).*
There is a trio of exhibition Fawn-and-white Indian Runners outside the window. One duck lays white eggs. The other lays pale green ones. That is how we can tell the parentage. Sometimes a duck will lay a slightly smaller egg than her sisters’, especially if she is a young bird. Sometimes the egg from one will be a slightly different shape from that of another. There is always slight variability within a flock, giving additional information about who lays what. These are useful management aids.
However, it’s been a bone of contention: ‘What colour should Indian Runner eggs be?’ Recently we had an enquiry from a breeder who was worried that his bird laid white eggs. He had been told that Runners’ should be green. Go back a hundred years and one might well have found breeders panicking that they should in fact be white. What is the answer, and why should it matter?
The nests belonging to wild mallards seem to contain pale green eggs. It might have been important for farmers to tell the difference between their domestic stock and wild intruders. Egg-shell colour might have become a badge of purity in some instances.
Egg Shape and Colour
Throughout the bird world there is tremendous variation in size, shape and shell colour. Birds that lay just one egg in a season can afford to expend more bodily resources (per egg) than those that lay a clutch. A kiwi, for example, can lay an egg about 25% of the female body weight. That is huge. I have seen an X-ray photograph of a kiwi in lay, and it was horrifying. A humming-bird can produce an egg of just 0.3g. An ostrich’s might be 1600g (a mere 1% of the mother’s weight).
All eggs are ‘rounded’. They have a smooth ‘bolus’ shape that allows easy passage along the oviduct. Owls tend to produce almost spherical eggs, but many birds have to maximize volume in relation to the geometry of the pelvic bones. The deeper the pelvis: the rounder the egg. To compromise, most eggs are ‘oval’, a word that means ‘egg shaped’. This is a variation of the ellipse, with one end more bulbous than the other. The evolution of this characteristic is easy to understand if you think of a sea-bird laying its clutch on an exposed cliff ledge. Round or elliptical eggs might roll off into oblivion. Pyriform (pear-shaped) ones tend to roll in tight circles! They are more likely to stay on the ledge. Additionally, pyriform eggs of ground-laying species fit into neat clutches of four, easily covered by the hen plover, for example.
The colour is laid down in the thick, calcium-rich testa of the shell. The pattern tends to be in the cuticle, the outer protective layer. It is only in the later stages of the egg’s passage along the oviduct that the markings are applied by pigment-secreting areas of the uterus. I imagine a sort of spiral production line with little paint guns firing at given intervals. If the production line is quite fast there will be streaks or elongated marks. If it is slow, there may be round spots or even bands.
Evolution has played a massive role in terms of correlating pattern with habitat. Birds that lay in caves or holes tend to have plain whitish eggs. Those laying in the open have often evolved cryptic patterns: they camouflage the eggs against a background of pebbles, leaves or plants. The colours essentially are made up of two groups of pigment: porphyrin (reds and browns) and cyanin (blues and green). Ducks frequently hide their nests in vegetation, hence there is little need for elaborate protective markings. [Ref. Brooke and Birkhead, Cambridge, 1991]
Indian Runner eggs: two from the Fawn-and-white ducks and the very dark one from a Black Runner who had just started to lay again after a short rest. You can see how easily the black pigment can be rubbed off the surface cuticle. The pale green egg is more deeply stained in the hard shell.